3.1. Biodegradation/Toxicity
Two of the key challenges currently faced in IL syntheses are related to using “green” synthetic techniques and minimizing the products’ ecological impacts. Many ILs are soluble in water, so their potential influence on the environment must be considered in case of any leakage or wastewater discharge leading to water or soil pollution [
50,
51]. According to Earle et al. [
52], some ILs can be distilled at low pressure; therefore, atmospheric contamination cannot be completely neglected either, especially when ILs are used at elevated temperatures. Using natural and bio-renewable building blocks (i.e., organic acids, amino acids, or choline) frequently leads to biodegradable ILs [
4,
5]; however, in some cases, the resulting compounds still do not undergo complete biodegradation [
53]. Additionally, as reviewed by Jordan and Gathergood [
5], the biodegradability of the ILs is notably influenced by their chemical structures, where unbranched alkyl chains containing ester, formyl, carboxylic, or hydroxyl groups support biodegradation processes because they are readily hydrolyzed or oxidized.
Ionic liquids with sugar moieties as their natural building blocks are rich in hydroxyl groups, which have been shown to enhance IL biodegradability, despite limited research in this area. Ferlin et al. [
37], investigated a series of tetrabutylammonium ILs containing anions based on natural organic acids, and they reported that ILs derived from
d-glucuronic (
1b) and
d-galacturonic (
1c) acids demonstrate higher biodegradability than analogous ILs derived from
l-lactic,
l-tartaric, malonic, succinic,
l-malonic, and pyruvic acids. Similarly, all of the bio-derived ILs investigated exhibited higher biodegradability in the Closed Bottle test (OECD 301D) than common tetrabutylammonium bromide and tetrabutylammonium hydroxide salts. However, the biodegradability of carbohydrate ILs is dependent on the length of the alkyl chain in the cation. In a series of quaternary ammonium salts based on
d-glucose cation, where alkyl chains of various length were introduced (–CH
3, –C
12H
25, –C
16H
33,
4a,
4b,
4c, respectively), a derivative with the shortest alkyl chain (
4a) was found to be readily biodegradable (74–83%), while
4b with C12 was close to fulfill the ready biodegradability criterion (60%), reaching 57% [
54]. In contrast, ILs with –C
16H
33 substituent (
4c) showed a longer lag phase of 10 days, after which it started to degrade, not reaching a plateau during the experiment time though.
Combining carbohydrates with other bio-derived molecules (i.e., amino acids) has also been shown to yield readily biodegradable ILs (
3a–
g), which can be decomposed in 5–6 days in activated sludge [
42]. This result highlights how carbohydrate-derived ILs can be used as alternatives to the cholinium family of ILs ([Ch]ILs), which have emerged as the most readily biodegradable ILs to date [
5]. Besides cholinium amino acid-based ILs (AAILs) exhibit lower viscosity and more facile synthesis, although a green synthetic pathway for obtaining sugar-derived ILs has been reported recently [
11], carbohydrates contain rich hydrogen bond structures which are favorable for certain applications.
The toxic effects of the ILs and/or their metabolites on living organisms must be considered when evaluating their environmental impacts. In this context, ILs derived from naturally occurring compounds are also expected to be less toxic, with [Ch]ILs again being considered some of the most promising [
55,
56,
57].
In addition to [Ch]ILs, carbohydrates have also gained attention as starting materials for synthesizing non-toxic and biocompatible ILs. Carbohydrate-derived ILs generally exhibit very low (eco)toxicity towards bacteria [
37], fungi [
37], human cancer cells [
17], mouse cells [
13,
39], rat cells [
54], and zebrafish eggs [
17], indicating a promising biocompatible character.
Among the aforementioned tetrabutylammonium ILs combined with natural organic acids (i.e.,
d-glucuronic (
1b),
d-galacturonic (
1c),
l-lactic,
l-tartaric, malonic, succinic,
l-malic, and pyruvic acids), carbohydrate-derived ILs were determined to be the least toxic [
37]. Interestingly, functionalization of conventional imidazolium ILs with sugar moieties decreased their toxicity or even removed it completely [
13,
17]. Hong et al. [
13] further confirmed the positive effect of sugar moieties on toxicity by incorporating them into PILs. Incorporation of a quaternary ammonium salt into the polymer backbone functionalized with pendent sugar units revealed significantly decreased cytotoxic activity against mouse fibroblast cells (L929).
However, Reiß et al. [
39] reported that the toxicity of the ILs can depend on their concentration, and they demonstrated this relationship with a series of pentose- (
6a–
h) and
d-glucopyranose-based ILs (
7a–
g). At a concentration of 0.1 M, almost all of the investigated ILs showed no cell viability; however, at 10 mM concentration, some differences were observed. Specifically, among pentoses-derived ILs, those based on ribose were the least toxic. Moreover, differences in toxicity between α- and β-anomers of glucose-derived ILs (
7a and
7b, respectively) were observed, with the IL based on the α-anomer demonstrating higher toxicity against mouse cells than the IL based on the β-anomer.
The length of the carbon chain is another factor that can influence the cytotoxicity of ILs. This effect was discussed in aforementioned work by Erfurt et al. [
54], where glucose-based ILs modified at anomeric position with various alkyl substituents (
4a–
c) were examined. Derivatives with the shortest alkyl chain (
4a) showed the lowest cytotoxicity against rat leukemia cells, in fact it revealed no effect on the viability of cells up to 0.584 mM concentration. For derivatives
4b and
4c, the EC
50 values were sufficiently lower: 0.085 for
4b and 0.010 mM for
4c, indicating an increase in cytotoxicity in the following order,
4a <
4b <
4c. These results confirm that higher cytotoxicity corresponds to the higher hydrophobicity of investigated ILs. Surprisingly replacing [Br]
− anion with [NTf
2]
− anion in
4c, showed very little influence on the toxicity. The authors compared also investigated ILs with analogue dimethyl-phenyl-ammonium chloride compounds. The latter showed similar relation between cytotoxicity and hydrophobicity; however, replacing phenyl ring with hydrophilic
d-glucose, significantly decreased their cytotoxicity [
58].
Considering the cation/anion influence on the toxicity, the toxicity of ILs with cationic sugar moieties mostly depends on the anion. As reported for
d-glucopyranose-derived ILs (
7b–
d), replacing [OTf]
− (
7b) or [OTs]
− (
7d) anions with [OMs]
− (
7c) anions decreased the toxicity significantly. In contrast, for ILs where a carbohydrate moiety was incorporated as the anion, the toxicity of the systems was influenced primarily by the nature of the cation. For example, in gluconate ILs, those combined with phosphonium cations (
2b,
2c) tended to be more toxic than their ammonium (
2a) or guanidinium (
2d) analogues. Moreover, in agreement with previous reports, the increasing length of alkyl chains in a tetraalkylphosphonium cation can also negatively influence their toxicity [
59,
60].
The results presented in this section demonstrate that modification of conventional ILs with carbohydrate motifs may lead to decreased toxicity, thus further highlighting the significant potential of using carbohydrate units to fabricate biodegradable, non-toxic ILs. Building on these promising perspectives, further studies into the biological activity of carbohydrate-based ILs could indicate highly desirable applications in areas of chemistry as well as in biology and ecology [
61].
3.2. Thermal Stability and Melting Point
The thermal properties of various ILs and salts determine their possible applications. ILs are frequently considered to be thermally stable because of their high decomposition temperatures (
Td). However, data related to these properties are usually collected using ramped-temperature thermogravimetric analysis (TGA), and Del Sesto et al. [
62] have reported that results obtained with this method can often be overestimated, even by hundreds of degrees, due to the high rates of temperature increase. The long-term stability of ILs can be evaluated using isothermal TGA (i.e., static TGA) measurements taken over a few hours. The
Td values obtained by such techniques are significantly lower than those measured with the ramped-temperature TGA method [
63]. To the best of our knowledge, such experiments have not yet been conducted for carbohydrate-derived ILs, but they would be very useful to investigate caramelization of saccharides [
64] which may also occur in saccharide-derived ILs.
Various structural features of the carbohydrate ILs may affect their thermal properties, including Td, melting point (Tm), and glass transition temperature (Tg). Here, we consider the influence of various cation modifications, including length of the alkyl chain on the quaternary ammonium group or alkyl spacer between carbohydrate moiety and quaternary ammonium group, additional functional/protecting groups, and the nature of both the cation and the anion.
3.2.1. Anion
The type of anion in carbohydrate-derived ILs and salts as well as in their common imidazolium analogues corresponds to their nucleophilicity and basicity and significantly affect their thermal stability [
65]. More nucleophilic and basic anions lead to less stable ILs and salts. This trend is clear for glucose-derived salts investigated by our group, with [Br]
−, [NCN]
−, and [NTf
2]
− anions (
8a,
8b, and
8c, respectively), where their thermal stabilities increased in the order,
8c >
8b >
8a, corresponding to decreasing anion nucleophilicity [
12,
41].
This trend is also evidenced in the open chain gluconamide derivatives (
5b,
5c) investigated by Billeci et al., [
17] where derivative
5b with [NTf
2]
− anions demonstrated higher thermal stability than
5c, with [Br]
− anions. Interestingly, derivatives with longer alkyl spacers in their cations (i.e.,
5d,
5e) followed the opposite tendency. The typical trend was maintained for isosorbide (generated via hydrogenation of glucose to sorbitol, followed by dehydration) derivatives,
14a–
d and
14e–
h, with [NTf
2]
− and [OTf]
−, respectively, where the latter exhibited higher thermal stability [
34]. In halide-based gluconamide derivatives (
5f and
5g, with [I]
− and [Br]
−, respectively), higher thermal stability was revealed for the derivative with the [I]
− anion, according to the decreasing basicity of halides ([F]
- > [Cl]
− > [Br]
− > [I]
−). However, some deviations from this trend may occur. For example, Kaur et al. [
40] reported a series of chiral salts derived from
d-galactopyranose and DABCO (
9a–
f), with
Td values between 180 and 250 °C, which rise in the order, [OTf]
− < [I]
− < [BrCH
2CH
2SO
3]
− < [SbF
6]
− < [BF
4]
− < [PF
6]
−.
Interestingly, a series of amino acid/carbohydrate-based ILs (
3a–
g) studied in our group revealed the profound influence of the amino acid structure on
Td [
42]. Among the studied anions ([Arg]
−, [Gly]
−, [Ser]
−, [His]
−, [Leu]
−, [Trp]
−, and [Tyr]
−), the thermal stability of the corresponding ILs increased with elongation of the amino acid side chain. The only exception was [Arg]
−, which caused reduced stability of
3a.
Comparing the
Td values for derivatives where the carbohydrate moiety was transformed into an anion (e.g., cyclic glucuronate (
1b) and open-chain gluconate (
2a)), combined with a tetrabutylammonium cation, the latter exhibited higher thermal stability (
Td = 136 and 161.8 °C, respectively). Derivative
2a also had a higher melting point and glass transition temperature than
1b [
17,
37]. The same trend was observed for ILs with glucuronate and gluconate anions combined with [EMIM]
+ cations. Specifically,
Td for the derivative with the glucuronate anion (
1a) < 200 °C, whereas
Td for the gluconate anion derivative (
2e) > 250 °C [
12,
22]. This is most likely due to the higher number of hydroxyl groups present in the gluconate anion (five, versus four in glucuronate) and the stronger H-bonding interactions.
The type of anion also influences the
Tm of carbohydrate-derived ILs and salts. Kumar et al. [
30] prepared a series of dicationic salts derived from isomannide (
15a–
f), where the
Tm values ranged from 60 to 251 °C, increasing in the following anion order: [NTf
2]
− < [TFA]
− < [OTf]
− < [I]
− < [BF
4]
− < [PF
6]
−.
Certain carbohydrate salts with halide anions have exceptionally high
Tm values, even higher than their analogues bearing [BF
4]
− or [PF
6]
− anions. In a series of chiral ammonium ILs and salts derived from isomannide (
16a–
f), the highest
Tm (170 °C) was measured for the derivative bearing the [I]
− anion (
16f) [
31]. Derivatives with [NTf
2]
− (
16a) and [TFA]
− (
16b) anions were liquids at room temperature; however, changing the anion to [OTf]
−, [PF
6]
−, or [BF
4]
− led to increases in their
Tm values, to 80, 95, and 150 °C, respectively. This was also the case for
d-ribose- (
10a–
f) and
d-galactose- (
11a–
e) derived ILs and salts [
26,
66]. Specifically,
d-ribose-based salts containing [NTf
2]
−, [OTs]
−, or [BF
4]
− anions were liquids at room temperature, while for the derivatives with [PF
6]
−, [I]
−, or [Br]
− anions, melting points were observed, rising in the following order: [PF
6]
− < [I]
− < [Br]
−. The corresponding
d-galactose derivatives were all solids with significantly higher
Tm values.
3.2.2. Cation
Both the origin and functionalization of an IL’s cation may affect the thermal properties of these sugar-based ILs. Poletti et al. [
21] studied the influence of modifications at the C6 position of methyl-α-
d-glucopyranoside using trimethylamine (
12a), diethyl sulfide (
12b), or tetrahydrothiophene (
12c). Both
12a and
12c are solids at room temperature, with
Tm = 137.5 and 110 °C, respectively. Additionally,
12c showed an exothermic crystallization peak at 52.6 °C, in contrast to
12b, which showed neither crystallization nor melting but formed a glass upon cooling to −53 °C. The thermal stability of these ILs increases in the order
12b <
12c <
12a.
Reiß et al. [
39] developed a series of pentose- (
6a–
h) and
d-glucopyranose-based ILs (
7a–
g) which showed significant differences in thermal properties. Although direct comparisons are difficult to make because they have different functionalization at the anomeric centers, comparing pentose-derived ILs with glucoside-derived ILs possessing the same anions and protection of hydroxyl groups reveals that the pentose-derived ILs were mostly liquids at room temperature, while glucoside-derived ILs were mostly solids under the same conditions. Additionally, pentose-derived ILs have higher
Td values (297–345 °C), relative to glucoside-derived ILs (
Td = 205 to 250 °C). Among the pentose-derived ILs, thermal stability increases in the following order:
d-lyxose <
l-arabinose <
d-xylose ≈
d-ribose. More exact comparisons can be made among glucoside-based products where differences in thermal properties were reported for α- and β-anomers. For example, the methyl-α-
d-glucopyranoside-derived IL with [OTf]
− anions (
7a) had a T
m of 95–100 °C, while the β-anomer (
7b) was a liquid at room temperature.
ILs where carbohydrate moieties were transformed into anions and combined with common alkylimidazolium, tetraalkylammonium, tetraalkylphosphonium, or
N,
N,
N’,
N’-tetramethylguanidium cations, demonstrate thermal stabilities in good correlation with the stability of cations reported for traditional ILs with non-carbohydrate-derived anions [
12,
17,
37,
63]. In a series of gluconate-based ILs with phosphonium, ammonium, or guanidium cations (
2a–
d), thermal stability depends on the nature of the cation and increases in the order, [tmgH]
+ < [P
4 4 4 4]
+ < [N
4 4 4 4]
+ [
17]. Moreover, their
Td values can be enhanced by elongation of the alkyl chain. This is evidenced by the fact that the derivative
2c, with [P
6 6 6 14]
+ cations, exhibits the highest thermal stability, with
Td even higher than that of
2a, with [N
4 4 4 4]
+ cations. Furthermore, as demonstrated for glucuronate-based ILs (
1a,
1b), the imidazolium salt (
1a) showed higher thermal stability than the corresponding ammonium salt (
1b) (i.e., [N
4 4 4 4]
+ < [Emim]
+) [
12]. This trend was also verified when imidazole was used as the functionalization moiety in a salt with a gluconamide-based cation (
5a), which demonstrated higher thermal stability than the corresponding ammonium-based cation salt (
5h) [
17].
3.2.3. Length of the Carbon Chain
The length of the carbon chain is another factor that can influence the thermal properties of carbohydrate-derived ILs and salts, where the carbon chain on the quaternary ammonium group and the spacer between the carbohydrate moiety and the quaternary ammonium group can be distinguished.
According to a report from Arellano et al. [
67] investigating traditional imidazolium ILs, increasing the alkyl chain length strengthens Van der Waals intermolecular interactions which impact on the organization of the molecules in the sample, leading to increased thermal stability [
68]. In contrast, increased chain lengths can also decrease the electrostatic interactions, thus causing lower thermal stability [
69]. Moreover, it was shown that longer alkyl chain lengths in piperidinium ILs may increase the stability of the corresponding carbocations and carbon radicals, making them better leaving groups and thereby promoting decomposition [
70]. In line with these observations, the thermal stability of carbohydrate-based ILs and salts show little dependence on elongations of alkyl chain length on the ammonium head.
In a group of herbicidal ILs (HILs) derived from
d-glucose, containing 4-chloro-2-methylphenoxyacetate anions and alkyl substituents at the ammonium head ranging from –CH
3 to –C
16H
33 (
4d–
h), no obvious trend in thermal stability was observed [
15]. However, the increase in thermal stability (i.e., C
1 < C
8 < C
4 < C
16 < C
12) corresponded well with the analogous group of HILs with 2,4-dichlorophenoxyacetate anions (
4i–
m). One exception was derivative
4j, with a C
4 substituent, which demonstrated the lowest stability.
Comparison of the
Td values of isosorbide-derived ILs with C
8 and C
12 alkyl chains (
14a,
14e, and
14b,
14f, respectively) suggests that the length of the alkyl chain imposes very little influence on the thermal stability of this type of IL [
34].
A more distinct influence of the alkyl chain length on the ammonium head could be determined considering the melting point data for investigated salts. For example, in the dicationic mannitol salts (
18a–
e), a systematic increase of
Tm was observed to correlate with elongation of the chain length from –C
3H
7 to –C
18H
37 [
25]. In addition, the isomannide-based salt with a
–C
4H
9 alkyl chain (
17a) has a higher melting point than its corresponding derivative with a methyl substituent (
17b), which is an oil [
32]. There is an observable, yet less profound influence of the alkyl chain length, which was also demonstrated for α-
d-glucopyranoside derivatives functionalized with DABCO amine and quaternized with C
4 to C
18 alkyl chains (
13a–
d)
[25].
The protection of hydroxyl groups by etherification provides a similar effect, as shown by glucose derivatives
7b and
7g, where hydroxyl groups have been methylated or ethylated, respectively [
39]. Specifically,
7b is a liquid at room temperature, while
7g has a
Tm of 118–120 °C.
In terms of the alkyl spacer between the carbohydrate moiety and the quaternary ammonium group, thermal stability can be influenced by its linear or branched nature. In a group of glucono-based derivatives, the branching of the alkyl chain caused a decrease in
Td values (
5c,
5i, and
5e,
5j) [
17]. Moreover, considering the length of the alkyl spacer, the thermal stability was dependent on both the nature of the anion and length of the alkyl chain. For glucose derivatives combined with [NTf
2]
− anions and modified at the C1 position with ethoxy or propoxy chains (
3h,
3i), a slight increase in thermal stability was reported with elongation of the chain (∆T = 2 °C) [
41]. This trend was also observed for glucono-based derivatives
5c and
5e, with ethyl and propyl linkers, respectively, combined with [Br]
− anions [
17]. In contrast, the opposite trend (i.e., decrease of thermal stability with lengthening of the alkyl spacer) was reported for corresponding derivatives combined with [I]
− and [NTf
2]
− anions (
5f,
5h and
5b,
5d, respectively). It is worth noting that, despite the opposite trend reported for derivatives with [NTf
2]
− anions, the effect of lengthening the alkyl chain is minor (∆T = 3 °C), as demonstrated by comparing glucose derivatives
3h and
3i [
41]. For halide derivatives with [I]
− (
5f, 5h) and [Br]
− (
5c, 5e) anions, the trend is noticeably more pronounced (∆T = 12 °C and ∆T = 27 °C, respectively) [
17].
Concluding, conventional ILs exhibit higher thermal stabilities than most of carbohydrate derived ILs. It is certainly due to the easier degradation of carbohydrate motifs at elevated temperatures.
Majority of ILs with hexose in the cation reveal
Td between 150 and 250 °C; however, additional stability can be achieved when combining sugar cation with [NTf
2]
− anion. On the other hand, some pentose-derived ILs reveal higher thermal stability of 296–345 °C [
39]. To compare, the conventional ILs with 1-ethyl-3-methylimidazolium cation and anions such as: [Cl]
−, [Br]
−, [I]
−, [OTf]
−, [OMs]
−, [N(CN)
2]
−, [C(CN)
3]
−, [BF
4]
−, [PF
6]
−, [AsF
6]
−, [NTf
2]
−, [Beti]
−, [EtSO
4]
− reveal
Td ranging from 282 to 462 °C, [
65], while ILs containing 1-butylpyridinium cation combined with: [Br]
−, [BF
4]
−, [PF
6]
−, [NO
3]
−, [OTf]
−, [N(CN)
2]
−, [HSO
4]
−, [H
2PO
4]
− reveal
Td ranging from 239 to 392 °C [
69]. Simultaneously, by changing the anion in the IL with 1-ethyl-3-methylimidazolium cation into either gluconate or glucuronate, considerably lower thermal stabilities up to 250 °C are achieved [
12,
22,
65]. These examples of pentose-based ILs show that certain carbohydrate ILs can be utilized in similar temperature ranges as the vastness of the conventional ILs, moreover, they are much less toxic.
3.3. Viscosity and Glass Transition Temperature
Viscosity (η) is a quantity describing a fluid’s internal flow resistance, and it is one of the key variables that determines an IL’s potential applications. This property is directly affected by different molecular interactions like repulsions, hydrogen bonding, short-range van der Waals interactions, and long-range electrostatic forces [
71]. Therefore, carbohydrate-derived salts with significant hydrogen bonding capabilities commonly also exhibit relatively high viscosities, even orders of magnitude higher than their corresponding imidazolium, ammonium, or cholinium analogues [
72]. Nevertheless, as reported by Billeci et al., it is still possible to design low viscosity carbohydrate-based ILs (e.g., η for
5g = 106.3 mPa·s), which are promising solvents with potential industrial applications. Moreover, the problem of exceptionally high viscosities can be overcame in some applications by using mixture of IL and molecular solvent [
73,
74]. It is also worth noting that carbohydrate-based ILs have recently been recognized as promising materials for applications where low viscosity is not a demand (i.e., as organocatalysts or PILs with pendent sugar moieties).
Currently, despite limited available data related to the viscosities of carbohydrate-based ILs, some structure–properties relationships can be inferred based on a survey published recently by Billeci et al. [
17]. In a series of glucono-based ILs, changing the anion from [Br]
− to [I]
− induced a drastic increase in viscosity (i.e., from 106.3 mPa·s for
5g, to 41,700 mPa·s for
5f). Surprisingly, for these halides, the viscosity also increased with shortening of the alkyl spacer (from propyl to ethyl) between the carbohydrate moiety and the quaternary ammonium group (
5h <
5f and
5e <
5c). The trend is usually the opposite in the case of imidazolium ILs (i.e., increasing the length of the alkyl chains typically increases the viscosity through stronger van der Waals interactions [
75]), and this opposite trend is also clear for glucono-based ILs with ethyl (
5b) and propyl (
5d) spacers and [NTf
2]
− anions. Elongation of the alkyl chain on the ammonium head (from butyl in
5g to octyl in
5c) also led to more viscous liquids, similar to the effect of a smaller degree of branching (
5e and
5j, respectively) [
17].
When combining carbohydrates with amino acids in ILs, the anion size also influences the viscosity. This relationship is evidenced by evaluating the effect of elongating the amino acid side chain. In general, the stronger intermolecular interactions accompanying such elongations resulted in increased viscosities of the derivatives
3a–
g [
42].
The viscosity values are frequently reflected in the
Tg, which is the temperature at which reversible transition in amorphous materials occurs, from a hard, brittle, glass-like state to a viscous state [
71,
76]. Therefore, for ILs bearing the same anion, an increase of the
Tg can be achieved by increasing the cation size, due to the expected stronger van der Waals interactions. These considerations relate to both cations and anions. Smaller anions with shorter alkyl chains commonly lead to lower
Tg values [
77,
78]. This behavior is evidenced in a series of
d-glucose-based ILs, where elongation of the alkyl spacer from ethyl to propyl (
3h and
3i, respectively) causes an increase in
Tg.
Further increase in
Tg is observed following introduction of an additional hydroxyl group on the hydrocarbon chain of the alkyl spacer in
3j [
41]. As expected, the additional hydroxyl group increased the overall strength of hydrogen bonding, which was then reflected in a higher
Tg value. In addition, a decrease in
Tg value was observed with greater branching of the alkyl side chain on the ammonium head in gluconamide-derived ILs (
5c >
5i and
5e >
5j) [
17].
Considering the protection of hydroxyl groups of the sugar moieties, the opposite tendency in terms of alkyl chain length has been reported, i.e., an increase of
Tg is observed with shortening of the alkyl chain length [
39]. Similarly, it was reported for ribose-based ILs, that their
Tg increased when changing propyl (
6c) and ethyl (
6d) ethers into methyl ethers (
6a) [
39].
The
Tg of gluconamide-derived ILs increases with increasing nucleophilicity of the anion. For example,
5g with [Br]
− anions have a reported
Tg value of −46.6 °C, while for
5f, [I]
− anions have
Tg = −7.6 °C. Comparing
Tg values of isosorbide-based ILs that contain the same cation and either [OTf]
− or [NTf
2]
− anions, revealed that those with [OTf]
− anions had higher T
g values (i.e.,
14a <
14e,
14c <
14g, and
14d <
14h) [
34].
3.4. Conductivity
The conductivity of IL is one of the key parameters determining its use in electrochemical applications. As long as conventional ILs have been extensively studied as electrolytes for lithium batteries, fuel cells [
79] or capacitors [
80], very little research has been done in this area among carbohydrate ILs.
Billeci et al. [
17] investigated the conductivity of gluconamide ILs (
5a–
f,
5h–
j) which were measured using [OMIM][BF
4] as a standard. The conductivity of carbohydrate ILs ranged from 0.002 to 0.162 S/m, while for [OMIM][BF
4] it was equal to 0.059 S/m. The lengthening of the alkyl spacer as well as its branching were the major factors influencing the decrease in the conductivity. Moreover, the degree of decrease was related to the anion coordination ability. The authors also compared the conductivity of
5c with analogous compound, where the gluconamide motif was replaced with a hexanamide one. The latter showed a significantly higher ability of charge transport due to the lack of –OH groups that reveal strong hydrogen bonding ability leading to increased viscosity and, hence, lower conductivity.
In another study, Chen et al. [
38] investigated the conductivities of [C
nMIM][Gluconate] (n = 2, 4, 6, 8, 10, 12 or 14) dissolved in various molecular solvents (water, ethanol and propyl alcohol). Again, regardless the solvent used, a decrease in conductivity was observed with the increase of alkyl chain length due to the lower mobility of such cations.