Enhancing hexachlorocyclohexane solubility with surfactants and ionic liquids

Journal of Molecular Liquids 399 (2024) 124329 Available online 27 February 2024 0167-7322/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/bync/4.0/). Enhancing hexachlorocyclohexane solubility with surfactants and ionic liquids Zoe Chaos a , * , María Balseiro-Romero a , b , Beatriz Calvi˜ no-V´ azquez a , Alba Somoza c , Ana Soto c , Carmen Monterroso a a CRETUS (Cross-Research in Environmental Technologies), Dpto. Edafoloxía e Química Agrícola, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain b Instituto de Recursos Naturales y Agrobiología de Sevilla (IRNAS-CSIC), 41012 Sevilla, Spain c CRETUS (Cross-Research in Environmental Technologies). Dpto. Enxe˜ naría Química, Universidade de Santiago de Compostela, 15782, Santiago de Compostela, Spain ARTICLE INFO Keywords: Hexachlorocyclohexane Organic contaminants Solubilisation Surfactants Ionic liquids ABSTRACT Surfactant-enhanced remediation of soil can increase the solubility of hydrophobic organic contaminants in water and improve their removal. In this study, the effect of 24 traditional surfactants and surface-active ionic liquids (SAILs) on the solubility of hexachlorocyclohexane isomers ( α -, β-, γ-, δ-HCH) was evaluated in batch-type experiments. The combination of two different proportions of a cationic SAIL and an anionic surfactant was also assessed. The solubilisation capacity of the surfactant varied depending on its concentration and structure, but also the HCH isomer considered. In general, the anionic surfactants yielded the best results, increasing solubilisation of HCH up to 2.4 times compared to the control (pure water). α -HCH and γ-HCH isomers were more easily solubilised than β-HCH and δ-HCH regardless of the surfactant’s type. The main advantage of ionic liquids being their tunability, some clues are given for formulating surfactants to enhance the solubilisation of HCH. The combination of a traditional surfactant and a SAIL influenced the critical micelle concentration, but not the solubilisation of the contaminant. Further studies on the use of blends for this application are required. The increase of solubility achieved with the studied surfactants encourages their application in washing HCHcontaminated soils through solubility enhancement. 1. Introduction Hexachlorocyclohexanes (HCHs) are a group of synthetic organochlorine compounds that were used as broad-spectrum pesticides worldwide during the mid-20th century. Originally, they were commercialised as a mixture of isomers, but due to the exclusive insecticidal efficacy of γ-HCH, this later became the only component in such formulations, sold under the name of lindane [1]. However, the process of isolating γ-HCH resulted in substantial quantities of residual HCH isomers, primarily α - and β-HCH, which were disposed in the proximities of production facilities lacking adequate control measures. α -, β-, and γ-HCH isomers were included in the Stockholm Convention persistent organic pollutants (POPs) list in 2009 due to their high persistence and considerable toxicity, leading to their prohibition in most countries [2]. It is well-established that these isomers, along with δ-HCH, present carcinogenic, neurotoxic, immunotoxic, and hepatotoxic risks to both wildlife and human populations [3]. Despite severe regulations and bans, trace amounts of these detrimental isomers persistently contaminate soil, water, animal tissues and food products, showcasing the historical legacy of these POPs [4–7]. This pressing situation demands an immediate and efficient solution for mitigating HCHinduced environmental contamination. In this context, bioremediation is emerging as a promising and sustainable approach for the decontamination of soils with POPs [8], including HCHs [9]. Microbial degradation, biosorption microbes, and phytoremediation are tested methods to remove lindane from soil [10]. They are low-cost, environmentally-sound, and they can be used in situ. However, the inherently low bioavailability of HCH compounds usually reduces the efficiency of bioremediation processes, decreasing the success of these methods, which are also characterised by requiring long treatment times. Physico-chemical treatments are the suitable alternative. Thermal desorption has been tested both inand ex-situ [10], but the energy requirements and the need to treat the generated off-gas make them high-cost methods. In situ chemical oxidation uses * Corresponding author. E-mail address: [email protected] (Z. Chaos). Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq https://doi.org/10.1016/j.molliq.2024.124329 Received 17 January 2024; Received in revised form 19 February 2024; Accepted 22 February 2024 Journal of Molecular Liquids 399 (2024) 124329 2 expensive strong chemical oxidizers and can lead to secondary contamination of soil and groundwater. Those problems are also associated to solvent extraction with traditional organic solvents. Nanoremediation may offer a more cost-effective option for lindane contaminated soil, but the strong negative effect of nanoparticles in the environment and human health is a serious drawback in the application of the method. Soil washing (ex-situ) or flushing (in-situ) with aqueous solutions are promising alternatives [11,12]. Currently there is an increasing interest in the use of surfactants for remediation of contaminated soils [13,14], as key components of washing formulations. The concentration of the surface-active agent in the aqueous solution is usually very low. However, its cost and retention in the soil are key factors in the feasibility of the process. Surfactants are amphipathic molecules, able to form micelles in aqueous solutions [15]. Thus, surfactants can encapsulate hydrophobic contaminants, such as HCH compounds, within these micellar structures. This encapsulation enhances the solubility of the contaminants in water, facilitating their treatment after leaching. Both synthetic surfactants and biosurfactants have demonstrated their capacity to solubilise diverse hydrophobic organic contaminants [16,17]. Within this framework, surface-active ionic liquids (SAILs) are emerging as a promising alternative to traditional surfactants. SAILs are characterised by their low melting point (below 100 ◦C) and a strong tendency for micellization [18]. In addition, they have a wide liquidus range, low volatility, and a reduced risk of flammability. However, their most attractive attribute is likely their tunability, as anions and cations of SAILs can be easily tuned to prepare task-specific compounds [19]. The ability of surfactants to dissolve contaminants depends on two key factors: the nature of the pollutant itself and the chemical structure of the surface-active agent [20]. It has been proven that different types of these chemicals, such as cationic, anionic, and nonionic surfactants, possess varying abilities to dissolve contaminants [21]. Other authors have also explored how the alkyl chain length and the counterion (in the case of ionic surfactants) can influence the effectiveness of the surfactants used [22–24]. The hydrophilic-lipophilic balance (HLB) of surfactants is a widely used parameter to measure the affinity of the surfactants for oil or water. Surfactants having an HLB value greater than 9 are water soluble and form oil in water emulsions, the HLB required being dependent on the type of oil or contaminant [25]. Unfortunately, the determination of the HLB of SAILs is still an undeveloped topic. Surfactant adsorption in the soil, biodegradability, toxicity, and cost, are considerations that must be thoroughly assessed when selecting a surface-active compound in surfactant-enhanced soil washing applications. However, the first step is finding a suitable surfactant able to improve the solubilisation in water of the specific contaminant. Nonionic surfactants are usually preferred in singular systems [21]. Mixed systems are being studied for their potential to diminish surfactant adsorption, lower the critical micelle concentration (CMC), and improve solubilisation capacity of the individual compounds [26]. Research in surfactant-enhanced soil remediation has been predominantly focused on Polycyclic Aromatic Hydrocarbons (PAHs) (see e.g. [27]) and Non-Aqueous Phase Liquids (see e.g. [28]). Studies on HCH are relatively scarce and combine different techniques such as washing and biodegradation or electrochemical oxidation [29–32]. Similarly, investigation on the solubilisation of different isomers (not exclusively HCHs) is also very limited [33,34]. For this reason, it is imperative to investigate how the application of surface-active compounds influences the apparent solubility of diverse HCH isomers for remediation applications. The aim of this study is to evaluate the impact of 24 surfaceactive compounds (comprising traditional surfactants and SAILs) with varying chemical natures (cationic, anionic, and nonionic) on the solubility of HCHs in water. Additionally, the synergistic effect of mixing a cationic SAIL and an anionic surfactant is thoroughly investigated. This paper sheds light on HCHs solubilisation enhancement with surfaceactive compounds, crucial for applying surfactant-based technologies to soil remediation. 2. Materials and methods 2.1. Materials HCH isomers (Fig. 1) with a purity of 98.0 wt% for α -HCH, 97.5 wt% for β-HCH, 98.6 wt% for γ-HCH and 98.6 wt% for δ-HCH were obtained from Dr. Ehrenstorfer. Cationic and anionic traditional surfactants and SAILs used in this work are shown in Table 1. The abbreviated name, source, purity, structure, and CMC of each compound are also presented. Table 2 shows these data for the nonionic surfactants. In the case of these chemicals, the HLB is available and it is also presented in the table. Some SAILs were synthesised in the laboratory. [C 8 C 1 im][OAc] was synthesised via metathesis reaction by mixing [C 8 C 1 im]Cl and potassium acetate in 2-propanol for 48 h under stirring. Precipitated KCl was removed via filtration, and 2-propanol was eliminated using a rotary evaporator. The sample was dissolved several times in cold acetone and filtered until the KCl was completely eliminated. The chloride content was determined employing ionic chromatography. A final value of 0.009 mass fraction of chloride content was obtained. A similar procedure was applied to the synthesis of 2-hydroxyethylammonium-Alfoterra®. In this case, Alfoterra®145-4S 90 and 2-hydroxyethylammonium chloride were mixed to obtain the desired product. The final chloride content was 0.001 mass fraction. Choline dodecanoate was synthesised by dropwise addition of dodecanoic acid (1:1) to an aqueous solution of cholinium bicarbonate while stirring at ambient temperature and pressure. Water was then removed using a rotary evaporator. In all cases, synthesised products were purified under high vacuum to remove residual volatile compounds. In addition, two combinations of C1EG (cationic SAIL) and SDBS (anionic surfactant) were studied: Blend 1 (55.3 wt% C1EG and 44.7 wt% SDBS) and Blend 2 (26.3 wt% C1EG and 73.7 wt% SDBS). These mixtures were selected due to their excellent capacity to solubilise oil and their low adsorption on carbonate rocks [38]. 2.2. Methods The CMC of surfactants that were not previously characterised in the literature was determined according to the Wilhelmy plate method. The surface tension of a series of different solutions of the surfactant at varying concentrations was measured with a Krüss K11 tensiometer at Fig. 1. Chemical structure of HCH isomers. Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 3 Table 1 Cationic and anionic traditional surfactants and SAILs used in this study. Compound Abbreviation Source Purity (wt%) Structure CMC (mg/L) Cationic surface-active compounds 1-Octyl-3-methylimidazolium chloride [C 8 C 1 im]Cl SigmaAldrich >97 438.48 [35] 1-Decyl-3-methylimidazolium chloride [C 10 C 1 im]Cl SigmaAldrich 96 14805.08 [36] 1-Octyl-3-methylimidazolium acetate [C 8 C 1 im] [OAc] Synthesised 99 11153 1 1-Dodecylpyridinium chloride [C 12 py]Cl Merck >93 4334.85 [37] Tributyltetradecylphosphonium chloride [P 4 4 4 14 ]Cl Solvay >90 435 [38] Methyltrioctylammonium chloride [N 1 8 8 8 ]Cl SigmaAldrich >93 808.32 [39] Benzyldimethyldodecylammonium chloride [N 1 1 12 Bz ]Cl SigmaAldrich >99 2991.91 [40] Poly[oxy(methyl-1,2-ethandiyl)] alpha-[2-(diethylmethylammonium) ethyl]- omega-hydroxy chloride 221PG Iolitec >95 31170 1 Cocosalkylpentaethoximethylammonium methylsulfate C1EG Iolitec >95 88.12 [41] Ethylbis(hydroxyethyl)tallow alkylethoxylated ethylsulfate T2EG Iolitec >95 36.32 [42] Anionic surface-active compounds Sodium dodecylsulphate SDS SigmaAldrich >99 2306.96 [43] Sodium dodecylbenzenesulphonate SDBS SigmaAldrich >99 453.02 [44] (continued on next page) Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 4 298.15 K. The CMC results from the intersection between the regression straight line of the linearly dependent region and the straight line passing through the plateau (see Table S1 and Figure S1 of the Supplementary Information). The solubilisation capacity of HCH isomers was determined for each surface-active compound. To that aim, stock solutions of the traditional surfactants or SAILs were prepared in pure water at concentrations of 5 g/L and 15 g/L. Additionally, in the case of nonionic surfactants and due to their low CMCs, a concentration of 15 mg/L was also tested. Stock solutions of all the HCH isomers were prepared with a concentration of 5 g/L in acetone. The required quantities of surfactant and HCH solutions were mixed to obtain a final concentration of 50 mg/L for α -, γ-, and δ-HCH and 2 mg/L for β-HCH in surfactant solutions. These concentrations were approximately ten times higher than reported solubility limit in water [51]. In addition, a control solution (without surfactant) was also prepared. The weighing was carried out on a Mettler AE-100 analytical balance. All test and control solutions were agitated for 24 h at room temperature until equilibrium was attained. Subsequently, the aqueous phase was separated via centrifugation (HettichTM Zentrifugen EBA 21; 3000 rpm, g Force =704, 10 min). The dissolved HCH was then extracted using hexane (1:1, v/v) in an ultrasound bath (Ultrasons, J. P. Selecta, S. A.) for 20 min. It is noteworthy that this extraction method yielded recovery rates higher than 95 % for all isomers (data not shown). Following extraction, the organic phase was separated, and any remaining water was removed using anhydrous Na 2 SO 4 . The resulting extracts were subsequently stored at −4◦C until analysis. Pyrex® screw cap test tubes with PTFE lined caps were used throughout the experiments to prevent the adsorption of HCH isomers to the laboratory material. All the tests were carried out in triplicate. HCH extracts were quantified by gas chromatography-mass spectrometry (Model 450 GC-Model 220 MS Agilent Technologies). Chromatographic separation was carried out on a Trace Gold 5SILMS column (30 m x 0.25 mm x 0.25 μ m; Thermo Scientific) under the following operational conditions: temperature 80 ◦C (1.5 min), increased in 30 ◦C/ min to 180 ◦C (maintained for 1 min). Subsequently, temperature was increased by 3 ◦C/min to 200 ◦C and then 40 ◦C/min to 290 ◦C (maintained for 0.25 min). Helium was used as the carrier gas at a constant flow rate of 1 mL/min. The injector temperature was 280 ◦C. To carry out the experiment, 1 μ L of sample was injected in split/splitless mode, and the mass spectrometer was operated in tandem mode. The molecules were ionized by electron impact, and the temperature of the ion trap was adjusted to 220 ◦C. Calibration standards with a concentration ranging from 1 to 5000 μ g/L were prepared from 5 g/L stock solutions, in accordance with the concentration of each HCH isomer. Internal standards, maintained at a constant concentration of 250 μ g/L, consisted Table 1 (continued) Compound Abbreviation Source Purity (wt%) Structure CMC (mg/L) Choline dodecanoate [Ch][ODo] Synthesised 99 7432.64 [45] Sodium N-lauroylsarcosinate Sarkosyl SigmaAldrich >99 3725.93 [46] Sodium dioctylsulfoccinate AOT SigmaAldrich >97 1182.53 [47] Alfoterra®145-4S 90 Na-Alfoterra Sasol NA 25.22 1 2-hydroxyethylammonium-Alfoterra® N 0 0 0 2OH - Alfoterra Synthesised NA 12.15 1 Poly(ethylene glycol) 4-nonphenyl 3sulfopropyl ether potassium salt Ralufon SigmaAldrich NA 207.65 1 NA: Not available. 1 Obtained from surface-tension measurements (Supplementary information; Table S-1). Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 5 of a mixture of deuterated compounds, including 1,4-dichlorobenzened4, acenaphthene-d10, chrysene-d12, naphthalene-d8, perylene-d12, and phenanthrene-d10 (Internal Standards Mix 33, Dr. Ehrenstorfer). Phenanthrene-d10 was selected as the internal standard due to its similarity in retention time to that of the HCH isomers. Finally, statistical analysis of the results was carried out. Unifactorial ANOVA and a post hoc Bonferroni test (p <0.05) were used to determine any differences in the solubility of the HCH relative to the control, for each HCH isomer and for the sum of all isomers (ΣHCH). The analyses were conducted using IBM SPSS Statistics 25.0 software. 3. Results The effect of surface-active compounds at the different studied concentrations in the solubility of ΣHCH (total HCH, considering all the isomers) was assessed. Moreover, the effect on the individual isomers was also analysed. All the results presented in this section were normalised relative to the control. Thus, a value of 1 indicates a solubility equal to that in water and values greater than 1 indicate an increase in solubility. Results are expressed as mean values ±standard error (n = 3). The experimental solubilities of the HCH isomers in the control system followed the previously reported order of magnitude: δ >γ > α >β, and the values obtained are within the range of values determined by other authors (mean values of 10.87 >6.09 >2.68 >0.86 mg/L, respectively) [51,52]. 3.1. Cationic surface-active compounds The effect of cationic SAILs (all the compounds show melting or glass transition temperatures below 100 ◦C) in the solubility of ΣHCH is presented in Fig. 2. The degree of solubilisation varied depending on the structure and concentration of the surface-active compound used. According to Fig. 2, most compounds increased the solubility of ΣHCH in comparison to the control. However, in the case of [N 1 8 8 8 ]Cl and [C 8 C 1 im]Cl (both tested surfactant concentrations were higher than CMC), the presence of the surfactant had a negative effect on the solubilisation of the contaminant. Similarly, [C 8 C 1 im][OAc] also showed a negative effect, even using a surfactant concentration of 15 g/L (higher than its CMC). The effect of 221PG was negligible but this was expected due to the high CMC of this compound (see Table 1). In the case of [C 10 C 1 im]Cl, only the highest concentration of surfactant (with a value slightly higher than CMC) led to a positive effect. Regarding the surface-active compounds with positive effect, in general higher concentrations were more effective (the only exception being the case of [P 4 4 4 14 ]Cl). The HCH solubilisation capacity of C1EG and T2EG increased by 30–45 % when increasing the concentration from 5 to 15 g/L. This effect is much more pronounced in the case of [N 1 1 12 Bz ]Cl. A concentration of 5 g/L of this surfactant resulted in a minimal increase regarding to water solubilisation. However, a higher concentration of 15 g/L, resulted in a solubility two times higher compared to the control. Table 2 Nonionic surfactants employed in this study. Compound Abbreviation Source Purity (wt%) Structure CMC (mg/L) HLB 2 Polyoxyethylene (10) cetyl ether Brij C10 SigmaAldrich >97 1.37 [48] 12 Polyethylene dodecyl ether Brij L4 SigmaAldrich >99 0.333 [49] 9 Polyoxyethylene (5) nonylphenlyether Igepal CO520 SigmaAldrich NA 208.55 [50] 10 Ethylenediamine tetrakis (ethocylateblock-propxylate)tetrol Tetronic 90R4 SigmaAldrich >99.9 28.96 1 1–7 Polyoxyethylenesorbitan monooleate Tween 80 SigmaAldrich NA 22 [43] 15 Polyethyleneglyco tert-octylphenylether Triton X-100 SigmaAldrich NA 187.85 [43] 13.5 NA: Not available. 1 Obtained from surface-tension measurements (Supplementary Information, Table S1). 2 Obtained from manufacturer. Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 6 Fig. 2. Effect of cationic surface-active compounds on the water solubility of ΣHCH. Asterisks indicate significant differences relative to the control (*p <0.05 and **p <0.01). Fig. 3. Changes in the aqueous solubility of α -, β-, γand δ-HCH isomers in the presence of cationic surfactants at 5 and 15 g/L. Asterisks indicate significant differences relative to the control (*p <0.05 and **p <0.01). Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 7 The head group of the surfactants but mainly the hydrophobic tails and the functional groups that they contain are significant parameters in the ΣHCH solubilisation. The effect of alkyl chain length is likely the most important parameter, as it affects the shape and size of the micelles and aggregations formed. Regardless of the type of head group, the utilization of SAILs with shorter alkyl chain lengths such as [N 1 8 8 8 ]Cl, [C 8 C 1 im]Cl, or [C 8 C 1 im][OAC], negatively affected the aqueous solubility of ΣHCH. In contrast, the use of surface-active compounds with longer alkyl chains improved the results. This is easily seen comparing solubilisations achieved with [C 8 C 1 im]Cl and [C 10 C 1 im]Cl. Results achieved with this last SAIL improved those found with [C 12 py]Cl or [P 4 4 4 14 ]Cl, with a longer alkyl chain length, so in principle, an imidazolium ring would be preferred. However, the best results were achieved with ammonium SAILs such as T2EG and C1EG (relative solubilities of 1.8 and 1.9, at 15 g/L, respectively), both a mixture of tail-branched surfactants of different lengths, and mainly with [N 1 1 12 Bz ]Cl (relative solubility was 2 at 15 g/L). The presence of an aromatic ring attached to the head group seems to positively influence the solubilisation capacity of the ammonium surfactant. Lastly, no differences were observed when altering the nature of the counterion, as evidenced by the comparison between [C 8 C 1 im]Cl and [C 8 C 1 im][OAc]. The effect of cationic surface-active compounds on the solubility of the different HCH isomers is presented in Fig. 3. In general, the α -HCH and to a lesser extent γ-HCH isomers were the most readily solubilised, reaching maximum solubility values in the presence of [N 1 1 12 Bz ]Cl ( α -HCH: ca. 6 times the control and γ-HCH: ca. 3 times the control). β-HCH was, in general, less soluble than the α and γ isomers. It was only solubilised in presence of [N 1 1 12 Bz ]Cl, 221PG, and [C 10 C 1 im]Cl. Surfactants leading to good results in the solubilisation of ΣHCH such as T2EG and C1EG, however, had a negative or negligible effect on this specific isomer. Worse results were found with δ-HCH: decrease or slight increase in solubilities were found with all the surfactants. 3.2. Anionic surface-active compounds The effect of anionic surfactants (SDS, SDBS, and AOT) and SAILs ([Ch][Odo], Sarkosyl, Alfoterra, N 0 0 0 2OH -Alfoterra, and Ralufon) on the solubility of ΣHCH is presented in Fig. 4. As in the case of cationic surfactants, the degree of solubilisation varied depending on the structure and concentration of the surface-active compound used, being favoured by the highest concentration (in the case of Ralufon and NaAlfoterra, a negligible effect of concentration was observed). The solubility of ΣHCH was increased by the presence of all the tested anionic surfactants except for [Ch][ODo] at a concentration of 5 g/L, which is below its CMC. SDBS, AOT, [Ch][Odo], and Sarkosyl were the anionic surfactants that, at a concentration of 15 g/L most increased the solubility of ΣHCH, showing relative solubilities of 2.4, 2, 2.4 and 2.2, respectively. Not only the length of the hydrocarbon groups, but also the presence of functional groups as –COOor aromatic rings, seems to play an important role in the performance of the surfactants. For instance, performance of SDBS is clearly better than SDS. Both have the same alkyl chain length, but the latter has a phenyl ring attached to the hydrophilic group. Na-Alfoterra and N 0 0 0 2OH -Alfoterra led to similar results, so the counterion did not appear to affect the solubilisation potential of the compounds. The effect of anionic surface-active compounds on the solubility of the different HCH isomers is presented in Fig. 5. In general, and as in the case of cationic surfactants, the solubility of α - and γ-HCH showed the most significant improvement. The efficacy of SDBS, AOT, [Ch][Odo], and Sarkosyl solubilising ΣHCH particularly applies to δ-HCH, increasing its solubility with respect to water from 1.4 to 1.6 times (15 g/L of surfactant). The only surfactants able to significantly increase the β-HCH solubility were AOT and SDBS with relative solubilities of 2 and 4.6, respectively, at 15 g/L. 3.3. Nonionic surfactants The effect of nonionic surface-active compounds on the solubility of ΣHCH is presented in Fig. 6. In this case, and due to the low CMC of this kind of surfactants, besides 5 and 15 g/L, a concentration of 15 mg/L was also tested. Even though this concentration is higher than the CMC of Brij C10, Brij L4, and Triton X-100, no enhancement of ΣHCH solubility in water was observed at this low surfactant concentration. Fig. 6 shows that the intermediate concentration (5 g/L) led to the highest relative solubilities in all the cases except for Tetronic 90R4. Brij C10 and Triton X-100 yielded the best results, increasing ΣHCH solubility in 1.7 and 2.1 times, respectively, compared to the control solution. Thus, an intermediate HLB seems to be favourable for solubilisation enhancement. The HLB values are 12 for Brij C10 and 13.5 for Triton X-100 (Table 2). Brij C10 and Brij L4 are both nonionic ethoxylated alcohols, and again the longest hydrophobic tail of the former led to better results. The highest number of ethylene-oxide units of Triton X100 in comparison to Igepal CO-520, increases its hydrophilicity (consequently its HLB) and seems to be the reason for its better performance. The effect of nonionic surface-active compounds on the solubility of the different HCH isomers is presented in Fig. 7. As in the case of the previously studied surfactants, α - and γ-HCH were the most efficiently solubilised isomers. With an intermediate concentration of surfactant (5 g/L) all the surfactants, except Tetronic 90R4, showed positive relative solubilities. It is worth mentioning that this surfactant at the highest concentration (15 g/L) was able to improve the performance of water solubilisation with all the isomers but α -HCH. The good performance of Triton X-100, at 5 g/L, regarding ΣHCH is not only due to its capacity for solubilising α - and γ-HCH isomers (with relative solubilities of 5 and 2.2, respectively) but also βand δ-HCH (with relative solubilities of 2.2 and Fig. 4. Effect of anionic surface-active compounds on the water solubility of ΣHCH. Asterisks indicate significant differences relative to the control (*p <0.05 and **p <0.01). Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 8 1.4, respectively). Nonetheless, for the recalcitrant isomers, Brij C10 only showed a significative positive performance with β-HCH, showing a relative solubility of 1.7. 3.4. Synergistic effects of a cationic SAIL and an anionic surfactant Two blends of the SAIL C1EG and the traditional surfactant SDBS were selected to enhance HCH solubilisation. The selected concentrations were 55.3 and 26.3 wt% C1EG. It has been shown in a previous work [41] that these two blends, in salt solutions, were able to successfully extract oil in enhanced oil recovery processes. Moreover, the formation of electrostatically neutral complexes (catanionic micelles) was found the reason for a low adsorption in rocks, this being a promising factor also for application in soil washing. The solubilisation capacity of blends 1 (55.3 wt% C1EG) and 2 (26.3 wt% C1EG) is shown in Table 3. The results show that an increase of the concentration from 5 to 15 g/L did not improve the results of the solubilisation. The ion pairs formed by blending cationic and anionic species exhibit unique properties such as lower CMC than the individual components. CMC of blend 1 and 2 are 10.5 mg/L and 16.9 mg/L, respectively [41]. The lowest surfactant concentration tested clearly exceeded these values. At 5 g/L, Blend 2 exhibited superior solubilisation of ΣHCH compared to Blend 1, with the relative solubility achieved (1.5) similar to that obtained with SDBS (1.5) and greater than that obtained with C1EG (1.3). Blend 1 only enhanced the results of Blend 2 in the case of α -HCH, yielding a relative solubility of 5.3, which also markedly surpassed the results achieved with the individual components at the same concentration (2.6 with SDBS and 2.3 with C1EG). Blend 2 performed better for the rest of the isomers, but it only slightly improved the results achieved with the pure surfactants for γ-HCH, and it worsened those of the most recalcitrant isomers (βand δ-HCH). Nevertheless, overall, Fig. 5. Changes in the aqueous solubility of α -, β-, γand δ-HCH isomers in the presence of anionic surfactants at 5 and 15 g/L. Asterisks indicate significant differences relative to the control (*p <0.05 and **p <0.01). Z. Chaos et al. Journal of Molecular Liquids 399 (2024) 124329 9 using the highest concentration (15 g/L) of either SDBS or C1EG generally resulted in higher relative solubilities than those achieved with the blends. 4. Discussion The solubilisation efficacy of the investigated surface-active compounds depended on their concentration and structure, as well as the specific HCH isomer ( α -, β-, γor δ-HCH). Surfactant concentration was fixed in order to identify the highest solubilisation at a given concentration of chemical. Below the CMC, as expected, the solubility of HCH isomers was not enhanced. At concentrations higher than CMC, micelles aggregate. At low concentrations, limited or discrete self-assemblies are expected (spherical, prolate, or cylindrical). However, at higher concentrations of surfactant, the assemblies become more ordered, and liquid crystal phases are formed in some cases. Structural parameters such as head group, chain length, counterion, and surfactant concentration affect the size and shape of micellar systems, and therefore the micellar solubilisation capacity. This last is also affected by the hydrophobicity of the solubilised compound [53]. HCH isomers show different hydrophobicity [54], leading to a different solubilisation using the same surfactant and concentration. Interestingly, the solubility of HCH did not increase consistently with surfactant concentration. In the case of 221PG, tested concentrations were lower than its CMC, which is why the solubilisation achieved with this compound was negligible. This, in general, discounts this surfactant for the application because a quantity of chemical too high would be required. Ideally, surfactants should operate at low doses in washing solutions to minimise remediation costs, especially considering that the effective CMC in the soil–water system tends to be higher than in aqueous solutions [55]. In the case of [Ch][Odo], the minor concentration tested (5 g/L) was also lower than its CMC, reason why only a significative effect was achieved with 15 g/L. In general, both the cationic and anionic compounds tended to produce higher solubility at the highest concentration (15 g/L) due to a larger number of micelles and aggregates formed. However, among surfactants leading to positive relative solubilities, in the case of the anionic Na-Alfoterra and Ralufon, and in the case of the cationic [P 4 4 4 14 ]Cl and 221PG, no positive effect was observed by increasing the concentration. In the case of nonionic surfactants, a concentration of 5 g/L led to the best results (except in the case of Tetronic 90R4). These results may be attributed to the lower CMC of nonionic compounds. In fact, the same behaviour was found with the blends of SDBS and C1EG that exhibit very low CMCs. Micelles of surfactant compounds are continually forming in equilibrium with free monomers. Exceeding a certain concentration may disrupt this equilibrium, destabilizing the forming micelles [20]. Surfactant compounds exhibit their optimal solubilisation capacity within a specific concentration range, consistently higher than the CMC, and with a welldefined upper limit. Previous reports have shown that nonionic surfactants displayed a greater capacity than cationic and anionic ones to solubilise PAHs [56]. Masrat et al. [57] compared the capacity of nonionic, anionic and cationic surfactants to solubilise naphthalene and pyrene, observing that the nonionic compounds produced the best results, as also reported by Sales et al. [26]. In this work, in general anionic surfactants performed better for the solubilisation of HCH, a polyhalogenated six-carbon ring. The best relative solubilities achieved for ΣHCH were about 2.4 and they could be achieved with SDBS and Sarkosyl, but close values were also achieved with other anionic (AOT and [Ch][Odo]), cationic [N 1 1 12 Bz ], and nonionic (Triton X-100) surfactants. The chemical structure of the surface-active compounds influences the micellar behaviour and, consequently, the solubilisation capacity. In general, the longer the alkyl chain length the greater the HCH solubilisation. This is due to the increase of the size of the hydrophobic nucleus that holds the contaminant. Chun et al. [58] showed that the presence of aromatic rings in the alkyl chains of the surfactants favoured the solubilisation of PAHs. That effect was also observed in this study. Additionally, it was also found that the presence of certain functional groups in the hydrocarbon tails (such as –COOor ethylene-oxide units) could improve solubilisation performance. However, the role of the hydrophilic region in micellization and solubilisation of contaminants remains unclear. Some authors have found that an increase in the length of the polar chain affects the solubilisation capacity of compounds [59], while others have found no such effect [60]. Among cationic surfactants, in this work ammonium ones seem to have performed best. Regarding SAILs, Wang et al. [24] noted that the nature of the anion on imidazolium SAILs affected the anion-cation interactions and consequently its CMC and aggregation. In the present study, a significant effect of counterions was not found, even when the surface-active compounds showed different CMC. In the case of nonionic surfactants at 5 g/L, solubilisation capacity went through a maximum as surfactant HLB increased, with higher relative solubilities at HLB values in the range 12–13.5 (Brij C10 and Triton X-100). Diallo et al. [61] found a similar behaviour investigating the solubilisation of aromatic hydrocarbons and chloroethene compounds in micellar solutions of dodecyl alcohol ethoxylates. According to the authors, this is due to two opposing effects: a decrease in solubility with increasing HLB (reduction of hydrophobic nucleus) and the enhancement by electron donor–acceptor complexation. The synergistic effects of surfactants have been extensively investigated [62] due to their behaviour in soils. Anionic compounds can cause precipitation and cationic surfactants generally show high adsorption. These drawbacks can be surpassed by blending surfactants. Most studies have focused on PAHs, commonly employing a mixture of ionic and nonionic surfactants. While the results in such cases have been very positive, mixtures of ionic-type surfactants are also promising. Mixtures Fig. 6. Effect of nonionic surface-active compounds on the water solubility of ΣHCH. Asterisks indicate significant differences relative to the control (*p <0.05 and **p <0.01). Z. Chaos et al.