Ionic Liquid-Based Surfactants: Recent Advances in Their Syntheses, Solution Properties, and Applications

Abstract

The impetus for the expanding interest in ionic liquids (ILs) is their favorable properties and important applications. Ionic liquid-based surfactants (ILBSs) carry long-chain hydrophobic tails. Two or more molecules of ILBSs can be joined by covalent bonds leading, e.g., to gemini compounds (GILBSs). This review article focuses on aspects of the chemistry and applications of ILBSs and GILBSs, especially in the last ten years. Data on their adsorption at the interface and micelle formation are relevant for the applications of these surfactants. Therefore, we collected data for 152 ILBSs and 11 biamphiphilic compounds. The head ions of ILBSs are usually heterocyclic (imidazolium, pyridinium, pyrrolidinium, etc.). Most of these head-ions are also present in the reported 53 GILBSs. Where possible, we correlate the adsorption/micellar properties of the surfactants with their molecular structures, in particular, the number of carbon atoms present in the hydrocarbon “tail”. The use of ILBSs as templates for the fabrication of mesoporous nanoparticles enables better control of particle porosity and size, hence increasing their usefulness. ILs and ILBSs form thermodynamically stable water/oil and oil/water microemulsions. These were employed as templates for (radical) polymerization reactions, where the monomer is the “oil” component. The formed polymer nanoparticles can be further stabilized against aggregation by using a functionalized ILBS that is co-polymerized with the monomers. In addition to updating the literature on the subject, we hope that this review highlights the versatility and hence the potential applications of these classes of surfactants in several fields, including synthesis, catalysis, polymers, decontamination, and drug delivery.

Note: Abbreviations and acronyms are listed after Acknowledgments

1. Introduction

Ionic liquids (ILs) are electrolytes whose melting points are, by operational definition, ≤100 °C. Ionic liquid-based surfactants (ILBSs) are ILs that carry hydrophobic “tails” and hence form colloidal aggregates in water, e.g., micelle and vesicles. Single-chain ILBSs can be covalently linked to form dimers (so-called gemini surfactants, GILBSs), trimers and, eventually, polymeric ILBSs. This structure versatility can be exploited to obtain different structures as shown in Figure S1 (in Supplementary Material) [1], and to obtain several colloidal morphologies, as can be seen in Figure 1 for a series of 1-hexadecyl-3-R-imidazolium bromides, allowing potentially interesting applications. Thus, the increase in the length of R from C₂ to C₁₆ leads to changes from isotropic solution to worm-like micelles, hexagonal liquid crystals, hydrogel and, eventually, surfactant precipitation [2]. Additionally, these surfactants also form thermodynamically stable water-in-oil (W/O) and oil-in-water (O/W) microemulsions that are used, e.g., in polymerization [3].

This review article is focused on ILBSs and GILBSs. Using literature data of (mostly) the last 10 years, we highlight the relationship between surfactant molecular structure and solution properties that are relevant to applications. Of these properties, we dwell on the adsorption parameters of the surfactants at the water/air interface and the characteristics of the formed aggregates. These data are important per se, and are fundamental for the development of novel applications. For example, ILBSs are employed as templates to fabricate nanoparticles (NPs) of different sizes and morphologies. Microemulsions (μEs) formed by these surfactants, both W/O and O/W, were also employed as templates for (free radical) polymerization, where the monomer acts as the “oil” component. Additionally, electrolytes and drugs, especially those with hydrophobic ions that carry opposite charge to the ILBS head-group, change the morphology of the aggregate, e.g., micelle → vesicle, with potential applications in drug delivery. ILBSs with a functional group in the long-chain (e.g., an ester or amide group) undergo reversible transitions—micelle ⇄ vesicle ⇄ organogel—on changing temperature and surfactant concentration. Vesicles and organogels have potential applications in drug delivery and waste-water decontamination (vide infra). ILBSs that carry a polymerizable group (usually a double bond) are advantageously employed in polymerization in μE media because the polymer core is covered with a surfactant shell, leading to enhanced NP stability. The hydrophilic/hydrophobic character of the NPs can be controlled by ion exchange of the anion of (co-polymerized) surfactant with other anions.

Our original premise was to limit the data discussed to ILBSs that conform to the m.p. criterion, i.e., ≤100 °C. A literature survey, however, showed that m.p.s are not reported for many compounds that are classified (by the authors) as ILBSs. In other words, our criterion for considering compounds such as ILBSs and GILBSs is either the availability of m.p. or classification of the surfactant as such by the authors. We included a few applications that use ILs because some of these are weakly surface-active [4].

The issue of surface-active purity of the surfactants employed should not be overlooked. Demonstrating this purity is important because uncertainty in the value of the critical micelle concentration (cmc) bears on the calculated adsorption and micellar parameters [5,6]; removing surface-active impurities from the surfactant solution is, at best, time-consuming and laborious [7]. Another aspect that should be considered when discussing ILBSs is their stability in aqueous media. In this regard, the purity and hence the data of aqueous solutions of ILBSs with BF₄⁻ and PF₆⁻ anions should be regarded with some reserve. The reason is that these ions are hydrolytically unstable in water, even at room temperature; this instability was demonstrated by several techniques [8,9,10,11]. This affects the physicochemical properties of the micellar solutions, e.g., cmc, the average aggregation number (N_agg), and the degree of counter-ion dissociation (α_mic). This problem was mentioned explicitly by some authors (precaution was taken to suppress its effect) [12] but not others [13], even when the ILBSs were heated with methanol at 85 °C for 8 h [14]. A literature survey using the search terms (HF, pH and hydrolysis) for the above-mentioned ILBSs showed that the time elapsed between preparing the ILBS solutions and the measurements/applications was not mentioned [15,16,17,18,19,20,21,22,23,24,25,26]. Therefore, we stress that this instability problem should not be overlooked; its potential effect on micellar parameters and other applications should be assessed. Based on these considerations, we feel justified in our decision to exclude from the parts of surfactant adsorption at the water/air interface and micellization of ILBSs with hydrolytically unstable ions, in particular BF₄⁻ and PF₆⁻.

Regarding the abbreviations/acronyms that we employed, we refer to each of the discrete structural moieties using two letters. For example, Im, Py and Vn refer to imidazole, pyridine and vinyl group, respectively. The alkyl moieties attached to the surfactant head-group are listed as C₁, C₂, C₃, and C₄ for methyl, ethyl, n-propyl, and n-butyl, respectively. Unless specified otherwise, the alkyl groups are n-alkyl. Usually, one of the two groups attached to the heteroatom is a long-chain. Therefore, C₁₆C₁ImBr, C₁₂C₁ImC₈SO₃ and C₁₂C₁ImDBS refer to 1-(1-hexadecyl)-3-methylimidazolium bromide, 1-(1-dodecyl)-3-methylimidazolium 1-octanesulfonate and 1-(1-dodecyl)-3-methylimidazolium dodecylbenzene sulphonate, respectively.

The presence of certain functional groups (e.g., amide and ester) in the hydrophobic tail is interesting because it may lead to reversible morphology transitions, e.g., micelle ⇄ vesicles ⇄ ionogel as a function of concentration of the ILBS and solution temperature, due to changes in the hydration of the functional group. Formation of ionogels can be exploited, e.g., in waste-water decontamination and drug delivery [27,28]. Equally important, however, is that the presence of these hydrolysable functional groups contributes to their aerobic biodegradation [29], an issue that is becoming important due to their increased applications, e.g., in high-temperature lubricants, for gas chromatography (stationary phases), in conductive polymer supercapacitors, and as gel polymer electrolyte for sodium-ion batteries [30,31,32,33,34,35].

The environmental impact (biodegradation and toxicity) of ILs and ILBSs should be assessed. Thus, several studies on the relationship between their molecular structures and toxicity showed that the most toxic (to aquatic life) are those carrying aromatic/heterocyclic cations and long alkyl chains; most anions play a minor role in toxicity. Therefore, the synthesis of a new generation of easily biodegradable ILs and ILBSs from renewable sources was studied [36,37,38]. It was shown that ester functionality enhances biodegradation of ILs; furthermore, adding a methyl group to the 2-position of the imidazolium cation and use of alkyl sulfate as a counter-ion also improves the biodegradability [36].

The importance of ILs and ILBSs can be readily assessed by examining Figure 2, which shows the number of publications on both classes of compounds from 2000 to 2020 based on a SciFinder database search. Figure 2 clearly shows an exponential growth of these numbers, a consequence of their molecular structure versatility, and hence potential applications in several fields.

2. Strategies for Synthesizing Mono-Cationic and Gemini Ionic Liquid-Based Surfactants

The synthesis of ILBS is usually carried out by two consecutive steps: quaternization of amines or phosphines, usually by the S_N2 mechanism (e.g., by the Menshutkin reaction) using alkyl halides, or alkyl sulphates, followed by anion exchange, where necessary, to yield the desired product. These quaternization reactions are simple and relatively efficient. The amine (or phosphine) is mixed with the desired alkyl halide, followed by stirring and heating. The effects of reaction variables on the yield are those known for S_N2 reactions. For example, for alkyl halides, the expected order is RI > RBr > RCl (R = n-alkyl group); an increase in the chain length of R decreases the reaction rate [39,40].

The most frequently employed procedures for the Menshutkin reaction include reflux in an appropriate molecular solvent, e.g., acetonitrile. The reaction between 1-methylimidazole and 1-chloroalkanes in acetonitrile under reflux generally requires 2 to 3 days [41,42,43,44,45,46,47]. ILBSs were alternatively synthesized in the absence of solvents, using microwave irradiation [48] or a combination of microwave and ultrasound irradiation [49]. The obtained products are termed first-generation ILs and ILBSs.

Second-generation ILs and ILBSs are obtained from their first-generation counterparts by a metathesis reaction, leading to ILBSs containing bulkier anions, e.g., BF₄⁻, PF₆⁻, C₆H₅CO₂⁻ and (CF₃SO₂)₂N⁻. The synthesis steps are summarized in Scheme 1.

The most common ILBSs are synthesized from 1-methylimidazole, which is commercially available at a low cost, along with a small number of other N-alkyl substituted imidazoles [40]. ILBSs that carry ester or ether groups are of interest especially because of their biodegradability [50]. Ester- and amide-containing ILBSs were synthesized by the S_N reaction of substituted imidazole with α-bromo ester or α-bromo amide. Examples are shown in Scheme 2. ILBS derived from other heterocycles, e.g., pyridine, pyrrolidine, and morpholine, were synthesized by the same general procedure [51,52,53].

GILBSs can be synthesized by two successive S_N reactions on imidazole. This requires protecting one of the nitrogen atoms, e.g., by reaction with acrylonitrile if the two attached alkyl groups are different. After the first alkylation, the protecting group is removed by E1cB-type elimination, followed by the second alkylation with a dihaloalkane [43,45], as shown in Scheme 3.

Alternatively, GILBSs were synthesized by reacting imidazole with a dihaloalkane, followed by alkylation of the two “outer” nitrogen atoms, see Scheme 4.

Functionalized GILBSs were synthesized by reacting 1-methylimidazole with diesters containing two bromo substituents and two long chains [54]. Similarly, thioether containing GILBSs were prepared from alkane-1,2-dithiol, alkenes and N-bromosuccinimide, the intermediate was then reacted with 1-methylimidazole to form the GILBS [55]. Non-imidazolium GILBSs were prepared by a straightforward one-step reaction, e.g., of tridodecylamine and dibromoalkanes. The class of gemini pyrrolidine-based ILs was synthesized by the consecutive reaction of the secondary amine with a long chain alkyl bromide, followed by reaction of the N-alkylpyrrolidine with 1,4-dibromobutane [56].

3. Relevant Properties of Aqueous Solutions of ILBS

3.1. Compilation and Discussion of the Properties of Aqueous Solution of ILBSs

As already mentioned, one of the most relevant aspects of ILs is their molecular structural versatility, as can be shown, e.g., by imidazolium-based surfactants. In addition to different anions (halides, alkyl sulphate, carboxylates, etc.), different substituents can be introduced at the two nitrogen atoms and at the three carbon atoms of the diazole ring.

Table 1. Literature data on aqueous solutions of ionic liquid-based surfactants at 25 °C. Adsorption and micellar parameters calculated from surface tension data.

Entry	Cation 1	Anion 2	cmc × 103 (mol L−1) 3	γcmc (mN m−1) ⁴	Πcmc (mN m−1) ⁵	Γmax (mol m−2) ⁶	Amin (Å2) ⁷	pC20 ⁸	ΔG0ads (kJ mol−1) ⁹
Cationic ILBSs
1	C8C1Im+	Cl−	116 [58], 101.7 [59], 190 [60], 108.9 [61], 170.2 [62]	28.3 [59], 33.5 [61], 37.3 [62]	43.3 [59], 32.2 [60], 38.5 [61]	2.682 [58], 1.6 [59], 1.52 [61], 1.24 [62]	104 [59], 84 [60], 109 [61], 133 [62]	1.8 [59], 1.67 [60], 1.92 [61]	−43.33 [58], −47.6 [61], −24.6 [62]
2		Br−	121 [45], 120.0 [59], 170 [63], 119.29 [64]	41 [45], 28.7 [59], 41.3 [63], 29.33 [64]	44.9 [59], 42.87 [64]	2.7 [59], 3.065 [64]	60 [59], 124 [63], 53 [64]	1.8 [59]	−36.5 [64]
3		I−	94.9 [59]	28.2 [59]	44.4 [59]	1.4 [59]	117 [59]	2.0 [59]
4		C1SO3−	220 [65]		29.0 [65]			1.0 [65]
5		C4SO3−	140 [65]		28.5 [65]			1.6 [65]
8	C10C1Im+	Cl−	39.90 [42], 45 [60], 40.00 [66], 40.0 [67], 40 [68], 40 [69], 27.7 [70]	27.3 [66]	33.7 [60], 44.5 [66]	1.9 [66], 1.84 [67], 1.84 [68], 1.84 [69]	85 [42], 92 [60], 85 [66], 90 [67], 90 [68], 90 [69]	2.55 [60], 2.5 [66], 2.60 [68]	−30.16 [42], −48.55 [67], −48.00 [68], −48.55 [69]
9		Br−	20 [45], 33 [63], 36.7 [71], 27.24 [72], 39.6 [73]	39 [45], 39.1 [63], 37.3 [71], 48.95 [72], 36.04 [73]	35.5 [71], 23.02 [72], 33.37 [73]	1.82 [71], 1.75 [72], 1.71 [73]	91 [63], 57.6 [74], 91.4 [71], 94.61 [72], 97.01 [73]	1.61 [72]
10		C1SO3−	60 [65]		28.5 [65]			1.5 [65]
12	C12C1Im+	Cl−	13.17 [42], 14.80 [66], 13.8 [70], 16.8 [75], 13.86 [76], 13.25 [77], 11.62 [78]	38.7 [66], 38.4 [75], 30.45 [76]	33.6 [66], 41.55 [76], 28.6 [77]	2.3 [66], 2.91 [75], 2.04 [76]	72 [42], 72 [66], 57 [75], 81.39 [76], 72 [77]	2.4 [66], 2.16 [75], 2.72 [76]	−32.03 [42]
13		Br−	4.3 [45], 9.0 [63], 9.19 [72], 10.6 [75], 9.29 [77], 10.35 [79], 8.22 [80], 11.21 [81], 8.73 [82], 9.68 [83], 9.0 [84], 9 [85]	35 [45], 37.2 [63], 46.71 [72], 36.8 [75], 38.96 [80], 42.9 [81], 38.68 [82], 37.40 [83]	25.26 [72], 29.3 [77], 33.06 [80], 33.94 [82]	1.80 [72], 3.03 [75], 4.48 [79], 3.36 [80], 3.095 [82]	67 [63], 92.49 [72], 64 [74], 55 [75], 71 [77], 37.1 [79], 49.42 [80], 53.7 [82]	2.35 [72], 2.16 [75], 3.06 [79], 2.54 [80], 2.45 [82]
14		I−	4.6 [75], 4.76 [77]	31.7 [75]	37.7 [77]	4.47 [75]	37 [75], 62 [77]	2.80 [75]
15		C1SO3−	14 [65]		28.5 [65]			2.1 [65]
17	C14C1Im+	Cl−	2.98 [42], 3.10 [70], 3.63 [86]	34.15 [86]	36.65 [86]	2.25 [86]	56 [42], 74 [86]	2.89 [86]	−33.81 [42], −55.55 [86]
18		Br−	1.9 [63], 2.69 [83], 2.76 [87], 2.6 [88], 2.8 [89]	37.2 [63], 37.70 [83], 37 [88], 39.2 [89]	41.4 [87], 33.8 [89]	1.74 [87], 1.26 [88], 1.96 [89]	67 [63], 64.8 [74], 95 [87], 132 [88], 84.7 [89]	3.6 [87], 3.03 [88], 3.33 [89]	−47.48 [87]
19	C16C1Im+	Cl−	0.87 [42], 1.14 [66], 0.89 [90], 0.87 [91]	37.0 [66], 40.9 [91]	34.8 [66], 28.8 [90]	3.4 [66], 2.06 [90], 3.4 [91]	49 [42], 49 [66], 80 [90], 49 [91]	3.2 [66], 3.4 [90], 3.39 [91]	−35.23 [42], −35.64 [91]
20		Br−	0.8 [45], 0.610 [72], 0.78 [79], 0.51 [83], 0.55 [89], 0.71 [91], 0.566 [92]	41 [45], 44.53 [72], 37.41 [83], 39.1 [89], 38.7 [91], 38.98 [92]	27.44 [72], 33.9 [89], 33.05 [92]	2.00 [72], 4.20 [79], 2.03 [89], 3.0 [91], 2.06 [92]	83.14 [72], 39.6 [79], 81.6 [89], 54 [91], 80.7 [92]	3.42 [72], 3.85 [79], 3.78 [89], 3.55 [91]	−36.83 [91]
21	C18C1Im+	Cl−	0.40 [66]	42.0 [66]	29.8 [66]	3.7 [66]	45 [66]	3.6 [66]
22	C12C2Im+	Br−	6.40 [80]	38.09 [80]	33.93 [80]	3.09 [80]	53.74 [80]	2.70 [80]
23	C16C2Im+	Cl−	0.88 [91]	35.4 [91]		2.6 [91]	63 [91]	3.62 [91]	−38.29 [91]
24		Br−	0.55 [91], 0.26 [93]	39.8 [91], 32.2 [93]	40.3 [93]	2.7 [91], 2.58 [93]	61 [91], 64.2 [93]	3.65 [91], 4.22 [93]	−38.19 [91], −66.32 [93]
25	C10VnIm+	Br−	27.20 [93]	34.5 [93]	37.3 [93]	1.86 [93]	89.3 [93]	2.44 [93]	−50.60 [93]
26	C12VnIm+	Br−	7.00 [93]	34.1 [93]	38.4 [93]	2.03 [93]	81.8 [93]	2.80 [93]	−56.31 [93]
28	C14VnIm+	Br−	1.85 [93]	33.8 [93]	38.6 [93]	2.18 [93]	76.2 [93]	3.40 [93]	−61.30 [93]
29	C16VnIm+	Br−	0.60 [91], 0.48 [93]	37.7 [91], 33.5 [93]	39.1 [93]	3.1 [91], 2.53 [93]	53 [91], 65.7 [93]	3.63 [91], 4.00 [93]	−37.09 [91], −65.47 [93]
30	C12C3Im+	Br−	5.05 [80]	37.56 [80]	34.46 [80]	2.20 [80]	75.48 [80]	2.85 [80]
31	C16C3Im+	Cl−	0.71 [91]	35.2 [91]		2.2 [91]	75 [91]	3.82 [91]	−40.78 [91]
32		Br−	0.44 [91]	39.7 [91]		2.3 [91]	73 [91]	3.80 [91]	−40.46 [91]
33	C16AlIm+	Br−	0.51 [91]	38.7 [91]		2.6 [91]	63 [91]	3.74 [91]	−38.90 [91]
34	C8C4Im+	Br−	41 [45], 75.8 [94]	40 [45], 33.2 [94]	37 [94]	1.3 [94]	126.9 [94]	2.3 [94]	−51.1 [94]
35	C10C4Im+	Br−	6.3 [45]	36 [45]
36	C12C4Im+	Br−	2.4 [45], 3.68 [80], 5.3 [94]	38 [45], 34.92 [80], 33.0 [94]	37.10 [80], 37.2 [94]	2.02 [80], 2.1 [94]	82.21 [80], 76.9 [94]	3.18 [80], 2.9 [94]	−53.2 [94]
37	C16C4Im+	Cl−	0.50 [91]	38.0 [91]		2.1 [91]	80 [91]	3.89 [91]	−41.86 [91]
38		Br−	0. 1 [45], 0.40 [91]	45 [45], 40.3 [91]		1.9 [91]	86 [91]	3.92 [91]	−42.73 [91]
39	C16C5Im+	Cl−	0.35 [91]	39.6 [91]		1.6 [91]	106 [91]	4.15 [91]	−46.37 [91]
40	C8C8Im+	Br−	5.6 [45], 8.0 [95]	32 [45], 25.9 [95]
41	C12C12Im+	Br−	0.1 [45]	28 [45]
42	C10C1C1Im+	Br−	43.0 [96]		30.9 [96]	1.7 [96]	99.4 [96]
43	C12C1C1Im+	Cl−	12.27 [97]	31.21 [97]	40.79 [97]	0.95 [97]	1.75 [97]	2.75 [97]	−78.78 [97]
44	C10C1C10Im+	Cl−	1.23 [98]	32.7 [98]		1.98 [98]	83.5 [98]		−46.66 [98]
45	C8Py+	Cl−	181 [99]	36.84 [99]	34.6 [99]	1.70 [99]	98 [99]	1.6 [99]
46		Br−	180 [63]	41.9 [63]			66 [63]
48	C10Py+	Cl−	65.5 [100]
49		Br−	30 [63]	40.7 [63]			61 [63]
50	C11Py+	Br−	19.5 [101]
51	C12Py+	Cl−	14.0 [101]
52		Br−	9.3 [63]	39.3 [63]			71 [63]
53	C13Py+	Br−	4.57 [101]
54	C14Py+	Cl−	3.20 [102]
55		Br−	2.2 [63], 2.65 [101]	38.0 [63]			86 [63]
56	C16Py+	Cl−	0.99 [103]	49 [103]	23.0 [103]	1.17 [103]	142.0 [103]	3.00 [103]	−25.7 [103]
57		Br−	0.62 [101], 0.9 [103]	49 [103]	23.0 [103]	0.91 [103]	142 [103]	3.03 [103]	−25.70 [103]
58	C8-(o-C1)Py+	Cl−	166 [99]	31.80 [99]	40.1 [99]	1.67 [99]	99 [99]	3.0 [99]
63	C8-(m-C1)Py+	Cl−	170 [99]	32.38 [99]	39.2 [99]	1.71 [99]	97 [99]	1.9 [99]
65	C10-(m-C1)Py+	Cl−	45 [85]		27.9 [85]		121 [85]	1.62 [85]
66	C12-(m-C1)Py+	Cl−	13 [85]		27.9 [85]		108 [85]	2.22 [85]
67		Br−	10 [85], 9.83 [104]	39.2 [104]	32.9 [104]	1.84 [104]	90.3 [104]	2.8 [104]	−55.00 [104]
68	C14-(m-C1)Py+	Cl−	3.1 [85]		28.0 [85]		92 [85]	2.82 [85]
69		Br−	2.26 [104]	38.8 [104]	33.3 [104]	2.09 [104]	79.4 [104]	3.1 [104]	−58.39 [104]
70	C16-(m-C1)Py+	Cl−	0.8 [85]		27.9 [85]		80 [85]	3.39 [85]
71		Br−	0.508 [104]	37.6 [104]	34.5 [104]	2.38 [104]	69.7 [104]	3.8 [104]	−63.00 [104]
72	C18-(m-C1)Py+	Cl−	0.3 [85]		27.8 [85]		76 [85]	3.87 [85]
73	C8-(p-C1)Py+	Cl−	175 [99]	31.00 [99]	40.7 [99]	1.65 [99]	101 [99]	2.2 [99]
79	C10C1Pn+	Br−	31 [105]	28.6 [105]		3.8 [105]	44 [105]
80	C12C1Pn+	Br−	16 [105]	25.1 [105]		4.4 [105]	38 [105]
81	C14C1Pn+	Br−	7.4 [105]	28.7 [105]		4.5 [105]	37 [105]
82	C16C1Pn+	Br−	3.3 [105]	31.2 [105]		5.2 [105]	32 [105]
83	C18C1Pn+	Br−	1.5 [105]	32.7 [105]		4.4 [105]	38 [105]
86	C12C1Pyrro+	Cl−	19.60 [106]	34.4 [106]	36.6 [106]	2.4 [106]	69 [106]	2.3 [106]
87		Br−	15 [107], 13.5 [108]	42.4 [108]	30.3 [108]	3.03 [108]	54.8 [108]
88	C14C1Pyrro+	Br−	3.30 [108]	42.7 [108]	30.0 [108]	3.55 [108]	46.8 [108]
89	C16C1Pyrro+	Br−	0.860 [108], 0.72 [109]	41.2 [108], 36.7 [109]	31.5 [108]	3.67 [108]	45.2 [108], 51 [109]
90	C18C1Pyrro+	Cl−	0.42 [66]	36.5 [66]	35.3 [66]	3.5 [66]	48 [66]	3.7 [66]
92	C8C4Pyrro+	Br−	150 [107]
93	C12C4Pyrro+	Br−	6 [107]
98	C12C1Pip+	Cl−	19.79 [110]	36.5 [110]	35.3 [110]	2.4 [110]	68.5 [110]	3.35 [110]
99		Br−	11 [85], 11.83 [111]	41.43 [111]	31.57 [111]	2.31 [111]	71.82 [111]	2.33 [111]
100	C14C1Pip+	Br−	3.22 [111]	41.23 [111]	31.77 [111]	2.35 [111]	70.65 [111]	2.90 [111]
101	C16C1Pip+	Br−	0.68 [109], 0.73 [111]	37.3 [109], 41.14 [111]	31.86 [111]	2.63 [111]	61 [109], 63.22 [111]	3.51 [111]
102	C18C1Pip+	Cl−	0.45 [66]	37.7 [66]	34.1 [66]	3.3 [66]	50 [66]	3.8 [66]
103	C16C1Aze+	Br−	0.590 [109]	37.9 [109]			65 [109]
104	C16C1Azo+	Br−	0.51 [109]	38.8 [109]			75 [109]
105	C10C1Mor+	Br−	30 [112]
106	C12C1Mor+	Cl−	21.80 [106]	34.6 [106]	36.4 [106]	2.5 [106]	66 [106]	2.3 [106]
107		Br−	9.6 [112]
108	C14C1Mor+	Br−	4.0 [112], 2.93 [113], 4.10 [114]		41.2 [113]	2.79 [113]	59 [113]	3.2 [113]	−15.4 [113]
109	C16C1Mor+	Br−	0.74 [109], 1.0 [112], 1.02 [113], 1.00 [114]	29.9 [109]	38.5 [113]	2.69 [113], 1.82 [114]	71 [109], 61 [113], 91 [114]	3.6 [113]	−9.5 [113], −42 [114]
110	C18C1Mor+	Br−	0.33 [112]
111	C8Gu+	Cl−	75 [53]	24.5 [53]		4.60 [53]	36.2 [53]
112	C10Gu+	Cl−	22 [53]	23.5 [53]		3.78 [53]	44.2 [53]
113	C12Gu+	Cl−	5.5 [53]	24.1 [53], 24.5 [115]		3.93 [53], 4.54 [115]	42.3 [53], 36.6 [115]
115	C8C1C1Gu+	Cl−	7.2 [53]	35.4 [53]		3.66 [53]	45.4 [53]
116	C10C1C1Gu+	Cl−	2.1 [53]	33.3 [53]		3.40 [53]	48.9 [53]
117	C12C1C1Gu+	Cl−	0.67 [53]	32.5 [53]		3.05 [53]	54.5 [53]
118	C8Ph3P+	Br−	32 [116]
120	C10Ph3P+	Br−	6.1 [116], 9.0 [117], 6.60 [118]	40.00 [118]
122	C12Ph3P+	Br−	2.0 [116], 1.51 [118], 2.35 [119], 2.00 [120], 2.00 [121]	40.80 [118], 46.0 [120]	26.5 [120], 32.4 [121]	2.24 [119], 1.36 [121]	74 [119], 122 [121]	3.01 [120]	−60.2 [119], −64.2 [121]
123	C14Ph3P+	Br−	0.33 [116], 0.61 [118], 0.61 [119], 0.60 [122]	40.60 [118], 41.3 [122]		1.88 [119]	88 [119], 82.2 [122]		−56.0 [119]
124	C16Ph3P+	Br−	0.10 [116], 0.15 [118], 0.24 [119], 0.14 [121], 0.15 [123], 0.10 [124], 0.10 [125]	40.25 [118]	25.8 [121], 27.2 [123], 31.4 [124]	1.02 [119], 1.38 [121], 1.39 [123], 1.40 [124]	163 [119], 120 [121], 119 [123], 118.0 [124]		−75.3 [119], −71.1 [121], −51.3 [123], −58.9 [124]
125	C18Ph3P+	Br−	0.018 [116]
Anionic ILBSs
127	(CH3)4N+	AOT−	2.90 [126], 2.48 [127]	29.4 [127]		1.60 [126]	104 [126], 96 [127]
129	(C2H5)4N+	AOT−	2.45 [126], 2.07 [127]	28.7 [127]		1.43 [126]	116 [126], 101 [127]
130	(C3H7)4N+	C12SO4−	1.46 [127]	31.8 [127]			67 [127]
131		AOT−	0.97 [126], 1.27 [127]	26.1 [127]		1.71 [126]	97 [126], 96 [127]
132	(C4H9)4N+	C8SO4−	24.5 [128]	32.0 [128]		3.75 [128]
133		C10SO4−	4.17 [128]	31.6 [128]		4.03 [128]
134		C12SO4−	0.525 [128]	31.2 [128]		4.47 [128]
135		C14SO4−	0.26 [129]			5.43 [129]	31 [129]
136		AOT−	0.77 [126]			1.63 [126]	102 [126]
137	C4Py+	DBS−	0.92 [130]	29.69 [130]	42.31 [130]	2.31 [130]	72.08 [130]	3.92 [130]
138	C4C1Pyrro+	C12SO4−	2.7 [131]	34.3 [131]	37.9 [131]	2.27 [131]	74 [131]	3.5 [131]
139	C4C1Im+	C8SO3−	135 [65]		40.0 [65]			1.6 [65]
140		C12SO3−	4.4 [132]	36.9 [132]	35.9 [132]	1.14 [132]	145 [132]	3.05 [132]
141		C8SO4−	33.4 [61], 23.0 [62], 34.9 [133], 30.5 [134], 30 [135], 34.5 [136], 31.9 [137], 30.0 [138]	26.1 [61], 30.5 [62], 26.1 [133], 29.6 [134], 31.5 [135], 30.3 [136], 32.8 [137], 34.5 [138]	45.9 [61], 45.9 [133], 42.4 [134], 41.0 [136], 37.5 [138]	2.44 [61], 1.63 [62], 1.9 [133], 2.18 [134], 1.87 [135], 1.39 [136], 2.079 [138]	68 [61], 102 [62], 87.1 [133], 76 [134], 89 [135], 12 [136], 69 [137], 131 [138]	2.60 [61], 2.5 [133], 2.56 [135], 1.52 [138]	−49.7 [61], −38.7 [62], −49.5 [134]
142		C10SO4−	8.8 [139]	34.7 [139]	37.9 [139]	2.81 [139]	59 [139]	2.7 [139]
143		C12SO4−	1.8 [131], 2.4 [133], 2.30 [140], 1.9 [141], 1.84 [142]	31.9 [131], 34.4 [133], 32.9 [140], 31.90 [142]	40.3 [131], 37.6 [133], 39.90 [142]	2.53 [131], 2.4 [133], 2.48 [142]	66 [131], 67.8 [133], 56 [140], 67 [142]	3.4 [131], 3.3 [133], 3.82 [142]
144		C14SO4−	0.5 [139]	30.5 [139]	42.1 [139]	1.66 [139]	10 [139]	4.2 [139]
145		DBS−	1.08 [130]	29.18 [130]	42.82 [130]	2.09 [130]	79.86 [130]	3.94 [130]
146		AOT−	1.78 [140]	25.7 [140]			86 [140]
147		TC−	0.55 [140]	24.8 [140]			111 [140]
148	C5C1Im+	C12SO4−	1.6 [143]	57.86 [143]	13.64 [143]	0.96 [143]	173.4 [143]		−59.23 [143]
149		DBS−	0.32 [144]	30.92 [144]	42.38 [144]	1.91 [144]	86.76 [144]	−1.03 [144]
150	C6C1Im+	C8SO4−	14.2 [133], 18.6 [137]	25.6 [133], 30.3 [137]	45.2 [133]	1.9 [133]	84.7 [133], 66 [137]	2.9 [133]
151		C12SO4−	1.1 [133], 0.8 [139]	27.1 [133], 30.0 [139]	44.9 [133], 42.6 [139]	2.4 [133], 2.08 [139]	68.5 [133], 80 [139]	4.1 [133], 4.0 [139]
152	C7C1Im+	DBS−	0.12 [144]	34.21 [144]	39.09 [144]	1.35 [144]	122.64 [144]	−1.15 [144]
Biamphiphilic ILBSs
153	C8C1Im+	C8SO3−	12 [65]		43.7 [65]			2.9 [65]
154		C8SO4−	4.1 [133]	24.4 [133]	47.6 [133]	2.5 [133]	66.0 [133]	3.3 [133]
155		C12SO4−	0.4 [133], 0.3 [139]	26.0 [133], 26.9 [139]	46.0 [133], 45.7 [139]	2.4 [133], 2.33 [139]	68.5 [133], 71 [139]	4.3 [133], 4.5 [139]
156	C10C1Im+	C12SO4−	0.1 [139]	25.4 [139]	47.2 [139]	2.36 [139]	70 [139]	5.0 [139]
157	C16Py+	C8SO3−	54 [145]
158		C8SO4−	24 [145]
162	C16(CH3)3N+	C8SO3−	88 [145]
163		C8SO4−	26 [145]

¹ Abbreviations: Imidazolium (Im⁺), pyridinium (Py⁺), 2-pyrrolidinonium (Pn⁺), pyrrolidinium (Pyrro⁺), piperidinium (Pip⁺), azepanium (Aze⁺), azocanium (Azo⁺), morpholinium (Mor⁺), guanidinium (Gu⁺), ammonium (N⁺) and phosphonium (P⁺). ² Acronyms: DBS, AOT, and TC refer to dodecylbenzene sulfonate, bis (2-ethylhexyl) sulfosuccinate, and aerosol-OT trichain analog, respectively. ³ Critical micelle concentration (cmc) from surface tension measurements. ⁴ Surface tension at cmc. ⁵ Surface pressure at cmc. ⁶ Surface excess concentration at the interface. ⁷ Minimum area per molecule at the water/air interface. ⁸ Surface tension reduction efficiency (by 20 mN m⁻¹). ⁹ Gibbs free energy of surfactant adsorption at the water/air interface.

displays the adsorption parameters of ILBSs in aqueous solutions, in the absence of electrolytes, at 25 °C, whereas Table S1 (in Supplementary Material) shows the micellization parameters of these surfactants. For ease of reading, we maintained the numbering of compounds the same in both Tables. For example, C8C₁ImCl is compound number 1 in Table 1 and Table S1 in both tables. A similar approach was also applied in

Table 2. Literature data on aqueous solutions of gemini ionic liquid-based surfactants at 25 °C. Adsorption and micellar parameters calculated from surface tension data. All surfactants have two bromides as counterions, except for entries 54 and 55, which have three bromides as counterions.

Entry	Cation 1	cmc × 103 (mol L−1)	γcmc (mN m−1)	Πcmc (mN m−1)	Γmax (mol m−2)	Amin (Å2)	pC20	ΔG0ads (kJ mol−1)
1	(C12Im)2C2)2+	0.55 [156]	33.6 [156]		1.26 [156]	135 [156]	4.54 [156]
2	(C16Im)2C2)2+	0.0341 [157]
3	(C16Im)2C3)2+	0.0048 [157]
4	(C10Im)2C4)2+	4.50 [43], 2 [45]	35.2 [43], 35 [45]		1.25 [43]	133 [43]	3.14 [43]
5	(C12Im)2C4)2+	0.72 [43], 0.76 [158]	35.7 [43], 35.0 [158]	37.8 [43]	1.19 [43], 16.30 [158]	140 [43], 101.43 [158]	3.94 [43]	−50.93 [158]
6	(C14Im)2C4)2+	0.10 [43]	37.2 [43]		0.88 [43]	188 [43]	5.04 [43]
7	(C16Im)2C4)2+	0.0222 [157]
8	(C16Im)2C5)2+	0.0269 [157]
9	(C12Im)2C6)2+	0.78 [156]	39.5 [156]		1.16 [156]	143 [156]	3.73 [156]
10	(C16Im)2C6)2+	0.0501 [157]
11	(C4Im)2C8)2+	32.3 [45]	47 [45]
12	(C16Im)2C8)2+	0.0512 [157]
13	(C1Im)2C10)2+	14.9 [45]	38 [45]
14	(C4Im)2C10)2+	11.7 [45]	38 [45]
15	(C16Im)2C10)2+	0.0607 [157]
16	(C4Im)2C12)2+	7.2 [45]	46 [45]
17	(C10Im)2C12)2+	0.6 [45]	48 [45]
18	(C16Im)2C12)2+	0.0619 [157]
19	((C12SMeIm)2C2)2+	0.32 [55]	39.7 [55]		2.60 [55]	63 [55]	3.93 [55]	−45.57 [55]
20	((C14SMeIm)2C2)2+	0.072 [55]	42.9 [55]		2.12 [55]	78 [55]	4.53 [55]	−47.80 [55]
22	((C12SMeIm)2C3)2+	0.26 [55]	40.7 [55]		2.13 [55]	77 [55]	4.06 [55]	−48.21 [55]
23	((C14SMeIm)2C3)2+	0.063 [55]	45.8 [55]		3.09 [55]	53 [55]	4.42 [55]	−42.31 [55]
25	((C12SMeIm)2C4)2+	0.22 [55]	40.8 [55]		2.06 [55]	80 [55]	4.12 [55]	−47.51 [55]
26	((C14SMeIm)2C4)2+	0.058 [55]	46.6 [55]		3.10 [55]	53 [55]	4.39 [55]	−42.82 [55]
28	((C12OHIm)2C3)2+	0.72 [159]	30.0 [159]		2.53 [159]	65 [159]	3.91 [159]	−49.67 [159]
29	((C12OHIm)2C4)2+	0.76 [159]	28.1 [159]		2.33 [159]	71 [159]	4.11 [159]	−50.63 [159]
30	((C12OHIm)2C5)2+	1.02 [159]	32.9 [159]		2.29 [159]	72 [159]	3.72 [159]	−52.46 [159]
31	((C12OHIm)2C6)2+	1.07 [159]	35.2 [159]		2.98 [159]	55 [159]	3.44 [159]	−44.78 [159]
32	((C12OHIm)2C8)2+	1.14 [159]	37.6 [159]		1.90 [159]	87 [159]	3.58 [159]	−49.45 [159]
33	((C12)3N)2C2)2+	1.995 [160] 2		39 [160]	0.769 [160]	251.7 [160]	5.1 [160]	−20.19 [160]
34	((C12)3N)2C3)2+	1.412 [160] 2		40 [160]	1.012 [160]	147.3 [160]	5.1 [160]	−19.30 [160]
35	((C12)3N)2C6)2+	1.445 [160] 2		42 [160]	1.131 [160]	146.8 [160]	6.9 [160]	−19.63 [160]
36	((C8C1C1N)2(OE)3Gly)2+	1.02 [161] 2	55.2 [161]		0.802 [161]			−47.5 [161]
37	((C10C1C1N)2(OE)3Gly)2+	0.859 [161]	32.55 [161]		1.466 [161]		3.863 [161]	−53.9 [161]
38	((C12C1C1N)2(OE)3Gly)2+	0.711 [161]	30.57 [161]		3 [161]		3.64 [161]	−37.1 [161]
39	((C14C1C1N)2(OE)3Gly)2+	0.243 [161]	34.5 [161]		0.947 [161]		4.727 [161]	−69.6 [161]
40	((C16C1C1N)2(OE)3Gly)2+	0.631 [161]	37 [161]		1.314 [161]		4.14 [161]	−55 [161]
41	((C8C1C1N)2(OE)4Gly)2+	1.822 [161]	49.08 [161]		0.519 [161]		3.417 [161]	−80.7 [161]
42	((C10C1C1N)2(OE)4Gly)2+	1.7 [161]	46.96 [161]		0.417 [161]		3.222 [161]	−85.3 [161]
43	((C12C1C1N)2(OE)4Gly)2+	1.239 [161]	28.54 [161]		1.06 [161]		4.124 [161]	−67.1 [161]
44	((C14C1C1N)2(OE)4Gly)2+	0.333 [161]	34.81 [161]		0.813 [161]		4.63 [161]	−75 [161]
45	((C16C1C1N)2(OE)4Gly)2+	0.389 [161]	36.51 [161]		0.785 [161]		4.71 [161]	−75.1 [161]
46	((C8C1C1N)2(OE)5Gly)2+	0.701 [161]	52.22 [161]		0.519 [161]			−83 [161]
47	((C10C1C1N)2(OE)5Gly)2+	0.649 [161]	48.28 [161]		0.827 [161]		3.539 [161]	−62.5 [161]
48	((C12C1C1N)2(OE)5Gly)2+	0.607 [161]	33.78 [161]		1.4 [161]		4 [161]	−55.1 [161]
49	((C14C1C1N)2(OE)5Gly)2+	0.502 [161]	33.61 [161]		1.04 [161]		4.389 [161]	−65.2 [161]
50	((C16C1C1N)2(OE)5Gly)2+	0.398 [161]	43.92 [161]		0.925 [161]		5.91 [161]	−60.0 [161]
51	(C10Pyrro)2C4)2+	3.3 [56]	43.2 [56]		1.37 [56]	121.2 [56]	2.96 [56]
52	(C12Pyrro)2C4)2+	0.5 [162], 0.53 [56]	41.7 [56]		1.46 [56]	113.3 [56]	3.80 [56]
53	(C14Pyrro)2C4)2+	0.1 [162], 0.108 [56]	40.4 [56]		1.59 [56]	104.4 [56]	4.41 [56]
54	((C8Im)3Am)3+	4.3 [163]	33 [163]		1.11 [163]	1.50 [163]	3.13 [163]
55	((C8Im)3Bn)3+	2.2 [163]	40 [163]		1.37 [163]	1.21 [163]	2.81 [163]

¹ Abbreviations: Imidazolium (Im), thioether-functionalized methylimidazolium (SMeIm), hydroxyl-functionalized imidazolium (OHIm), quaternary ammonium (C_xC_yC_zN), ethylene oxide units (OE), glycol (Gly), pyrrolidinium (Pyrro), triethylamine (Am) and 1,3,5-trimethylbenzene (Bn). ² Measurements done at 20 °C.

(GILBSs) and Table S2 (in Supplementary Material).

The data reported cover the period between 2010 and 2020, unless the information is only available before 2010. Only ILs with alkyl chain ≥C₈ carbons are included, because these surfactants present spherical aggregates at surfactant concentration ≥cmc [57]. The ILBSs are listed by the charge of the group with the longest hydrophobic chain, namely cationic and anionic. ILBSs are listed as biamphiphilic when the alkyl chains of the anion and cation are longer than n-octyl. In Table 1 and Table S1, entries 1–125 refer to cationic, entries 126–152 refer to anionic, and entries 153–163 refer to biamphiphilic ILBSs.

Cationic ILBSs are divided according to the structure of the head group (HG), including Imidazolium (Im⁺), pyridinium (Py⁺), 2-pyrrolidinonium (Pn⁺), pyrrolidinium (Pyrro⁺), piperidinium (Pip⁺), azepanium (Aze⁺), azocanium (Azo⁺), morpholinium (Mor⁺), guanidinium (Gu⁺), ammonium (N⁺) and phosphonium (P⁺). Cationic ILBSs carrying the same HG were ordered by the number of carbons of the side chain(s). When two alkyl chains are attached to the heteroatom, e.g., the halides of 1-C_x-3-C_y-imidazolium, the classification is based on the length of C_y. Accordingly, all ILBSs with C_y = methyl are listed before those carrying C_y = ethyl, independent of the length of C_x. Additionally, saturated alkyl groups take precedence over unsaturated ones, e.g., ethyl comes before the vinyl group (both with two carbon atoms). In a few cases, the heterocyclic ring carries substituents attached to the ring carbon atoms, C_z, e.g., when the surfactant precursor is 1,2-dimethylimidazole. In this case, we still list the surfactant according to the length of C_x and C_y, giving priority to the surfactant without C_z, e.g., C₁₀C₁C₁Im⁺ comes after C₁₀C₁Im⁺. The molecular structures and acronyms for the ILBSs’ cationic groups are depicted in Scheme 5.

Knowledge of the adsorption and aggregation behavior of ILBSs is required to develop and improve their applications. In face of some relatively large differences between the data reported for the same ILBS, we took a conservative approach by comparing data obtained by the same technique, e.g., surface tension for surfactant adsorption at the water/air interface, and (mostly) conductivity measurements for micelle formation.

Regarding the adsorption parameters, we note that these have greater variation in relation to those obtained by conductivity. This is due to the fact that surface tension measurements are more sensitive to some experimental factors, such as time to reach the surfactant equilibrium at the water/air interface, and the presence of surface-active impurities. This problem can be minimized by analyzing data from articles separately. The data show that as the surfactant hydrophobic chain-length increases, the effectiveness of surface adsorption, given by surface tension at the cmc (γ_cmc), varies slightly, whereas the efficiency of surface adsorption (pC₂₀) increases significantly. As expected, A_min decreases as the size of the hydrophobic chain increases, due to the concomitant closer packing of monomers at the interface. The transfer of the surfactant monomer from bulk aqueous solution to the interface is favored by the increase of the hydrophobic chain length, which explains the increase in the values of the corresponding ∣ΔG⁰_ads∣.

Regarding micelle formation, we comment on the values of cmc of Table 1 and Table S1 because this is the main parameter employed to calculate several adsorption and micellization properties. Figure 3 shows the dependence of log cmc on the number of carbon atoms (C_x) of the hydrophobic chain (HC). The observed linear relationship is expected because the value of the free energy of transfer of a CH₂ group from bulk aqueous pseudo-phase to the interior of the micellar aggregate (ΔG⁰_CH2) should be independent of the nature of HG. Consequently, the slope is expected to be independent of the charge and nature of the surfactant head-ions.

Figure 3 is an example of the Stauff–Klevens rule,

log cmc = A − B x,

(1)

where A is a constant that depends on the experimental conditions, the structure of the surfactant monomer and counterion, and B refers to the effect of each additional CH₂ (in x) on cmc. Application of Equation (1) to the results of Figure 3 yields Equation (2) for cationic ILBSs:

log cmc = 1.46 ± 0.04 − 0.282 ± 0.003 × R² = 0.975

(2)

Application of Equation (1) to the data of Table S1, for anionic and biamphiphilic ILBSs, yield Equations (3) and (4), respectively:

log cmc = 1.12 ± 0.17 − 0.327 ± 0.017 × R² = 0.954

(3)

log cmc = −0.4 ± 0.3 − 0.30 ± 0.03 × R² = 0.966

(4)

The most relevant point is that the values of the slopes are of the same order, in agreement with the above-mentioned independence of ΔG⁰_CH2 of the nature of the head-ions.

Other aggregation parameters, such as Gibbs free energy of micellization (ΔG⁰_mic), enthalpy of micellization (ΔH⁰_mic), degree of counterion dissociation of the micellar aggregate (α_mic) and average micellar aggregation number (N_agg), are also important. Unlike the values of cmc, which are calculated directly and precisely from solution conductivity, the above-mentioned parameters are published less frequently, and their calculation is subject to uncertainties. We dwell on this point because of its relevance to the calculated aggregation parameters that are employed in the correlation between surfactant molecular structure and solution properties. The usual procedure is to calculate the value of ΔG⁰_mic is Equation (5), where cmc is given on the mole fraction scale, χ_cmc [146]:

ΔG⁰_mic = (2 − α_mic) RT ln χ_cmc

(5)

The value of ΔH⁰_mic is then calculated from the dependence of cmc on the temperature; the value of ΔS⁰_mic is calculated from Gibbs free energy relationship:

ΔG⁰_mic = ΔH⁰_mic − TΔS⁰_mic

(6)

As argued elsewhere, this sequence of calculations can be problematic because the value of α_mic is calculated using Frahm’s (simple) equation. The latter disregards the contribution of the micelle (a macroion) to solution conductivity. Evans’ equation takes into consideration the micelle contribution to solution conductivity above the cmc [147]. The result is that α_mic (Frahm) > α_mic (Evans); error in α_mic is reflected in the calculated values of ΔG⁰_mic, and ΔS⁰_mic; see Equations (5) and (6). Note that Evans’ equation requires knowledge of N_agg, whose value can be calculated, e.g., from static light scattering data, or from the volume of the surfactant monomer. As has been shown, a relatively large uncertainty in N_agg leads to a negligible effect on the value of α_mic [148]; i.e., the use of Evans’ equation is recommended.

On the other hand, the values of ΔH⁰_mic that are calculated indirectly by the van Hoff treatment and directly by ITC usually do not agree. The reason is that there is no provision in the former treatment for the effects of increasing temperature on micellar parameters, e.g., α_mic, N_agg and monomer dehydration [149]. Effects of these variations are “embedded” in the value of ΔH⁰_mic calculated by ITC. As shown by Equation (6), the above-mentioned uncertainties in ΔH⁰_mic are carried over to ΔS⁰_mic.

Analysis of the available data from ILBS shows that ΔG⁰_mic decreases as a function of increasing the number of CH₂ groups in the HC; i.e., the micellization becomes more favorable. Regarding α_mic and N_agg, it is seen that the former decreases and the latter increases as a function of increasing the length of the hydrophobic chain; these effects are consequences of the smaller surface area of surfactants with longer HC.

Besides the length of the hydrophobic chain, the effects of some other structural variables in the HG were also probed. Schnee and Palmer [150] studied the effect of the size of heterocyclic ring structures (5- to 8-membered rings: C₁₆C₁PyrroBr, C₁₆C₁PipBr, C₁₆C₁AzeBr and C₁₆C₁AzoBr, respectively) on their aggregation properties. Increasing the size of HG led to a decrease in the value of cmc, from 0.83 to 0.67 mmol L⁻¹; see Figure 4. The reason is that increasing the size and hydrophobicity of the HG results in energetically unfavorable surfactant–solvent interactions in the bulk aqueous pseudo-phase, as well as stronger interactions at the micelle surface, resulting in lower cmc values.

Keppeler et al. [91] studied the effect of the length of alkyl side chain (C_y) of imidazolium-based surfactants (C₁₆C_yImBr and C₁₆C_yImCl) on their adsorption and aggregation properties. It was found that increasing the length of C_y from methyl to n-pentyl led to a linear decrease in the values of log cmc; see Figure 5. The slope of log cmc versus C_y (change in the HG) is about 3 times smaller than the corresponding slope for introducing methylene groups in the HC (hydrophobic chain). This behavior is expected, because on micellization, there is more dehydration of most of the CH₂ groups in HC (whose micelle interior is oil-like) than any CH₂ in the head group. Increasing the length of the alkyl side chain results in an increase in Amin, probably due to steric repulsion between the increasingly voluminous C_y chains. A corollary to this statement is that the increase in the length of C_y leads to less surfactant molecules at the water/air interface, in agreement with the decrease in Γmax and increase in γ cmc. The adsorption of a more hydrophobic HG is favored, as shown by the increase in |ΔG⁰ads| and pC₂₀.

Another structural variable that can be analyzed is the effect of the position of methyl groups in heterocyclic ring on the value of cmc. Sastry et al. [99] studied the aggregation behavior of 1-octylpyridinium and 1-octyl-2-, -3- or -4-methylpyridinium chlorides. The data show that the ILs with methyl-substituted pyridinium cations have lower cmc values than the parent pyridinium cation, indicating that the presence of methyl group in pyridine ring increases its hydrophobicity, in agreement with published values of log P (the partition coefficient of a substrate between mutually saturate water and n-octanol), 0.73 and 1.2 for pyridine and 4-methylpyridine. The position of the methyl group in the 1-ocylypyridinium ring has a small effect (6%) on the value of cmc.

The effect of the counterion was also analyzed. Since this topic was not investigated in detail, we limit our analysis to ILBSs with halide anions. Kim and Ao [75] studied the properties of aqueous solutions of ILBSs with different halide anions (C₁₂C₁ImCl, C₁₂C₁ImBr and C₁₂C₁ImCl). The order of cmc and αmic at 25 °C was (cmc in mmol kg⁻¹; αmic calculated by Frahm’s equation): C₁₂C₁ImCl(15.1; 0.44) > C₁₂C₁ImBr(10.6; 0.25) > C₁₂C₁ImCl (5.2, 0.15). Counterions are adsorbed at the micellar interface primarily by strong electrostatic interactions. For halide anions, this adsorption depends on the balance between anion polarizability [153] and radius of the hydrated anions [154]. As a consequence of the increase in the size of the hydrated anions, the chloride counterions are located further away from the micellar interface than the hydrated iodide counter ion [155]. That is, the micellar surface potential decreases in the order ILBS-Cl > ILBS-Br > ILBS-I, in agreement with the above-mentioned values of cmc and αmic.

3.2. Compilation and Brief Discussion of the Properties of Aqueous Solution of GILBSs

The GILBSs in Table 2 and Table S2 are listed in a similar way to the ILBSs. They are first divided according to the structure of cationic headgroup: imidazolium (Im), thioether-functionalized methylimidazolium (SMeIm), hydroxyl-functionalized imidazolium (OHIm), quaternary ammonium (C_xC_yC_zN) and pyrrolidinium (Pyrro). Within each category, they are ordered by the number of carbon atoms of the “spacer” and then by the number of carbon atoms of the hydrophobic chain(s). Accordingly, imidazolium-based GILBSs with a spacer containing two carbon atoms are presented before those with a spacer containing three carbon atoms. At the bottom of Table 2, we present two examples of ILBSs containing three long chains. To the best of our knowledge, there are no reports on GILBSs containing unsaturated alkyl groups. The molecular structures for the GILBSs cationic groups are depicted in Scheme 6.

The first members reported in the literature that conform to the m.p. criterion (≤100 °C) are quaternary ammonium surfactants that carry the hydroxyl group; the latter was considered important to promote intermolecular hydrogen bonding that lowers the melting point [161]. As compared to their single-chain counterparts, the GILBSs have an increased propensity to form aggregates and efficiently reduce surface tension. The same trend is observed for conventional single chain and gemini surfactants [1,164].

As can be seen from Table 2, the general trends for ILBSs are also observed for GILBSs. For example, for the same spacer, the cmc values are expected to be lower with increasing the length of the hydrophobic chain(s). One example is the quaternary ammonium surfactants with HC from 8 to 16 carbon atoms and 1–3 ethylene oxide units (EOs) as spacers [161]. Figure 6 shows the dependence of cmc on the number of carbon atoms in HC for one series. They are not in the expected order (e.g., cmc for C₁₆ > cmc for C₁₄), probably due to the possibility of self-coiling or formation of pre-micellar aggregates [161].

The effect of the spacer length on cmc is complex because it is a sum of several factors, including rigidity of the molecule, hydrogen bonding (where applicable), hydration of HG, Coulombic repulsion between HGs. This complex behavior was shown by Pal et al. [157] for a series of GILBSs containing two imidazolium rings in the HG and spacer from 2 to 12 methylene groups. They observed a lower cmc value for the (CH₂)₃ spacer, after which the cmc values increased and then reached a plateau (Figure 7). This was explained in terms of rigidity and planar nature of the imidazolium HG, which interfere with the spacer packing, leading to independent behavior of each single chain beyond a spacer of (CH₂)₃.

The possibility of “tuning” the morphology of the colloidal aggregate by adjusting the length of the two component ions of biamphiphilic compounds is nicely shown in Figure 8 for dimeric and trimeric surfactants with ring-containing cation and anion. The multitude of possibilities is relevant to applications of these surfactants that may require, e.g., a vesicle, a bilayer, or a wormlike aggregate. These aggregates can be obtained by a judicious choice of (in Figure 8) the length of the spacer in the cation and the HC of the anion [1].

4. Applications of IL-based Surfactants

4.1. Nanotechnology

ILBSs have been explored in various fields, including chemical synthesis and catalysis [165,166,167,168,169,170], drug delivery [171,172,173,174,175,176,177], biomass conversion [178,179,180,181,182,183], liquid crystal development [184,185,186], decontamination [187,188,189,190,191,192,193] and formation and stabilization of metal NPs [61,194,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216]. We dwell here on the synthesis and stabilization of mesoporous nanoparticles (MNPs), including mesoporous silica (MSNPs).

MNPs have small sizes and large surface areas that make them important materials in various fields, e.g., medicine, electronics, electrical and magnetic materials, catalysis and fabrication of novel chemical and biological sensors. Apart from synthesis, controlling the average size, surface area, porosity and stability of MNPs is a challenging task and, if achieved, contributes to their wider applications. The synthesis of uniform-sized MNPs with controlled morphology is feasible, thanks to the relative ease of tailoring the properties of ILBSs to play a required role. Consequently, various morphological architectures, e.g., micelles and vesicles, were employed as soft templates for the formation and stabilization of these MNPs. The latter particles are, however, only kinetically stable and will aggregate to thermodynamically more stable larger particles due to Ostwald ripening. This spontaneous process occurs because larger particles are energetically favored, due to their lower surface-to-volume ratio. Therefore, stabilization of the formed NPs is essential for any application. The effect of the colloidal template depends, inter alia, on the length of its HC, the nature of the HG and the counter ion. The reason is that these structural factors determine the value of cmc and the morphology of the colloidal species (spherical micelle, vesicle, etc.).

This dependence was nicely shown by studying the effect of the length of the hydrophobic group on the average particle size and stability of Ag NPs, by a series of 1-R-3-MeImX (R = C₈, C₁₀, C₁₂; X = Cl⁻, Br⁻), C₁₂Me₃NBr and sodium dodecyl sulfate (SDS). It was found that the length of the surfactant HC is determinant to the stability of the NPs. For example, (cationic) micelles of the surfactant with R = C₈ did not provide enough stabilization, so the synthesized Ag NPs coalesced immediately. Increasing the length of R from C₈ to C₁₂ led to an increase in stability and concentration of the formed Ag NPs due essentially to hydrophobic interactions between surfactants and surface of the Ag NPs [217].

As shown by Figure 9, stabilization of the NPs by ILBS is achieved mainly through (i) electrostatic interactions of their ions with the NP and (ii) steric repulsion between the sheaths covering the generated NPs. Both mechanisms create a protective coating around the NPs, thereby hindering their aggregation and control the distribution of their sizes. Judicious selection of the molecular structures of ILBSs is required for fabricating NPs with controlled sizes, shapes and porosity that can be useful in various biological applications and catalysis [218,219,220].

Aqueous micellar solutions of dodecyltrimethylammonium bromide were found to stabilize α-FeO₂H NPs and decrease their average size, when compared with those prepared in absence of the surfactant. An electrochemical method was used to synthesize nano-sized α-FeO₂H particles (average diameter = 5–10 nm) in the presence of the surfactant. The proposed reaction pathway for the electrosynthesis of ILBS-FeO₂H NPs is shown in Figure 10, where FeO₂H NPs were formed inside inverse micelles [194].

Because of their surface and optoelectronic properties, nano-sized α-FeO₂H particles with an average diameter of 1–100 nm are used as a semiconductor catalyst in the degradation of chlorinated compounds [99,100], e.g., 2-chlorophenol (2-CP). Note that α-FeO₂H is practically inactive in the absence of oxidizing agents (e.g., H₂O₂) that provide the hydroxyl radicals (·OH) necessary for 2-CP oxidative-degradation. IL-FeO₂H degraded 56% and 85% of 2-CP when the catalyst was irradiated with visible light (350–600 nm), in the absence and presence of H₂O₂, respectively. It is presumed that the surfactant head-groups in the IL-FeO₂H reverse micelle trapped the photogenerated electrons at the conduction band and simultaneously decreased the recombination rate of photo-induced electron-hole pairs at the valence band; resulting in the enhancement of the 2-CP degradation.

A similar magnetite (Fe₃O₄) NP stabilization mechanism by the reverse micelles of the GILBS (16-2-16), α,ω-bis(3-decylimidazolium-1-yl) ethane dibromide, and other gemini cationic surfactants were suggested. The Fe₃O₄ NPs were synthesized by a hydrothermal treatment of an equimolar mixture of FeCl₃ plus FeSO₃ in the presence of GILBS (150 °C, 24 h), followed by removal of the excess surfactant by extraction with hexane. The positively charged surface-active Fe₃O₄ NPs thus obtained were used to extract Au and Ag NPs from their aqueous solutions. The Au and Ag NPs were solubilized in water by single-chain surfactants (cetyltrimethylammonium bromide, CTABr or SDS); hence, they have positive and negative charges, respectively. Consequently, the extraction was favored by NP–NP interactions, including electrostatic, in the case of SDS-stabilized Au- and Ag NPs, and hydrophobic, for CTABr and SDS stabilized metal NPs [221].

Han et al. [195] used a novel sol-gel method to synthesize hollow silica spheres and tubes with disordered and ordered mesopores by using C₁₀C₁ImCl and the nonionic surfactant P123 (a copolymer between PEG and PPG, of the average composition PEG₂₀PPG₇₀PEG₂₀) as the template and co-template, respectively. The micelle/P123 co-assembly is supposed to be responsible for the formation of the silica morphology and mesostructure. Two strategies were explored for the fabrication of SiO₂ hollow spheres, namely using the ILBS alone, and using the ILBS plus P123 as co-template. It was observed that the shape and size of the SiO₂ nanospheres depend on the IL concentration; i.e., at low dosage (0.0025 mol), flake-like silica was observed which was converted into bulk-like silica when the IL concentration was increased to 0.005 mol. Increasing the IL concentration to 0.015 mol resulted in uniformly sized (average radius = 3.8 nm) hollow SiO₂ spheres. In the second strategy, it was observed that increasing the molar ratio of P123/C₁₀C₁ImCl from 0.03 to 0.04 resulted in the SiO₂ morphology changing from spheres with an average diameter of 5 μm to a long curved tube. Upon a further increase of the above-mentioned ratio to 0.07, the silica tubes become longer and prism morphology began to appear. Addition of P123 strengthened the binding between adjacent spheres and allowed them to adhere to each other more tightly. It is interesting to note that using P123 alone resulted in the formation of bulk silica without any tubes, demonstrating the significant impact of both P123 and C₁₀C₁ImCl on the morphology of silica tubes; see Figure 11.

Based on Figure 11, these authors explained the role of the template (C₁₀C₁ImCl) and the co-template (P123) on the morphology of the fabricated SiO₂ NPs. When only ILBS was present at a concentration less than its cmc (0.062 mol L⁻¹), the tetraethyl orthosilicate silane (TEOS) hydrolyzed quickly, while the hydrolysate condensed slowly. As a result, stable small oligomers Si(OC₂H₅)_4−x(OH)_x were formed. At [C₁₀C₁ImCl] less than its cmc, the Si(OC₂H₅)_4−x(OH)_x dissolved into the spherical micelles present; these coalesced into larger spheres that, upon washing and calcination, produced hollow SiO₂ nanospheres. In the second case, i.e., in the presence of the co-template, the tail of the C₁₀C₁ImCl interacted hydrophobically with the PPO block of the P123. This led to the formation of P123/C₁₀C₁ImCl mixed micelles. At higher concentrations of P123, the PEO blocks of the P123 interacted with the oligomers through hydrogen bonding to form the cylinder-like micelles. Through aging, the small oligomers crystallized on the surface of the long cylindrical micelles that, on washing and calcination, formed the hollow prism-like tubes.

Therefore, the formation of ILBS-additive mixed micelles offers a versatile approach for the fabrication of MNPs of controlled geometry suitable for many applications. In addition to co-polymers (e.g., P123), the additive can be a relatively hydrophobic ion with an opposite charge (to that of the micellar surface), leading to the growth of the spherical micelles and the incorporation of TEOS hydrolysis oligomers therein. This approach was used to prepare MSNPs containing the drug ibuprofen (2-(4-(isobutylphenyl)propanoic acid; pKa = 4.54) by hydrolysis of TEOS in the presence of the ILBS C₈C₁ImCl. At the working pH (=7.5) the (relatively hydrophobic), ibuprofen anions were incorporated into the micelles (by ion exchange with Cl⁻), leading to the formation of MSNPs with large surface area (as high as 812 m² g⁻¹) and pore volumes (1.25 cm³ g⁻¹) with ibuprofen loading of 46 wt.%. The residual ILBS in the ibuprofen-MSNPs was relatively high (13%); its release under drug delivery conditions should be assessed [196].

MSNPs were successfully synthesized by acid hydrolysis of TEOS in the presence of two ILBSs, C₁₆C₁ImBr and C₁₈C₁ImBr. The average particle sizes (nm), surface area (BET; m² g⁻¹) and pore volume (cm³ g⁻¹) of the formed MSNPs were affected by the alkyl chain length of the ILBS template. Namely, smaller, more porous NPs were obtained using the C₁₈C₁ImBr template. Thus, the ratios of the above-mentioned properties (C₁₈C₁ImBr/C₁₆C₁ImBr) were found to be 0.68, 1.05 and 1.70, respectively [197]. Therefore, changing the length of the hydrophobic tail of the ILBS is another variable that can be exploited for controlling the properties of MNPs.

Similar results were observed when pyridine-based ILBSs (C₁₂PyBr, C₁₄PyBr, C₁₆PyBr and C₁₈PyBr) were employed as templates for fabricating MSNPs, using triethanolamine (TEA) to get well-dispersed MSNPs. The authors employed two strategies: (i) The template was fed with the premixed and preheated TEA and TEOS. (ii) TEA, template and water were preheated and stirred before the addition of the TEOS. With both strategies, the authors assessed the role of alkyl chain length of pyridinium ILBSs in the formation of MSNPs. It was observed that with both strategies, except for a minor change in the results, the size of the MSNPs decreased with increasing the length of HC, whereas the porosity and surface area of MSNPs increased in the same direction. This is shown by the reported results of C₁₈PyBr/C₁₂PyBr, for the mean particle size (nm), BET surface area (m² g⁻¹) and pore volume (cm³ g⁻¹), respectively: 0.31, 5.63 and 3.57 [198]. The same authors employed experimental design to optimize the average surface area and particle size of the fabricated MSNPs and studied the loading and release of the drug quercetin (a flavonoid employed to prevent/treat some cancer types). The loaded drug was then checked for its release profile using a dialysis bag technique. It was observed that, due to the drug–MSNP interaction, the crystallinity of quercetin changed to amorphous, which increased its bioavailability. The 32% cumulative release of the drug was obtained in the MSNP-loaded drug against the unloaded drug. These results suggest that with MSNPs, it is possible to have a slower release of drugs [199]. Drug release profile is illustrated in Figure 12 [198].

C₁₆PyBr was used as the soft template for the production of uniformly shaped spherical MSNPs with a particle diameter of 35 to 40 nm. These MSNPs were then functionalized to obtain MSNP-NH₂, MSNP-SH and MSNP-COOH surface-functionalized particles through a post-grafting technique, during which the functional groups were covalently bonded to the silanol group (Si–OH) on the external or internal pore surface, see Figure 13. The functionalized MSNPs thus obtained were used as carriers to load drugs employed for cancer treatment, including hydrophilic gemcitabine, and hydrophobic quercetin [200].

In their efforts to enhance the desulfurization of diesel oil and gasoline, Zhang et al. [201] explored the oxidative desulfurization of fuels using an ILBS with polyoxometalate as anion. The ILBS (1-hexadecyl-3-methylimidazolium phosphomolybdic compound ([C₁₆C₁Im]₃PMo₁₂O₄₀)) was used to fabricate the functional molybdenum-containing ordered mesoporous silica, as shown in Figure 14. Herein, the C₁₆ chain acted as the template of ordered mesoporous, whereas the polyoxometalate anion acted as the source of active metal sites. Under optimal conditions, dibenzothiophene was completely removed in 50 min, and the catalyst efficiency was found to be 91% after recycling nine times [201].

C₁₂C₁ImCl based template was used to fabricate zeolite nanocrystals by using a hydrothermal technique. In the absence of the ILBS, unstable nanocrystals in the micrometer range were obtained. In the presence of the ILBS, however, much smaller particles were obtained, with average size and surface area of <30 nm and 443.6 m² g⁻¹, respectively. The post treatment to remove the ILBS template included direct calcination at 550 °C and extraction with refluxing ethanol for 12 h; the latter treatment resulted in smaller particles with a larger average surface area. For example, the following ratios (ethanol extracted sample/directly calcinated sample) were observed for the micropore volume: 0.90 and 0.94 for hydrothermal heating times of 54 and 102 h, respectively. The ratios for the surface area were 2.19 and 6.82 for hydrothermal heating times of 54 and 102 h, respectively [202].

Nano-particle aggregation/growth during the reaction that causes their catalytic deactivation was hindered through stabilizing the MNPs using ILBSs. In their work on the catalytic hydrochlorination of acetylene, Hu et al. [203] used tetra(n-butyl)phosphonium carboxylates (octanoate, dodecanoate, tetradecanoate, hexadecanoate and octadecanoate). The self-assembling of the ILBSs along with the high reactivity of NPs was used to reduce the deactivation of the metal catalysts through establishing the effective redox cycle between Pd⁰ and Pd^II. The ILBSs form a protective layer around the NPs, hindering their aggregation. Herein, the authors observed no obvious disparity in dispersion degree or particle size of Pd NPs (narrow size distribution in the range of 2.4–4.4 nm and the mean size ~3.2 nm) when the alkyl chain of the anions of ILBSs was changed from C₇ to C₁₇.

Pt and Au catalysts were fabricated using the respective NPs/ILBSs with the stearate anion simply by blending the surfactant with the precursor PtCl₂ and HAuCl₄·4H₂O at 120 °C, respectively. The authors observed highly ordered lattice fringes in a Pt NP with particle sizes in the range of 1.5 nm, whereas in the case of Au, the particle size was >20 nm. When tested for their catalytic performances, Pd NPs/ILBSs systems with the longer carboxylate showed maximum acetylene conversion into vinyl chloride of 93.04%, whereas the corresponding ILBS with octanoate anion showed 76.23%, suggesting the impact of alkyl chain length of the anion on the catalytic performance. For the Pt and Au NP/ILBSs systems, the catalytic performance of the stearate ILBS was 74.25% and 64.56% respectively. Furthermore, the basicity of the carboxylate anion was effective in absorbing and activating acetylene and HCl. It was observed that 1 mol of ILBS with the stearate anion absorbed approximately 2 mol of HCl and 0.6 moles of C₂H₂ at the reaction temperature. This study shows that metal NPs/ILBS systems are promising as substitutes for toxic mercury catalysts in the hydrochlorination of acetylene [203].

To explore the application potential of a rare earth oxide in its various morphologies, Huang et al. [204] synthesized monodisperse Nd₂O₃ nanoparticles using ILBS as a template. The surfactants employed included (cationic) C₁₄C₁ImCl, C₈C₁ImCl and (zwitterionic) N-(3-cocoamidopropyl)-betaine (CAPB). Nd₂O₃ nanoparticles were prepared from its precursor NdCl₃ in the absence and presence of ILBS at concentrations greater than cmc. Nd₂O₃ forms multifarious shaped nanocrystals (short nanorods, nanospheres, irregular flakes, highly regular leaf-shaped to torispherical) when CAPB concentrations were changed from 1 to 20 times its cmc (=0.01 M). The short nanorods prepared in the absence of surfactant have good homogeneity with diameters of about 100 nm, (Figure 15e), changing to nanospheres with better homogeneity and an average diameter of 50 nm when the CAPB was added at its cmc (Figure 15a). Increasing the concentration of CAPB to 5 times its cmc led to a mixture of nanospheres and irregular flakes with diameters about 50 nm (Figure 15b). At [CAPB] = 10 × cmc, regular leaf-shaped Nd₂O₃ nanoparticles with lengths of 12 mm, widths of 6 mm and thicknesses of 50 nm were obtained. These NPs are composed of aggregated nanorods with lengths of about 200 nm and some nanospheres (Figure 15c). At [CAPB] = 20 × cmc, the shape of the Nd₂O₃ particles changed to torispherical with diameters about 50–100 nm with a certain extent agglomeration; see Figure 15d [204].

When CAPB was replaced by C₁₄C₁ImCl, Nd₂O₃, NPs with different morphologies were obtained, namely leaf-shaped nanosheets and nano-blocks. Thus, different surfactants form different micellar templates, leading to different morphologies of the fabricated NPs; the surfactant with a lower cmc value (C₁₄C₁ImCl; 0.003 mol L⁻¹) [222] forms a more stable micelles template. The authors explained the mechanism of the formation of variously shaped nanoparticles through the schematic representation shown in Figure 16 [204].

Chitosan forms complexes with ILBSs (C₄C₁ImC₈OSO₃ and C₈C₁ImCl) above their respective cmc values, at pH = 3, i.e., where the biopolymer is protonated. The difference between these surfactants is that the hydrophobic part of the former is the anion (C₈OSO₃⁻), whereas it is the cation in the latter (C₈C₁Im). Electrostatic and hydrophobic interactions between chitosan and these ILBSs lead to the formation of positively charged spherical chitosan NPs, with sizes ranging from 300 to 600 nm from the aqueous biopolymer (0.2 wt.%) solutions. Chitosan NPs prepared using C₄C₁ImC₈OSO₃ have better sphericity and show less agglomeration than those prepared using C₈C₁ImCl. For the latter ILBS, the chloride counter-ions at the surface of micelles induce interactions between chitosan and C₈C₁ImCl complexes, leading to the agglomeration of biopolymer–micelle complexes. The relatively hydrophobic octyl sulfate anion at the micellar interface prevents the agglomeration of the chitosan-ILBS aggregate complexes; see Figure 17 [61].

To limit the use of organic solvents, and to reduce the number of preparation steps, Komal et al. [205] used ILBSs with tetrachloroferrate anion, namely, 1-R-3-methylimidazolium FeCl₄, R = n-butyl, n-octyl and n-hexadecyl as the templates for the preparation of α-Fe₂O₃ NPs, via a grinding followed by calcination. It was observed that upon going from n-butyl to n-hexadecyl, the size of the NPs decreases. These NPs are interconnected in the form of nano-sheets, where the void spaces in the interconnected network and solution viscosity increase upon going from n-butyl to n-hexadecyl, preventing agglomeration of the NPs. At the same time, the concomitant decrease in the surface tension reduces the energy barrier to nucleation that causes an increase in the rate of nucleation as compared to the growth rate of NPs. This also decreases the size of the NPs upon going from n-butyl to n-hexadecyl. Pictorial presentation of the nano-segregated polar and non-polar domains of the ILBSs employed in this study is depicted in Figure 18. The synthesized NPs showed ILBS dependent structural, photo-physical and magnetic properties.

The fabricated α-Fe₂O₃ NPs were further explored as catalysts for the photo-degradation of the organic dye Rhodamine B by sunlight. The availability of the larger voids between the interconnected network influences the catalytic activity of the synthesized NPs with those fabricated using n-hexadecyl ILBSs showing the highest and n-butyl the lowest. Furthermore, ILBSs with n-hexadecyl chain length show excellent recyclability and can be used without losing their catalytic character even after four dye-degradation cycles [205].

Li et al. [206] used 1-(10-bromodecyl)-3-methylimidazolium bromide as the morphology-controlling agent to synthesize icosahedral gold NPs. These were then electrochemically deposited onto a glassy carbon electrode surface. A highly ordered and dense monolayer of the Au NPs was formed at the interface through self-assembling the 1,3-di-(3-mercaptopropyl)-imidazolium bromide IL. The prepared modified glassy carbon electrode was used as the electrochemical immunosensor for selective and sensitive determination of Squamous cell carcinoma antigen [206].

Xu et al. [207] used vesicles of the PEGylated ILBSs (surfactants with polyethylene glycol (PEG) side chain) to stabilize the Pd nanoparticles. The polyethylene glycol ether moiety was CH₃O-(CH₂CH₂O)₁₁-CH₂CH₂- and the other group was methyl, benzyl, n-octyl and n-hexadecyl, the counter ion was iodide (first ILBS) or bromide. The prepared Pd NPs in the presence of hydrazine hydrate were used as an efficient catalytic system for the chemoselective reduction of nitroarenes. ILBSs with C₁₆ chain gave the best result with a 99% yield, as compared to no reduction in the case of the ILBSs with R = methyl. It was observed that in absence of aqueous ILBS solution, the reduction was inhibited, because of poorly stabilized Pd NPs. As observed above, the NP sizes decreased with increasing the alkyl chain length and concentration of the ILBS. The increased reaction yield is due to the smaller-sized NPs that increase the surface area, leading to their better stabilization. The authors suggested three stabilizing effects (i) electrostatic, through the cations and anions of the ILBS; (ii) steric, via protection of the NPs through the PEG chain; and (iii) chemical, due to the formation of N-heterocyclic carbene palladium complex between the C-2 hydrogen of the surfactant imidazolium ring [207].

A similar approach of using PEGylated GILBS was employed for the fabrication of catalytically active Pd suspension that was employed in hydrogenation. The PEGylated GILBS was synthesized by reacting 1-(n-dodecyl)imidazole with Cl(CH₂)₂O-(CH₂CH₂O)₄₃-Cl to get the poly(ethylene glycol) functionalized gemini surfactant. The Pd NPs were fabricated by a treatment of palladium acetate with the surfactant solution (12 h, room temperature), followed by treatment with hydrogen (0.1 MPa) at 60 °C for 20 min. The catalyst obtained was successfully employed for the hydrogenation of several classes of organic compounds at room temperature and a pressure of 1 MPa. The following are examples of compounds that were hydrogenated with 100% product yield: styrene, cyclooctene, ethyl acrylate, allyl alcohol, nitrobenzene and 4-nitrotoluene [223].

C₁₀C₁ImBr was used as a template for fabricating hollow spherical PtCu alloy NPs (with the size of 124 ± 16 nm) supported on reduced graphene oxide (PtCu/rGO). The rGO was used as the supporting material because of its unique structure, large specific surface area and excellent electrical conductivity. The synthesized PtCu/rGO exhibited a high electrocatalytic activity and good poisoning-resistant ability during methanol oxidation in acidic medium. In order to investigate the influence of the ILBS anion on the formation of PtCu/rGO nanoparticles, three C₁₀C₁ImCH₃CO₂ and C₁₀C₁ImBF₄ were tested. Among these, C₁₀C₁ImBr resulted in less NP agglomeration. Further, the impact of the alkyl chain length of the ILBSs was studied. Unlike other studies, irregularly shaped hybrid PtCu/rGO NPs were obtained when C₆C₁ImBr and C₁₄C₁ImBr were employed, and hollow spherical PtCu/rGO was observed when C₁₀C₁ImBr was used. This dependence was attributed to the difference in the amphiphilicity of these cations, which can produce ordered self-organized structures. The electrochemical active surface area of the fabricated supported PtCu/rGO (in m² g⁻¹) was 1.64, i.e., larger than that of commercial Pt/rGO. The material obtained was employed for the catalytic oxidation of a solution of methanol in 0.5 mol L⁻¹ H₂SO₄. The high electrocatalytic activity and a good tolerance for methanol oxidation of PtCu/rGO were attributed to the unique hollow spherical nanostructures that enlarge the specific surface area and provide more active sites for the electrooxidation of methanol, and the two dimensional and nanostructures of rGO promote electron transfers during the reaction [208].

Abbaszadegan et al. [209] prepared C₁₂C₁ImCl-protected positively charged Ag NPs that were further studied as a promising disinfectant in root-canal dental treatment. Ag NPs with different surface charges (negative, neutral and positive) were synthesized, and their antibacterial activity and cytotoxicity were evaluated and compared with two widely used endodontic irrigates, namely NaOCl and chlorhexidine gluconate. Among the synthesized NPs, positively charged Ag NPs completely prevented the growth of Enterococcus faecalis, even after 5 min of contact time, whereas negatively charged Ag NPs started to inhibit their growth only after 1 h of contact time; neutral Ag NPs had a moderate inhibitory effect. The greater affinity of the positively charged Ag NPs to sulfur- and phosphorus-containing proteins of bacteria leads to the higher antibacterial activity of it amongst the three Ag NPs. Results of the antibacterial activity suggest that a 70-fold concentration of NaOCl was required to achieve an antibacterial activity equal to the positively charged Ag NPs. When tested for their cytotoxicity against L929 fibroblast cell lines in vitro, positively charged Ag NPs showed significantly lower cytotoxicity than the negatively charged and neutral charged Ag NPs. The positively charged Ag NPs exhibited even less cytotoxicity than the NaOCl and CHX [209].

4.2. Polymerization

As expected, weakly surface-active ILs and ILBSs form emulsions and microemulsions (μEs), both W/O and O/W. Microemulsions are isotropic, transparent or translucent solutions, usually formed by water, oil and an amphiphile. Interest in μEs stems from the small diameters of the (W or O) nanodroplets formed (3–30 nm) and, most importantly, their thermodynamic stability, essentially because of the very low O/W interfacial tension [224,225]. Windsor [226] classified μEs into the four types, as shown in Figure 19. In Windsor type I μEs, oil nanodroplets are stabilized in an aqueous continuous phase by the surfactant, in addition to excess oil phase. The inverse situation exists in type II μEs, i.e., water nanodroplets stabilized in oil, in addition to the excess aqueous phase. In type III, the μE is composed of a bicontinuous (BC) O/W μE in equilibrium with W and O, whereas type IV is composed solely of BC phase, i.e., W and O mix in all proportions. Depending on the type of continuous pseudo-phase, μEs are classified into aqueous and nonaqueous systems. μEs have important applications because the diameter of the formed NPs, including metals, oxides, and polymers can be controlled by adjusting, e.g., the molar ratio of W and O. This, in turn, controls the diameter of the formed nanodroplets and hence that of the NPs therein.

Regarding polymerization, ILs and ILBSs were employed to form μEs that contain monomers as the “oil” component; this is usually followed by a controlled polymerization. Additionally, “polymerizable” ILBSs can be employed in latex production instead of nonpolymerizable surfactants, leading to latex with enhanced stability. Thus, μEs were prepared from oil (methyl methacrylate, MMA), water and either the IL C₄C₁ImBr, or the ILBS C₁₂C₁ImBr. The free-radical polymerization of MMA (by atom transfer radical polymerization; ATRP) produced polymethylmethacrylate (PMMA) NPs with diameters between 40 and 60 nm. The IL and ILBs were recycled and reused, producing PMMA NPs with reproducible size distribution, average molar mass (MM) and low polydispersity [227,228].

The polymerization of MMA was carried out at 25 °C by AGET (activators regenerated by electron transfer)-ATRP in C₄C₁ImBF₄-based μE with polyoxyethylene sorbitan monooleate as surfactant; CCl₄ as initiator; the complex FeCl₃·6H₂O/N,N,N′,N′-tetramethyl-1,2-ethanediamine as catalyst, and ascorbic acid as reducing agent. The polymerization kinetics showed the feature of controlled ″living″ process as evidenced by linear first-order plots. The produced PMMA had narrow polydispersity indices (1.22 to 1.35) and an average particle size <30 nm. The IL-based μEs were transparent throughout the polymerization process, whose rate increased with increasing the [MMA]/[CCl₄] molar ratio and the concentration of the surfactant [229].

The use of C₁₂C₁ImBr/C₄C₁ImBF₄-based μEs for AGET-ATRP polymerization of MMA was investigated. The produced PMMA NPs had a small average diameter (∼5 nm) and narrow MM distribution (M_w/M_n = 1.24), probably due to the low initiation efficiency in IL/ILBS-μE polymerization. After isolation of the formed PMMA, the mixture containing the catalyst and the IL/ILBS was recycled four times with convenient results in terms of the average MM (5748 ± 398) and M_w/M_n, 1.24 to 1.37. Figure 20 shows the complete cycle of AGET-ATRP of MMA, polymer precipitation and microemulsion regeneration in the system C₁₂C₁ImBr/C₄C₁ImBF₄/MMA [230].

The effect of ILBS (C₁₂C₁ImBr) and GILBS (C₁₄Im)₂C₄Br₂ on the polymerization at 60 °C of MMA in O/W μEs was investigated to delineate the effects of the molecular structure of the surfactant. The latex PMMA NPs obtained with the ILBS had a smaller diameter (30–40 nm) and higher M_n (=442,600) than that synthesized in the presence of GILBS, (50–90 nm and 262,400, respectively). This was attributed to the difference between the cmc values of the two surfactants, which were larger for C₁₄C₁ImBr. Therefore, at the same surfactant molar concentration, there is a smaller number of micelles in the C₁₄C₁ImBr μE, leading to the formation of more MMA oligomeric radicals in the bulk aqueous phase, before they adsorb the surfactant molecules [231]. A PMMA/TiO₂/IL photocatalyst was fabricated by polymerization of MMA in μE of C₄C₁ImBF₄/1-butanol/Triton X-100 nonionic surfactant (2-[4-(2,4,4-trimethylpentan-2-yl)phenoxy]ethanol), with TiO₂ loading from 0.006 to 0.14 wt.%. For all samples, no visible phase separation was observed during and after the polymerization reaction. The recyclability of the polymerization catalyst was demonstrated through a series of subsequent polymerization reactions. The efficiency is significantly reduced, however, after the 5th polymerization cycle due to progressive surface poisoning and particle aggregation. Films were fabricated from the NPs and employed for the photodegradation of methylene blue as a model pollutant. The results indicated that the PMMA/TiO₂ NPs are more efficient in dye elimination than pure TiO₂. The efficiency of photooxidation first increased, then decreased as a function of increasing the TiO₂ content of the NPs. The effect of pH on the photodegradation of MB dye was investigated; the NP efficiency was maximum at pH = 8. The reason is that at this pH, a positive charge develops on the surface of the catalyst, leading to increased NP-cationic dye electrostatic attraction that increases the photocatalytic activity [232].

The separation of aromatic/aliphatic hydrocarbons is industrially important; it is carried out by extractive distillation, azeotropic distillation and liquid–liquid extraction. The efficiency of membrane separation of these hydrocarbons is increased by using silver salt complexes adsorbed onto the membrane. On use, however, the adsorbed silver ions “leach” into the liquid medium, leading to a decreased membrane separation efficiency and selectivity. This problem can be avoided by incorporating Ag⁺ into a polymeric matrix. Thus, AgCl NPs were incorporated into MMA–acrylamide (AM) co-polymer. The latter was fabricated by co-polymerization of MMA/AM (molar ratio 3:1) in μE using C₁₂C₁ImCl. The AgCl/poly(MMA-co-AM) hybrid membranes were 20 ± 2 μm thick, composed of a core (AgCl; average diameter = 20 nm)-shell (MMA-co-AM) structure. Figure S2 shows a schematic representation of the formation of AgCl that results from the addition of an aqueous solution of AgNO₃ to the ILBS-μE, via an anion-exchange reaction (AgNO₃ + C₁₂C₁ImCl → AgCl + C₁₂C₁ImNO₃); see Figure S2. AgCl was fixed into the MMA-AM co-polymer matrix via a –(H₂N)C==O⋯⋯Ag⁺ coordination. The swelling/sorption behavior of the hybrid membranes in benzene and cyclohexane and the separation ability of these hydrocarbons were investigated. The membranes showed preferential sorption of/swelling by benzene and demonstrated better pervaporation performance than that of the membrane without Ag⁺ NPs in separating the benzene/cyclohexane mixtures [233].

Styrene was polymerized (free radical, RAFT) in 1-R-3-methlyimidazolium bromide-based mini-emulsions (R = C₁₂ and C₁₆). Polystyrene (PS) nanoparticles were obtained (average diameter 80–125 nm, depending on the experimental conditions), demonstrating the efficiency of the ILBSs. The surfactant stabilized mini-emulsions were employed to fabricate PS-based magnetic NPs, as shown in Figure 21. The enrichment of styrene phase with oleic acid (OA)-coated magnetic NPs is due to phase separation between magnetic NPs and developing PS phase during mini-emulsion polymerization of PS [234].

OA-coated magnetic NPs were fabricated by stirring an aqueous mixture of FeCl₂ + FeCl₃ with OA, followed by neutralization of OA with NH₄OH aqueous solution, washing and drying of the magnetic OA-coated NPs. The magnetic PS NPs were then fabricated by styrene polymerization in the presence of OA-coated NPs. The authors assumed a preferential migration of the magnetic NPs to the PS particle surface, due to their immiscibility with the final PS phase, as shown in Figure 21. Finally, the magnetic properties of the materials were determined by vibrating sample magnetometer analysis [234].

An interesting extension of the use of ILBSs in polymerization is where the surfactant itself carries a polymerizable group (usually a double-bond), leading to its inclusion in the formed polymer. Thus O/W μE-mediated polymerization of MMA was carried out using (non-polymerizable) C₁₂C₁ImBr (ILBS-a) and polymerizable 1-(2-acryloyloxyundecyl)-3-methylimidazolium bromide (ILBS-b, for structure see Figure S3). Unlike the PMMA particles produced using ILBS-a, those obtained using ILBS-b did not show aggregation, probably due to the formation of polymerized surfactant shell around the PMMA core NPs, thus rendering them more stable. The anion (Br⁻) of the polymerized surfactant can be exchanged with other, less hydrophilic and stimuli-responsive anions, e.g., BF₄⁻, PF₆⁻ and N(CN)₂⁻. Figure 22 shows SEM images of the fabricated PMMA, before and after the above-mentioned anion exchange [227].

The same polymerizable surfactant-based μE was employed to fabricate PMMA NPs that were employed to obtain polymeric films. These can be used as starting material for the production of coatings whose porosity and transparency can be “fine-tuned” according to the duration of their treatment in an aqueous KPF₆ solution; see Figure S4 in Supplementary Material. Ion-exchange with the surfactant anion (Br⁻) with BF₄⁻, PF₆⁻, and N(CN)₂⁻ leads to stimuli-responsive films, due to the pairing of the anion to the imidazolium ion [235].

Anion exchange of the ILBS can be also employed to confer magnetic properties to the fabricated polymers. For example, C₁₂VnImBr was homopolymerized and the anion (Br⁻) of the produced polymer was transformed into FeBrCl₃⁻, CoBrCl₂⁻ and MnBrCl₂⁻ by reaction with FeCl₃, CoCl₂ and MnCl₂, respectively. As expected, these polymerized ILBSs showed magnetic properties. Additionally, C₁₂VnImX (X = FeBrCl₃⁻, CoBrCl₂⁻ and MnBrCl₂) were copolymerized with mixtures of MMA and n-butyl acrylate to give stable latexes that showed paramagnetic behavior with weak antiferromagnetic interactions between the adjacent metal ions; i.e., they are candidates as optically transparent microwave absorbing materials [236].

The same strategy, i.e., polymerization in the presence of a colloidal stabilizer, followed by ion-exchange was employed to fabricate so-called “liquid marbles”. This term refers to liquid droplets (generally water) coated with an exterior layer of a hydrophobic material. They display non-adhesive and nonwetting behavior toward many surfaces, so that these droplets “float” on the surface of water. When liquid marbles exist in macroscale, they appear as free-flowing powders, referred to as “dry liquids” (e.g., “dry water”) because they appear as dry, solid materials rather than continuous fluids. Thus, styrene, and mixtures of MMA and benzyl methylacrylate were polymerized (free radical emulsion polymerization) in the presence of cationic surfactants, e.g., CTACl, and poly(1-vinyl)-3-methylimidazolium bromide as particle stabilizers. The fabricated liquid marbles (diameter up to 500 nm) can have magnetic properties by polymerization in the presence of magnetite (Fe₃O₄). The same strategy was employed to fabricate fluorescent liquid marbles, namely, emulsion polymerization in the presence of fluorescein O-methacrylate or acrylate functionalized Rhodamine B1. The fabricated liquid marbles can be flocculated by ion exchange of the stabilizer anions (Br⁻ or Cl>⁻) with (hydrophobic) bis(trifluoromethanesulfonyl)-imide. The properties (magnetic and fluorescent) of the above-mentions materials can be exploited in different applications, e.g., gas and pH sensing, microreactors, microfluidics, biotechnology, drug delivery and also cosmetics and personal care products [237,238].

5. Conclusions and Perspectives

We identify molecular structure versatility as the main reason for the sustained interest in single- and multiple-chain ILBSs. This offers a window of opportunities for diverse applications, including the fabrication of NPs thfighuat are employed, inter alia, in catalysis, decontamination and drug delivery. Dimeric and polymeric ILBSs have an additional structural dimension, the spacer, whose length can be varied as required, and may contain a heteroatom. Another interesting class is biamphiphilic surfactants, because the electrostatic and hydrophobic interactions that lead to aggregate formation can be “fine-tuned” by controlling the length and hydrophobicity of both surfactant ions; see Figure 8. Table 1, Table S1, Table 2, Table S2 show the recent data available (last 10 years) on the adsorption at the water/air interface and micelle formation by single- and multiple-chain ILBSs. This should help in choosing not only the molecular structure of the ILBS, but also the optimum surfactant concentration for the intended application. The values of α_mic are related to the surface potential of the micelle and hence help in applications where electrostatic interactions substrate-micelle are important. Among many applications, we chose those relying on the use of ILBSs as soft templates for the fabrication of NPs and polymers. We hope that our effort highlights these points and serves to increase the awareness of the enormous potential of ILBSs in science and technology.

Figure 2 shows the sustained and expanded interest in ILs and ILBSs. In this review, we focused on their use in the fabrication of NPs, both metallic and polymeric. Although it is outside the scope of the present review to cover all important applications of ILBSs, it is worthwhile to mention other applications, including their use in enhanced oil recovery, e.g., by flooding with μEs [239,240], and in analytical chemistry, including liquid–liquid extraction [241], voltammetry and amperometry as organic electrolytes for carrying out electrochemical processes [242], solid-phase microextraction [243] and in chromatography [244]. The ease with which the properties of weakly surface-active ILs and ILBSs can be fine-tuned to the researcher’s need means that the ascending curves shown in Figure 2 is likely to continue in the future. The somewhat “exotic” uses of ILs by NASA are just an example (www.nasa.gov/oem/ionicliquids (accessed on 29 March 2021)).