Electrochemical Analysis of Aqueous Benzalkonium Chloride Micellar Solution and Its Mediated Electrocatalytic De-Chlorination Application

Abstract

The physicochemical properties of biologically important benzalkonium chlorides (BKCs) and the effects of its structure on the de-chlorination of allyl chloride was studied by electrogenerated [Co(I)(bipyridine)₃]⁺ (Co(I)) using an electrochemical technique. The results of [Co(II)(bipyridine)₃]²⁺ (Co(II)) cyclic voltammetry in the presence of BKC demonstrates Co(II)/Co(III) redox couple for physicochemical analysis of BKC and Co(II)/Co(I) redox couple for catalytic application. Cyclic voltammetry over a range of scan rates and BKC concentrations revealed the BKC-bound Co(II)/Co(III) micelles showed that the identification of cmc and association of the probe Co(II) species, associated more in the hydrophobic region. In addition, change in diffusion coefficient value of Co(II)/Co(III) with BKC concentration demonstrates the association of Co(II) in micellar hydrophobic region. The beneficial effects of BKC could be accounted for by considering the benzyl headgroup-Co (II) precatalyst-volatile organic compounds (VOCs) (allyl chloride here) substrate interaction. Chromatography/mass spectroscopy (GC/MS) revealed 100% complete de-chlorination of allyl chloride accompanied by three non-chloro products. This is the first report of benzyl headgroup-induced micellar enhancement by an electrochemical method, showing that it is possible to use hydrophobic benzyl headgroup-substitution to tune the properties of micelles for various applications.

1. Introduction

Benzalkonium chloride (BKC) is used primarily as a preservative and antimicrobial agent, as well as a surfactant. The compound has potent biocidal activity, killing microorganisms, and inhibiting their growth [1,2], but is friendlier to human skin, which is why it is used as an ingredient in antibacterial hand wipes, antiseptic creams, and anti-itch ointments. Other than the medicinal field, BKC has a beneficiary role as a surfactant particularly in the corrosion resistance of carbon steel because of its strong adsorption and hydrophobicity [3]. To stabilize and increase the porosity during the preparation of porous carbon, BKC is used greatly as porosity stabilizer [4]. In solubilized media, BKC is used as the supporting electrolyte during the electrochemical reduction of CO₂ reduction in methanol at room temperature [5,6]. Benzalkonium chloride has been applied to remove biofilms on marine-based sensors or optical components [7,8], which highlights the need to determine the diffusion coefficient for BKC in water. Smith et al. [9] developed a membrane diffusion method and derived the diffusion coefficient for BKC with C12 and C14 in water. Ultrafiltration or ultracentrifugation methods have been adopted to determine the partition coefficient of BKC and the free BKC surfactant concentration in micellar solutions [10,11]. In the biological and chemical fields, both solubilization and stabilization play important roles in their specific applications. A detailed mechanistic aspect to understanding BKC micelles is still lacking, particularly in the electrochemical field.

With the exception of BKC, many other self-aggregated surfactant monomers have been used to solubilize redox species in solution, and they have been analyzed by the electrode surface [12,13]. Advantageous of this phenomena used to harvest the physicochemical properties of the various surfactant solutions by single potential step chronoamperometry [14,15] and diffusion limiting current at micro-electrodes techniques [16,17]. Despite the interesting physicochemical identification of surfactants in electrochemistry, the mediated catalytic reduction of halo compounds, such as 4-bromobiphenyl, 4,4’-dichlorobiphenyl, allyl chloride, and PCB (Polychlorinated biphenyls) mixtures, catalyzed by different organic and inorganic mediators, e.g., 9-phenylanthracene [18], [Co(bpy)₃]²⁺ (where bpy = 2,2’-bipyridine) [19,20], zinc phthalocyanine [21], etc., have been observed in aqueous solutions of various surfactants, such as CTAB (Cetyltrimethylammonium bromide), SDS (Sodiumdodecyl sulfate), and film-forming dodecyl dimethyl ammonium bromide. In a recent study, the cobalt polypyridine complex-mediated de-chlorination of several chlorinated pesticides with significantly enhanced catalytic rates was achieved in a surfactant-containing aqueous solution [22]. In addition, our previous study reported the electrocatalytic dehalogenation of allyl chloride and its methyl-substituted derivative β-methyl allylchloride mediated by electrogenerated [Co(I)(bpy)₃]⁺ at a glassy carbon electrode in aqueous solutions of cationic micelles [23,24,25].

In the present investigation, physicochemical analysis of BKC with an alkyl chain length of C16 was analyzed using an electrochemical method. For this purpose, in the first part of the study, the [Co(II)(bpy)₃]²⁺/[Co(III)(bpy)₃]³⁺ redox reaction in BKC surfactant was studied by CV (Cyclic voltammetry) and some key parameters, such as the cmc (critical micellar concentration), micellar diffusion coefficient, and relative association constants of the oxidized species were estimated, as done for other surfactants [26,27,28,29,30,31]. Following this, the effects of BKC on allyl chloride catalysis were examined by product analysis and discussed.

2. Results and Discussion

2.1. Selection of Redox Couple

According to Figure 1, two redox couples for [Co(II)(bpy)₃]²⁺ redox system were found. One was related to Co(II) to Co(III) oxidation and another one was for Co(II) to Co(I) reduction. One mM of [Co(II)(bpy)₃]²⁺ in 0.1 M Na₂SO₄ medium shows highly reversible redox behavior of Co(II) to Co(III) (A1 and C1) oxidation (Figure1, curve a), but complicated redox peaks were observed in the case of Co(II) to Co(I) reduction (C2 and A2). At the same time, addition of 10 mM BKC led to the reversible Co(II)/Co(III) redox couple becoming quasi-reversible or diminished according to the BKC concentration. Surprisingly, the reduction couple Co(II)/Co(I) became reversible without any complications (Figure1, curve b).

It is more realistic that the Co(II)/Co(III) redox couple can be considered for the BKC aggregates and its physicochemical analysis due to effective change in peak current and potential with BKC concentration, Equation (1). Because of the reversible redox behavior in presence of BKC, the Co(II)/Co(I) redox couple can be considered for catalytic reduction analysis, as the reversibility is one of the mediated electrocatalytic processes [32] and solubilities of the VOCs in the micellar medium (Equation (2)).

[Co(II)(bpy)₃]²⁺ + BKC ⇌ M-[Co(II)(bpy)₃]²⁺ ⇌ M-[Co(III)(bpy)₃]³⁺ + e⁻

(1)

M-[Co(II)(bpy)₃]²⁺ + e⁻ ⇌ M-[Co(I)(bpy)₃]⁺  (M = BKC micelles)

(2)

2.2. cmc and Solubilization Constant Evaluation of BKC by CV using [Co(II)(bpy)₃]²⁺/[Co(III)(bpy)₃]³⁺ Redox Couple

In Figure 2, the peak current initially shows constant from 0.01 mM up to 0.48 mM, then there is a sudden drop in current when the concentration of BKC (c_BKC) is increased to 10 mM. A similar trend is followed at all scan rates studied. This sudden variation in i_pA1 with c_BKC may identify the aggregation of monomer amphiphiles forming higher oligomers of BKC, which in turn predicts the cmc values for BKC. The break point value ca. 0.48 mM is near to the cmc of BKC observed in water from surface tension measurements, viz. 0.35 mM [33], and from light scattering measurements, viz. 0.4 mM [34]. The light scattering study also reports a value 0.02 mM for the cmc of BKC in 0.04 M NaCl [34]. The present study almost correlates with cmc of the light scattering measurements.

According to Figure 3, there is almost no change in E_1/2 (difference in A1 and C1 peak potential) values with increasing BKC concentration up to 0.48 mM, but there is a positive E_1/2 value shift in subsequent concentrations from 1 to 4.8 mM of BKC (Figure 3 shows a definite E_1/2 value of 2.2 mM). The shifts in potential can be correlated directly with the relative extent of the solubilization of the redox components within the micelles [26,27,30,31] using Equation (3):

E_1/2,M-W^r = E_W^0’ + (RT/nF)ln(K_R/K_O)

(3)

The experimentally measured E_1/2M-W in the presence of BKC and E_1/2W without BKC difference is 25 mV (E_1/2M-W = 95 mV and E_w^0’ = 70 mV), which is giving rise to the partition coefficient ratio K_Co(II)/K_Co(III) for Co(III)/Co(II) redox couple to be 2.9. This implies a preferential partitioning of [Co(II)(bpy)₃]²⁺, than [Co(III)(bpy)₃]³⁺, into the BKC micellar phase. The larger K_Co(II) value for the association of complex than for its oxidized counterpart can be attributed to greater hydrophobic nature of the divalent complex, supported by the reduced charge on the cobalt center, which provides electrostatic advantage of the 2+ charge, since its electrostatic repulsion with the cationic micellar entity is smaller. The predominance of hydrophobic interactions has also been observed for [Co(III)(phen)₃]³⁺/[Co(II)(phen)₃]²⁺ couple by Davies and Hussam [30] in SDS micelles, which collectively tells that BKC solubilize also the cationic counterpart into its micelle.

2.3. Partition Coefficient and Association Constant by Diffusion Coefficient of the Co(II)/Co(III) Redox Couple

Figure 4 shows the variation of i_pA1 with v^1/2 for various c_BKC. The linearity is decreased with increasing the BKC concentration, but at low scan rate, the peak current value obey linearity with passes through the origin. The condition predicted by the Randle–Sevcik equation for a diffusion controlled electron transfer process, namely, peak current passing through the origin and also varying linearly with v^1/2 is strictly followed only in the absence of BKC, and at low concentrations of BKC, confirms that BKC allows C(III)/Co(II) via diffusion controlled electron transfer process. The diffusion coefficient of the micelle-solubilized Co(II) complex, D_M-W-2/3¸ was calculated from i_pa1 vs. v^1/2 slope using Equation (4), which is valid for an electrode process controlled by diffusion [32]:

$$\frac{\partial \left(i_{pc}\right)}{\partial \left(v^{1/2}\right)}=2.69\times {10}^5{n}^{3/2}{A}_{geo}{D}_{M-W2/3}{}^{1/2} c_{M-W-Co\left(II\right)}$$

(4)

where n is the number of electrons of the redox couple (n = 1 here); A_geo is the electrode area, and c_M-W-Co(II) is the bulk concentration of [Co(II)(bpy)₃]²⁺ in the presence of defined BKC concentration. D_M-W2/3 values slightly decreased with BKC concentration up to 1 mM, as predicted by the linear interaction theory [17], then sharply decreased in additional BKC concentration (Figure 5).

A decrease in diffusion coefficient value when the increasing BKC concentration associates with the larger micelle aggregates formation, which means solubilization of the Co(II)/Co(III) within the micellar region. In the present case of the preferably polar head-confined Co(II) complex-bound BKC micelles, their effective size could also increase due to the additional electrostatic repulsion-induced Co(II) among the adjacent surface sites acting along the surface periphery at the micelle head/solution boundary. These larger BKC micelles would allow the slower diffusion of Co(II) probe-solubilized aggregates with smaller diffusion coefficients. In addition, the diffusion coefficient value change with BKC concentration can be used to identify the cmc of BKC.

The measured diffusion coefficient of the micelle-bound Co(II) complex in the aqueous/micellar mixture, D_M-W2/3, is contributed by free complex and the bound complex. Thus, considering the partitioning of the complex between the free and the micelle bound forms, D_M-W2/3 is described by a concentration (mole fraction) weighted average of the two forms [35,36,37,38] for a partitioning of [Co(II)(bpy)₃]²⁺ between micelle bound (S_n.R) and free (R_W) form in the BKC micellar medium (S_n), which can be written as

$${\mathrm{R}}_{\mathrm{W}}+{\mathrm{S}}_{\mathrm{n}}\underset{{\mathrm{K}}_{\mathrm{out}}\cdot \mathrm{R}}{\overset{{\mathrm{K}}_{\mathrm{in}}\cdot \mathrm{R}}{{\displaystyle \rightleftharpoons }}}{\mathrm{S}}_{\mathrm{n}}\cdot \mathrm{R}$$

(5)

D_M-W2/3 = F_f⋅D_WR + F_b⋅D_BKC

(6)

Here, D_WR is the diffusion coefficient of free [Co(II)(bpy)₃]²⁺ measured in a 0.1 M Na₂SO₄ aqueous solution without surfactant. D_BKC is the diffusion coefficient of BKC micelle.

$${\mathrm{F}}_{\mathrm{f}}=\mathrm{mole}\mathrm{fraction}\mathrm{of}\mathrm{free}{\left[\mathrm{Co}\left(\mathrm{III}\right){\left(\mathrm{bpy}\right)}_3\right]}^{2+}=\frac{\left[{\mathrm{R}}_{\mathrm{W}}\right]}{\left[{\mathrm{R}}_{\mathrm{W}}\right]+\left[{\mathrm{S}}_{\mathrm{n}}\cdot \mathrm{R}\right]}$$

(7)

$${\mathrm{F}}_{\mathrm{b}}=\mathrm{mole}\mathrm{fraction}\mathrm{of}\mathrm{micelle}\mathrm{bound}{\left[\mathrm{Co}\left(\mathrm{II}\right){\left(\mathrm{bpy}\right)}_3\right]}^{2+}=\frac{\left[{\mathrm{S}}_{\mathrm{n}}\cdot \mathrm{R}\right]}{\left[{\mathrm{R}}_{\mathrm{W}}\right]+\left[{\mathrm{S}}_{\mathrm{n}}\cdot \mathrm{R}\right]}$$

(8)

Since F_f + F_b=1, Equation (6) can be expressed in terms of diffusion parameters as

F_f = (D_M-W2/3 − D_BKC)/(D_WR − D_BKC)

(9)

F_b = (D_M-W2/3 − D_WR)/(D_BKC − D_WR)

(10)

K_R = [S_n⋅R]/[R_W] = F_b/F_f = (D_M-W2/3 − D_WR)/ (D_BKC − D_M-W2/3)

(11)

Thus, Equations (9)–(11) are a method to calculate F_f, F_b, and K_R, respectively, from diffusion coefficient values. Mandal and Nair [39] have adopted this method to calculate the partition parameters and the mole fractions in the micellar phase for three redox forms of [Co(en)₃](ClO₄)₃ in CTAB and CPC (Cetylpyridinium chloride) micellar solution. By substituting experimentally measured diffusion coefficient values in Equations (7)–(9), the micellar solubilization constant, K_Co(II), fractions of solubilized and free probe molecules are calculated to be 3.8 × 10⁴ M⁻¹ using the values of D_WR = 3.42 × 10⁻⁶ cm² s⁻¹ and D_BKC = 0.39 × 10⁻⁶ cm² s⁻¹ [40]. The calculated thermodynamic parameters are presented in

Table 1. Experimentally measured diffusion coefficient of [Co(II)(bpy)₃]²⁺ complex in micellar solutions by CV (D_M-W2/3), mole fraction of micellar bound Co(II) complex (F_b), mole fraction of unbound Co(II) complex (F_f), and partition coefficient of Co(II) complex (K_Co(II)) in 0.1 M Na₂SO₄ containing different concentrations of BKC.

cBKC/mM	DM-W2/3/10−6 cm2 s−1	Fb a	Ff b	KCo(II) c
0.00	3.42	-	-	-
0.01	3.29	0.05	0.96	0.04
0.022	3.63	−0.07	1.07	−0.06
0.048	3.43	−0.04	1.00	−0.04
0.10	3.29	0.04	0.96	0.04
0.22	3.03	0.12	0.88	0.14
0.48	2.97	0.14	0.86	0.17
1.00	2.82	0.19	0.81	0.24
2.20	2.01	0.45	0.55	0.83

^a F_b = (D_M-W2/3 − D_WR)/(D_BKC − D_WR); ^b F_f = (D_BKC − D_M-W2/3)/(D_BKC − D_WR); ^c K_Co(II) = (D_WR − D_M-W2/3)/(D_M-W2/3 − D_BKC).

. The micelle solubilized fraction of Co(II) species is negligible up to 0.1 mM of BKC, and gradually increases up to 2.2 mM, where Co(II) is partitioned equally into the micellar phase. The partition coefficient, K_Co(II), increases with c_BKC and shows linear segment with slope (Figure 4) {K_Co(II)/ [BKC]} = 3.8 × 10² M⁻¹. The corresponding association constant is, therefore, 3.9 × 10⁴ M⁻¹ (K_Co(II) × n, n = 102 for BKC), which confirms the BKC allows the probe molecule (Co(II)) equally distributed in the micellar system.

2.4. BKC Effect on Catalysis by Product Analysis

As shown in Figure 1, curve b, the C2 and A2 redox couple in the presence of 0.1 M BKC was adopted to check the catalytic reduction of allyl chloride (a model VOC solution). The CV of [Co(II)(bpy)₃]²⁺ in different surfactant with various substrates proved its mediated catalytic reduction [19,23,24,25]. The product identification may lead to the effectiveness of BKC in catalytic reaction on allyl chloride. Figure 6 shows the current response during the bulk electrolysis of allyl chloride (5 mM), [Co(II)(bpy)₃]²⁺ (1 mM, the 5:1 ratio of allylchloride to cobalt enables to follow pseudo first order condition), and BKC (100 mM) in 0.1 M Na₂SO₄ solution. The current sharply increased and reached to 4 mA in 2 h and started to decrease a little, which indicates the reduction reaction was enabled without blocking and the reaction was completed in 2 h. Figure 7A’,B’ presents GC/MS data of the reduction products collected from a 3-h electrolyzed solution of allyl chloride. They show three GC peaks at three different rt (3.59, 4.28, and 6.12) (Figure 7A’) corresponding to the fragment ions at m/z = 82, 138, and 136 in their respective mass spectrum (Figure 7B’). The three compounds predicted by these fragment ions at the above m/z are likely to be 1,5-hexadiene (3) (m/z = 82), 1,4,8-nonatriene (4) (m/z = 138), and 1,8-nonadiene (5) (m/z = 136). The absence of an allyl chloride substrate GC peak (rt = 3.22) in the GC spectrum for the electrolyzed solution confirms the complete degradation of allyl chloride by the BKC micelle solubilized Co(I) mediation process. Scheme 1 presents the possible allyl chloride reduction mechanism based on the results of both CV and bulk electrolysis, accounting for these three reduction products.

As shown in Scheme 1, the mechanism for the formation of degradation products (3), (4), and (5) begins with heterogeneous electron transfer to convert the BKC micelle bound-Co(II)(bipyridine) precursor to the BKC-Co(I) complex catalyst in Reaction (1’), which is followed by a reaction with the substrate allyl chloride to form a {M-[Co(I)(bpy)₃]⁺∙∙∙ally chloride} complex according to Reaction (3). The Co(I) within the mediator–substrate complex could transfer an electron electrocatalytically to the adjacent allyl chloride to give the allyl radical (2) and a Cl⁻ ion and regenerate the precursor Co(II) species in a slow rate-determining step (Reaction (4’)). The radical (2) formed could dimerize immediately to form the GC/MS identified 1,5-hexadiene (3) (m/z = 82) (Reaction (5)). Alternatively, the radical (2) could react with the product, i.e., 1,5-hexadiene (3) in Reaction (6), to give an intermediate radical (6), which upon eliminating a hydrogen radical (Reaction (7)) forms a product, i.e., 1,4,8-nonatriene (4) (m/z = 136). The same intermediate radical (6), upon abstracting a hydrogen radical (reaction 8), affords the other GC/MS identified product, 1,8-nonadiene (5) (m/z = 138). Note that TDTAB (tetradecyltrimethlyammonium bromide) surfactant solubilized b-methyl allyl chloride found a chlorinated product [24] and TS-1 (Titanium silicate-1) in ethanol with H₂O₂ produced 99% epichlorohydrin [41], but the unusual and complete non-chlorinated products formation could be due the stabilization by the BKC micellar solution, evidenced by the high partition coefficient and diffusion coefficient, into its hydrophobic region. Also, the stabilization of chemicals in BKC is evidenced by slow drug delivery in antibacterial drugs [42]. The above ally chloride reduction mechanism satisfactorily accounts for the GC/MS-identified degradation products, and for the stoichiometry coefficient σ = 1, because one mole of mediator is needed to reduce 1 mole of the substrate. On the other hand, the source for OH^∙ radical (in Reaction (8)) could not be accounted for, and the reaction intermediates have not been identified or characterized independently.

3. Material and methods

3.1. Chemicals

Benzalkonium chloride was purified by repeated washing with anhydrous ether [43]. Allyl chloride was distilled twice (BP = 46 °C) prior to use. The Co(II)-tris-bipyridine perchlorate complex was prepared using the procedure reported elsewhere [44]. All other reagents and solvents were of AR grade and used as received.

3.2. Electrochemical Studies

The CV experiments were carried out using a Wenking potentiostat (ST 72, Gerhard Bank Elektroniks, Göttingen, Germany), a Wenking signal generator (VSG 83, Gerhard Bank Elektroniks, Göttingen, Germany) and a X-Y-t recorder (WX 2300, Graphtec, Tokyo, Japan) in combination. A conventional gas-tight three-electrode electrolysis cell was used with a glassy carbon disk, a Pt plate, and a dip type saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. Before each experiment, the GCE surface was polished using a fine mesh alumina on a velvet cloth followed by ultrasonic cleaning in double distilled water and washing with methanol. Charcoal-treated double distilled water was used to prepare all experimental solutions. Dry nitrogen was used to degas the solutions and the temperature was maintained at 25 ± 0.2 °C for all experiments.

Bulk electrolysis for the de-chlorination products was carried out using a large-sized carbon plate cathode in a divided cell containing 250 mL of a stirred electrolyte solution under a N₂ atmosphere with Pt as the anode at a constant potential that was 100 mV negative of the reduction peak of the catalyst (−1300 mV) in the presence of the substrate. After approximately 3-h electrolysis, the products from the catholyte were extracted into ether and analyzed using a Perkin-Elmer Auto System XL gas chromatograph with a Turbo Mass Spectrometer (EI.70 eV, Perkin-Elmer, Waltham, MA, USA). A 30 m × 0.25 mm × 1 μm capillary column (Perkin-Elmer Elite series PE-5, Perkin-Elmer, Waltham, MA, USA) with helium as the carrier gas at a flow rate of 1.0 mL min⁻¹ and an oven temperature programmed between 80 and 250 °C at a rate of 10 °C min⁻¹.

4. Conclusions

The paper reported that the benzyl headgroup-bearing BKC micelles exerted significantly different effects on the oxidation of [Co(II)(bpy)₃]²⁺ to [Co(III)(bpy)₃]³⁺ as well as on the effect of electrocatalysis by the BKC bound electrogenerated Co(I)(bpy) complex. The CV analysis with different BKC concentrations demonstrates the identification of cmc and diffusion coefficient of BKC surfactant. The presence of bulkier hydrophobic benzyl headgroups in BKC led to the preferred solubilization of the Co(II) complex (K_Co(II)/K_Co(III) = 2.9), a more positive onset potential for Co(II) oxidation to Co(III), and considerably larger micellar aggregates with decreasing D_M-W2/3 values with concentration. These studies clearly correlated the behavioral differences of the BKC surfactant on the different redox couples of the same mediator. The highly stable micelle-solubilized Co(I) mediator and the total de-chlorination of allyl chloride accompanied by three non-chloro organics as the only de-chlorination products in the BKC solution highlight the efficiency of the hydrophobic benzyl headgroup-bearing surfactant towards the de-chlorination process. This study also highlights the power of the CV technique as the diagnostic electrochemical tool in characterizing the micellar systems as well as in in electrocatalytic systems.