- Email: [email protected]
Aggregation of an ionic-liquid type hydrotrope 1-Butyl-3-methylimidazolium p-toluenesulfonate in aqueous solution
Colloids and Surfaces A 564 (2019) 95–100
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Aggregation of an ionic-liquid type hydrotrope 1-Butyl-3methylimidazolium p-toluenesulfonate in aqueous solution Tianxiang Yina,1, Yaling Chena,1, Weiguo Shena,b, a b
T
⁎
School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, China Department of Chemistry, LanzhouUniversity, Lanzhou, Gansu, 730000, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Hydrotrope Ionic liquid Aggregation behavior
In this work, the aggregation behavior of 1-butyl-3-methylimidazolium p-toluenesulfonate (BmimTOS), a new type of cationic hydrotrope, was investigated, where two distinct inflection points C1 and C2 were reflected by the change of conductivity, micro-polarity, surface tension. Therefore, it is speculated that small aggregate was formed in BmimTOS aqueous solution after C1 and structure change of these aggregates may be presented to form larger aggregates after C2. The formation of aggregate and the change of the aggregate size were further confirmed by dynamic light scattering (DLS) measurements. However, as suggested by micro-polarity measurements, these aggregates are not typical micelle-like ones. Furthermore, such two-step aggregation behavior was shown to be presented at a wide temperature region from 288.15 K to 318.15 K; however, the phenomenon cannot be observed when the alkyl chain length on imidazolium cation was changed into ethyl-, hexyl-, or octyl-.
1. Introduction Hydrotropes are substances that can significantly increase the solubility of hydrophobic solutes in water [1], whose molecular structure usually consists of both hydrophilic and hydrophobic parts [2,3]. Hydrotropes are widely used in many industrial fields like drug design, cleaning, cosmetics, etc [2–6]. Unlike surfactants, their hydrophobic
parts are much smaller, thus, they can’t form traditional micelle-like aggregates in aqueous solutions [2,3,7]. However, recent researches showed that the addition of third hydrophobic component to hydrotrope aqueous solution can lead to the formation of mesoscale domain, whose structure can change with the composition of hydrotrope [8–18]. This mesoscale structure has been named as “ultraflexible microesmulsion” [13,14] or “surfactant-free microemulsion” [15–18] by
Corresponding author at: School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, China. E-mail address: [email protected] (W. Shen). 1 These authors contributed equally as first authors. ⁎
https://doi.org/10.1016/j.colsurfa.2018.12.032 Received 13 August 2018; Received in revised form 10 December 2018; Accepted 14 December 2018 Available online 19 December 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
some researchers, which further broadens the application of hydrotropes. In addition to the traditional hydrotropes like sodium salts of short alkylbenzene sulfonates, short-chain alcohols, and some small organic molecules like urea [19–21], a recent study suggested that ionic liquid, like 1-alkyl-3-methylimidazolium salt, can act as a new type of catanionic hydrotropes [22], where both cationic and anionic ions can synergistically increase the solubility of poor water-soluble solutes. As a newly developed solvent, ionic liquids are easily designable and show good soluble ability for a wide range of substance, which would enormously enrich the hydrotrope types. It has been shown that the presence of aggregate in binary solution of hydrotrope may influence the solubilization mechanism of hydrophobic substance [12]. For ionic liquid consisting long alkyl chain, usually considered as surface active ionic liquid, can form various aggregates like traditional surfactant, for instance, imidazolium salt with long alkyl chain is such a typical type of surface active ionic liquid [23–30]. However, for those with short alkyl chain, the situation is complicated. For instance, some studies [31–33] showed that small and loose aggregates may be formed in 1-butyl-3methylimidazolium tetrafluoroborate, while no aggregate was detected in aqueous solution of 1-butyl-3-methylimidazolium bromide [32]. Therefore, in this work, we systematic investigated the aggregation behaviors of ionic liquid 1-butyl-3-methylimidazolium p-toluenesulfonate (BmimTOS) in aqueous solution, which was reported as a powerful cationic hydrotrope to enhance the solubility of hydrophobic substance gallic acid (solubility enhancement of 40-fold with BmimTOS being 2 mol.kg−1) [22], by means of fluorescence, conductivity, surface tension, resonance light scattering (RLS), and dynamic light scattering (DLS) measurements. The results reported herein suggested small aggregate is formed in aqueous solution of BmimTOS, which showed further structure change to form larger aggregate at high concentration.
solutions with various BmimTOS concentrations were prepared by the similar procedure described above. Each of the saturated solutions was diluted 400 times by mixing it with the corresponding BmimTOS aqueous solution. The absorbance of TB at the wavelength λ = 433 nm and 298.15 K for each of the BmimTOS solutions (A) or for the aqueous solution (A0) was measured by using a UV spectrometer (UV-2450, Japan) and the value of A/A0 was recorded to quantify the enhancement of TB solubility in the presence of BmimTOS. 2.2.2. Fluorescence measurements The steady-state fluorescence measurements were conducted in the fluorimeter supplied by Edinburgh Instrument (Model FLS 920), equipped with a 450 mW Xe arc lamp as a light source and a PMT detector (R928 P Hamamatsu) [34]. The sample was filled in a quartz cuvette with a path length of 1 cm and the temperature in the cuvette was controlled with a water-circulating bath with the stability of ± 0.1 K. Pyrene was used as the probe to investigate the micropolarity of the system characterized by the ratio (I1/I3) of its first peak (373 nm) to the third peak (383 nm) of the emission spectrum. Each of the samples for the micropolarity measurements was prepared by adding a certain volume of the stock solution of pyrene in ethanol into a volumetric flask followed by removing the solvent with a gentle flow of N2, and then transferring a certain volume of BmimTOS solution into the flask and diluting the solution with water to the required volume. The concentration of pyrene in each sample was about 5 × 10−7 M. All samples were stirred under the room temperature overnight. The sample was excited at 335 nm and the emission spectra were scanned from 350 nm to 500 nm with an interval of 1 nm at 298.15 ± 0.1 K. 2.2.3. Surface tension measurements The surface tensions of BmimTOS aqueous solutions at a series of concentrations were measured using the hanging platinum plate method by a tensionmeter supplied by Shanghai Hengping Instrument Company (Shanghai, China) with an accuracy of ± 0.1 mN·m−1 [34,35]. The temperature was controlled by a water circulating bath at 298.15 ± 0.1 K. The tensionmeter was calibrated with pure water, and the plate was washed and burned over flame before each measurement to ensure its cleanness. Typically, it took 0.5–1 h to reach equilibrium before the measurement.
2. Experimental section 2.1. Chemicals 1-alkyl-3-methylimidazolium p-toluenesulfonate (alkyl- being ethyl-, butyl-, hexyl-, and octyl-) (purity ≥ 99%) was purchased from ChengJie (Shanghai, China). The chemical structure of 1-butyl-3-methylimidazolium p-toluenesulfonate is shown in Scheme 1. Water used in this work was from a Leading Ultra-pure water system. All chemicals were used without further purification.
2.2.4. Conductivity measurements Conductivity measurements were performed with a digital conduct meter supplied by Leici Co. (Shanghai, China) using a titration method [34]. The conduct meter was initially calibrated by a standard KCl solution with the concentration of 0.01 mol.L−1. The concentrated BmimTOS aqueous solution was prepared and transferred into a cell, which was placed in a water bath with temperature being controlled at 298.15 ± 0.1 K. A certain amount of water was titrated into the cell by a micro syringe. The cell was shaken after each titration and kept undisturbed to reach thermal equilibrium before measurement. The uncertainty in conductivity measurement was estimated to be 2%.
2.2. Methods 2.2.1. Solubility measurements Water and excess amount of BmimTOS were weighed and mixed together, followed by rigorous stirring at 298.15 K for at least 24 h to reach equilibrium. The sample then was centrifuged at the same temperature for 20 min at 6000 rpm to separate the undissolved solid and obtain a BmimTOS saturated solution. The solid was dried and weighed for calculation of the solubility of BmimTOS in water. This measurement was repeated for three times and an average value of the solubility of BmimTOS in water at 298.15 K was determined to be 3.85 ± 0.05 M. In order to determine the enhancement of the thymol blue (TB) solubility in water due to addition of BmimTOS, a series of TB saturated
2.2.5. Resonance light scattering measurements The resonance light scattering (RLS) measurements were conducted by a fluorimeter (Model FLS 920) supplied by Edinburgh Instrument with a 450 mW Xe arc lamp as a light source and a PMT detector (R928 P Hamamatsu) [36]. In a typical measurement, the sample was filled in a quartz cuvette with a path length of 1 cm and the temperature in the cuvette was controlled by a water-circulating bath with a stability of ± 0.1 K and the RLS spectrum was recorded by synchronously scanning with λem=λex in the wavelength range from 200 nm to 700 nm. 2.2.6. Dynamic light scattering Hydrodynamic radius of the aggregates in aqueous solutions of BmimTOS were measured by the dynamic light scattering using
Scheme 1. Chemical structure of 1-butyl-3-methylimidazolium p-toluenesulfonate. 96
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 2. Plot of conductivity κ against concentration of BmimTOS; the inset c versus concentration, from which the inflect points are shows the plot of determined.
Fig. 1. Plot of I1/I3 against the concentration of BmimTOS.
instrument supplied by ALV (model ALV-CGS-5022 F) at a fixed angle of 90° and a laser of 633 nm wavelength. Measurements were taken in a batch mode at 298.15 K using a quartz cuvette with a path length of 1 cm. 3. Results and discussion The fluorescence probe method was widely used to detect the change of microscopic microenvironment in surfactant solution. Herein, using pyrene as a probe, we studied the change of ratio (I1/I3) with the concentration of BmimTOS, which was shown in Fig. 1. It is clearly shown that the value of I1/I3 decreases with the concentration of BmimTOS, suggesting the formation of hydrophobic micro-structure in aqueous solution of BmimTOS. However, two plateaus were presented with concentrations of inflect points being C1 = 0.64 M and C2 = 1.41 M, respectively, which are summarized in Table 1. Thus, we may speculate that only small aggregates form at low concentration and experience structure change to form larger and more compact aggregate at high BmimTOS concentration. However, it should be mentioned that the largest change of I1/I3 value in the studied concentration range was only about 0.2, much smaller than those in traditional surfactant solutions, which indicates that the aggregate is not a typical micelle-like structure. In order to shed more light on the detailed information of the structure changes at these two critical concentrations, conductivity and surface tension measurements were carried out and the results are shown in Figs. 2, 3, respectively. Two inflection points are clearly shown in Fig. 2 with the concentrations being C1 = 0.54 M and C2 = 1.13 M, which are in reasonable agreements with those determined from the fluorescence measurements. As shown in Fig. 2, the growth of conductivity with the BmimTOS concentration after C2 declines more obviously than that after C1, which is reflected by the ratio of slopes before and after the inflection point, i.e. 2.8 and 4.3 for C1 and C2. This may be due to that small aggregate formed after C1 thus still a great number of ions can move freely instead of being bound to the aggregate, while more ions take part in and bound to larger aggregate after C2. It should be
Fig. 3. Plot of surface tension σ against concentration of BmimTOS.
mentioned that the viscosity of the solution change slightly in the studied concentration range (from 1.4 cp to 4.8 cp), which contributes little to the change of conductivity. Different from that shown in Fig. 2, only one inflection point C1 = 0.57 M is suggested in Fig. 3, which reflects the formation of small aggregate. The structure change of aggregate occurs at C2 possibly makes negligible influence on the distribution of BmimTOS between the aggregate phase and the air-solution interface, hence does not result in the second inflect point on the plot of surface tension against concentration of BmimTOS. The aggregation and the structure change may further suggested from resonance light scattering measurements, which is an effective and sensitive way to detect aggregate in solution [37]. A typical spectrum of RLS of BmimTOS aqueous solution is shown in Fig. 4(a), where maximum scattering wavelength appears at 330 nm and the corresponding light intensity is recorded as IRLS. A plot of IRLS against the concentration of BmimTOS is presented in Fig. 4(b), indicating two
Table 1 Critical concentrations C1 and C2 obtained from different methods.
C1 /M C2 / M a
Fluorescence method
Conductivity
Surface tension
Resonance light scattering
Average Value a
0.64 ± 0.02 1.41 ± 0.02
0.54 ± 0.06 1.13 ± 0.04
0.57 ± 0.03 /
0.66 ± 0.08 1.06 ± 0.08
0.60 ± 0.05 1.22 ± 0.04
the average values are also shown in Figs. 1–4. 97
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 4. (a) Typical RLS spectrum of BmimTOS aqueous solution; (b) plot of IRLS against BmimTOS concentration.
mentioned that Coutinho et. al. didn’t find the presence of aggregate by dynamic light scattering in their work [22]. It may be due to that the concentration of BmimTOS used in their experiment was 0.64 mol.kg−1 which is in the close vicinity of the C1 value, thus the aggregates are hard to be detected. In order to shed deeper insight on the two-step aggregation process, the influence of temperature and the alkyl length on imidazolium cation were investigated by means of RLS measurements. The results at different temperatures and various alkyl lengths are shown in Figs. 6 and 7, respectively. It is indicated that the two-step aggregation process is presented in the temperature region studied herein, while such aggregation behavior is not observed in 1-alkyl-3-methylimidazolium p-toluenesulfonate with alkyl being ethyl-, hexyl- or octyl-. Finally, we determined the solubility enhancement of a model hydrotropic dye thymol blue (TB), quantified by A/A0, in BmimTOS aqueous solutions with different concentrations, which is shown in Fig. 8. It is clearly indicated that when the concentration of BmimTOS is above 0.7 M (close to the average value of C1), the solubility of TB increases rapidly till C = 1.8 M, where the maximal enhancement reaches 80-fold. This result suggests that the formation of aggregate greatly favors the solubilisation of hydrophobic TB. As demonstrated by Kunz et. al. that for a very hydrophobic solute, the presence of a prestructure in the aqueous solution of a hydrotrope may be propitious to the solubilisation of the hydrophobic solute [12].Our results further support this conclusion.
Fig. 5. Size distributions of BmimTOS aqueous solutions with concentrations being (a) 0.8 M and (b) 1.6 M.
peaks at C1 = 0.66 M and C2 = 1.06 M, which is close to those observed above. It is clearly shown in Fig. 4(b) that the first peak is much smaller than the second one, which is similar to those reported by Shi et. al [37]. for sodium dodecyl sulfate (SDS) aqueous solution that the peak corresponding to the formation of micelle is significantly larger than that of pre-micelle (small aggregate). In order to show the size of the aggregates in the BmimTOS solutions with concentrations being 0.8 M and 1.6 M, DLS measurements were conducted. The results of DLS measurements were shown in Fig. 5, which indicates that small aggregates are formed when BmimTOS concentration is above C1 and the size of the aggregates becomes a little larger as the concentration is above C2. However, it should be
4. Conclusion In this work, we systematically investigated the aggregation behavior of BmimTOS, a new type of cationic hydrotrope, by methods of fluorescence, conductivity, surface tension, resonance light scattering (RLS), and dynamic light scattering measurements. Small aggregate is
Fig. 6. Plots of IRLS against BmimTOS concentration at 288.15 K (a), 308.15 K (b), and 318.15 K (c). 98
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 7. Plots of IRLS against 1-alkyl-3-methylimidazolium p-toluenesulfonate concentration with alkyl being (a) ethyl-, (b) hexyl-, and (c) octyl- at 298.15 K. [6] S.E. Friberg, C. Brancewicz, O/W microemulsion and hydrotropes: the coupling action of hydrotropes, Langmuir 10 (1994) 2945–2949. [7] V. Srinivas, G.A. Rodley, K. Ravikumar, W.T. Robinson, M.M. Turnbull, D. Balasubramanian, Molecular organization in hydrotrope assemblies, Langmuir 13 (12) (1997) 3235–3239. [8] D. Subramanian, M.A. Anisimov, Phase behavior and mesoscale solubilization in aqueous solutions of hydrotropes, Fluid. Phase Equilib. 362 (2014) 170–176. [9] M. Sedlak, D. Rak, On the origin of mesoscale structures in aqueous solutions of tertiary butyl alcohol: The mystery resolved, J. Phys. Chem. B 118 (10) (2014) 2726–2737. [10] D. Subramanian, J.B. Klauda, P.J. Collings, M.A. Anisimov, Mesoscale phenomena in ternary solutions of tertiary butyl alcohol,Water, and propylene oxide, J. Phys. Chem. B 118 (20) (2014) 5994–6006. [11] A.E. Robertson, D.H. Phan, J.E. Macaluso, V.N. Kuryakov, E.V. Jouravleva, C.E. Bertrand, I.K. Yudin, M.A. Anisimov, Mesoscale solubilization and critical phenomena in binary and quasi binary solutions of hydrotropes, Fluid. Phase Equilib. 407 (2016) 243–254. [12] T. Buchecker, S. Krick, R. Winkler, I. Grillo, P. Bauduin, D. Touraud, A. Pfitzner, W. Kunz, The impact of the structuring of hydrotropes in Water on the mesoscale solubilisation of a Third hydrophobic component, Phys. Chem. Chem. Phys. 19 (3) (2017) 1806–1816. [13] T. Lopian, S. Schöttl, S. Prévost, S. Pellet-Rostaing, D. Horineek, W. Kunz, T. Zemb, Morphologies observed in ultraflexible microemulsions with and without the presence of a strong acid, ACS Cent. Sci. 2 (2016) 467–475. [14] S. Schöttl, D. Horinek, Aggregation in detergent-Free ternary mixtures with microemulsion-like properties, Curr. Opn. Colloid Interf. Sci. 22 (2016) 8–13. [15] J. Marcus, D. Touraud, S. Prévost, O. Diat, T. Zemb, W. Kunz, Influence of additives on the structure of surfactant-Free microemulsions, Phys. Chem. Chem. Phys. 17 (2015) 32528–32538. [16] V. Fisher, J. Marcus, D. Touraud, O. Diat, W. Kunz, Toward surfactant-Free and Water-Free microemulsions, J. Colloid Interf. Sci. 453 (2015) 186–193. [17] S. Schöttl, J. Marcus, O. Diat, D. Touraud, W. Kunz, T. Zemb, D. Horinek, Emergence of surfactant-ffree micelles from ternary solutions, Chem. Sci. 5 (2014) 2949–2954. [18] W.G. Hou, J. Xu, Surfactant-Free microemulsions, Curr. Opn. Colloid Interf. Sci. 25 (2016) 67–74. [19] M.H. Hatzopoulos, J. Eastoe, P.J. Dowding, S.E. Rogers, R. Heenan, R. Dyer, Are hydrotropes distinct from surfactants? Langmuir 27 (2011) 12346–12353. [20] J.J. Booth, M. Omar, S. Abbott, S. Shimizu, hydrotrope accumulation around the drug: the driving force for solubilization and minimum hydrotrope concentration for nicotinamide and urea, Phys. Chem. Chem. Phys. 17 (2015) 8028–8037. [21] B.K. Roy, S.P. Moulik, Effect of hydrotropes on solution behavior of amphiphiles, Curr. Sci. 85 (8) (2003) 1148–1155. [22] A.F.M. Cláudio, M.C. Neves, K. Shimizu, J.N.C. Lopes, M.G. Freire, J.A.P. Coutinho, The magic of aqueous solutions of ionic liquids: ionic liquids as a powerful class of catanionic hydrotropes, Green. Chem. 17 (7) (2015) 3948–3963. [23] R. Kamboj, S. Singh, A. Bhadani, H. Kataria, G. Kar, Gemini imidazolium surfactants: synthesis and their biophysiochemical study, Langmuir 28 (33) (2012) 11969–11978. [24] C. Cao, T. Huang, L. Zhang, F.P. Du, Interfacial rheological behavior of ionic liquid type imidazolium surfactant, Colloids Surf. A 436 (2013) 557–562. [25] A. Klee, S. Prevost, M. Gradzielski, Self-assembly of imidazolium-based surfactants in magnetic room-temperature ionic liquids: binary mixtures, Chemphyschem 15 (18) (2014) 4032–4041. [26] T.X. Yin, J.Q. Wu, S.Z. Wang, W.G. Shen, Structural rearrangement in aqueous solution of surface active ionic liquid 1-butyl-3-methylimidazolium bis(2-ethylhexyl)sulfosuccinate, Small 11 (23) (2015) 4717–4722. [27] A. Bhadani, T. Misono, S. Singh, K. Sakai, H. Sakai, M. Abe, Structural diversity, physicochemical properties and application of Imidazolium surfactants: recent advances, Adv. Colloid Interface Sci. 231 (2016) 36–58. [28] S.B. Wang, X.L. Yin, Y. Yan, Z.Y. Xiang, P. Liu, Y. Chen, X. Xin, Y.Z. Yang, Gold extraction through vesicles self-assembled by cationic gemini surfactant and sodium deoxycholate, Ind. Eng. Chem. Res. 55 (29) (2016) 8207–8214. [29] X.H. Zhao, D. An, Z.W. Ye, Adsorption and thermodynamic properties of dissymmetric gemini imidazolium surfactants with different spacer length, J. Disper. Sci. Technol. 38 (2) (2017) 296–302.
Fig. 8. Effect of BmimTOS concentration C on the solubility of thymol blue quantified by (A/A0) at 298.15 K (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
formed after C1, which results in the decrease of conductivity and the micropolarity of the microstructure where probe pyrene located. Besides, the surface tension decreased much slower thereafter. When the concentration of BmimTOS is above C2, the small aggregate may experience structure change and get together to form larger aggregate. Thus, the conductivity increased much slower and the micropolarity of the microstructure showed further decrease. To sum up, small aggregate was formed in BmimTOS aqueous solution at low concentration and structure change of these aggregates were presented to form larger aggregates at high concentration. However, the micro-polarity measurements suggested that these aggregates were not typical micelle-like ones. Besides, it is showed that the temperature effect on such two-step aggregation behavior is not significant in the region from 288.15 K to 318.15 K, while change of alkyl length on imidazolium cation diminished this phenomenon. Acknowledgement This work was supported by the National Natural Science Foundation of China (Projects 21,773,063). References [1] C. Neuberg, Hydrotropic phenomena. I, Biochem. Z. 76 (1916) 107–176. [2] J. Eastoe, M.H. Hatzopoulas, P.J. Dowding, Action of hydrotropes and alkyl-hydrotropes, Soft. Matt. 7 (13) (2011) 5917–5925. [3] W. Kunz, K. Holmberg, T. Zemb, Hydrotropes, Curr. Opn. Colloid Interf. Sci. 22 (2016) 99–107. [4] S.E. Friberg, R.V. Lochhead, I. Blute, T. Wärnheim, Hydrotropes performance chemicals, J. Disp. Sci. Tech. 25 (3) (2004) 243–251. [5] T.K. Hogdgon, E.W. Kaler, Hydrotropic solutions, Curr. Opn. Colloid Interf. Sci. 12 (3) (2007) 121–128.
99
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al. [30] P.P. Sun, S.S. Zhang, J.Y. Pang, Y.B. Tan, D. Sun, C.X. Xia, X.H. Cheng, X. Xin, Selfassembly of ionic liquid type imidazolium gemini surfactant with polyoxometalates into supramolecular architectures for photocatalytic degradation of dye, J. Mol. Liq. 272 (2018) 180–187. [31] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, R.K. Heenan, Aggregation behavior of aqueous solutions of ionic liquids, Langmuir 20 (2004) 2191–2198. [32] J.J. Wang, H.Y. Wang, S.L. Zhang, H.C. Zhang, Y. Zhao, Conductivities, volumes, fluorescence, and aggregation behavior of ionic liquids [C4mim][BF4] and [cnmim]Br (n=4,6,8,10,12) in aqueous solutions, J. Phys. Chem. B 111 (2007) 6181–6188. [33] S. Dorbritz, W. Ruth, U. Kragl, Investigation on aggregate formation of ionic liquids, Adv. Synth. Catal. 347 (2005) 1273–1279. [34] S.Z. Wang, T.X. Yin, W.G. Shen, Comparative investigations on mixing behaviors of
cationic gemini surfactant with surface active ionic liquid in Water and in ethylammonium nitrate, Ind. Eng. Chem. Res. 53 (2014) 18202–18208. [35] C.F. Du, D.X. Cai, M. Qin, P.Z. Zheng, Z.G. Hao, T.X. Yin, J.H. Zhao, W.G. Shen, Thermodynamics of mixed surfactant solutions of N,N’-bis(dimethyldodecyl) -1,2ethanediammonium dibromide with 1-dodecyl- 3-methylimidazolium bromide, J. Phys. Chem. B 118 (2014) 1168–1179. [36] T.X. Yin, M. Qin, Y. Yang, P.Z. Zheng, D.S. Fan, W.G. Shen, The interactions of lcarrageenan with cationic surfactants in aqueous solutions, Soft. Matt. 10 (2014) 4126–4136. [37] Y. Shi, H.Q. Luo, N.B. Li, Determination of the critical premicelle concentration, first critical micelle concentration and second critical micelle concentration of surfactants by resonance Rayleigh scattering method without any probe, Spect. Acta Part. A 78 (2011) 1403–1407.
100
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Aggregation of an ionic-liquid type hydrotrope 1-Butyl-3methylimidazolium p-toluenesulfonate in aqueous solution Tianxiang Yina,1, Yaling Chena,1, Weiguo Shena,b, a b
T
⁎
School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, China Department of Chemistry, LanzhouUniversity, Lanzhou, Gansu, 730000, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Hydrotrope Ionic liquid Aggregation behavior
In this work, the aggregation behavior of 1-butyl-3-methylimidazolium p-toluenesulfonate (BmimTOS), a new type of cationic hydrotrope, was investigated, where two distinct inflection points C1 and C2 were reflected by the change of conductivity, micro-polarity, surface tension. Therefore, it is speculated that small aggregate was formed in BmimTOS aqueous solution after C1 and structure change of these aggregates may be presented to form larger aggregates after C2. The formation of aggregate and the change of the aggregate size were further confirmed by dynamic light scattering (DLS) measurements. However, as suggested by micro-polarity measurements, these aggregates are not typical micelle-like ones. Furthermore, such two-step aggregation behavior was shown to be presented at a wide temperature region from 288.15 K to 318.15 K; however, the phenomenon cannot be observed when the alkyl chain length on imidazolium cation was changed into ethyl-, hexyl-, or octyl-.
1. Introduction Hydrotropes are substances that can significantly increase the solubility of hydrophobic solutes in water [1], whose molecular structure usually consists of both hydrophilic and hydrophobic parts [2,3]. Hydrotropes are widely used in many industrial fields like drug design, cleaning, cosmetics, etc [2–6]. Unlike surfactants, their hydrophobic
parts are much smaller, thus, they can’t form traditional micelle-like aggregates in aqueous solutions [2,3,7]. However, recent researches showed that the addition of third hydrophobic component to hydrotrope aqueous solution can lead to the formation of mesoscale domain, whose structure can change with the composition of hydrotrope [8–18]. This mesoscale structure has been named as “ultraflexible microesmulsion” [13,14] or “surfactant-free microemulsion” [15–18] by
Corresponding author at: School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, China. E-mail address: [email protected] (W. Shen). 1 These authors contributed equally as first authors. ⁎
https://doi.org/10.1016/j.colsurfa.2018.12.032 Received 13 August 2018; Received in revised form 10 December 2018; Accepted 14 December 2018 Available online 19 December 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
some researchers, which further broadens the application of hydrotropes. In addition to the traditional hydrotropes like sodium salts of short alkylbenzene sulfonates, short-chain alcohols, and some small organic molecules like urea [19–21], a recent study suggested that ionic liquid, like 1-alkyl-3-methylimidazolium salt, can act as a new type of catanionic hydrotropes [22], where both cationic and anionic ions can synergistically increase the solubility of poor water-soluble solutes. As a newly developed solvent, ionic liquids are easily designable and show good soluble ability for a wide range of substance, which would enormously enrich the hydrotrope types. It has been shown that the presence of aggregate in binary solution of hydrotrope may influence the solubilization mechanism of hydrophobic substance [12]. For ionic liquid consisting long alkyl chain, usually considered as surface active ionic liquid, can form various aggregates like traditional surfactant, for instance, imidazolium salt with long alkyl chain is such a typical type of surface active ionic liquid [23–30]. However, for those with short alkyl chain, the situation is complicated. For instance, some studies [31–33] showed that small and loose aggregates may be formed in 1-butyl-3methylimidazolium tetrafluoroborate, while no aggregate was detected in aqueous solution of 1-butyl-3-methylimidazolium bromide [32]. Therefore, in this work, we systematic investigated the aggregation behaviors of ionic liquid 1-butyl-3-methylimidazolium p-toluenesulfonate (BmimTOS) in aqueous solution, which was reported as a powerful cationic hydrotrope to enhance the solubility of hydrophobic substance gallic acid (solubility enhancement of 40-fold with BmimTOS being 2 mol.kg−1) [22], by means of fluorescence, conductivity, surface tension, resonance light scattering (RLS), and dynamic light scattering (DLS) measurements. The results reported herein suggested small aggregate is formed in aqueous solution of BmimTOS, which showed further structure change to form larger aggregate at high concentration.
solutions with various BmimTOS concentrations were prepared by the similar procedure described above. Each of the saturated solutions was diluted 400 times by mixing it with the corresponding BmimTOS aqueous solution. The absorbance of TB at the wavelength λ = 433 nm and 298.15 K for each of the BmimTOS solutions (A) or for the aqueous solution (A0) was measured by using a UV spectrometer (UV-2450, Japan) and the value of A/A0 was recorded to quantify the enhancement of TB solubility in the presence of BmimTOS. 2.2.2. Fluorescence measurements The steady-state fluorescence measurements were conducted in the fluorimeter supplied by Edinburgh Instrument (Model FLS 920), equipped with a 450 mW Xe arc lamp as a light source and a PMT detector (R928 P Hamamatsu) [34]. The sample was filled in a quartz cuvette with a path length of 1 cm and the temperature in the cuvette was controlled with a water-circulating bath with the stability of ± 0.1 K. Pyrene was used as the probe to investigate the micropolarity of the system characterized by the ratio (I1/I3) of its first peak (373 nm) to the third peak (383 nm) of the emission spectrum. Each of the samples for the micropolarity measurements was prepared by adding a certain volume of the stock solution of pyrene in ethanol into a volumetric flask followed by removing the solvent with a gentle flow of N2, and then transferring a certain volume of BmimTOS solution into the flask and diluting the solution with water to the required volume. The concentration of pyrene in each sample was about 5 × 10−7 M. All samples were stirred under the room temperature overnight. The sample was excited at 335 nm and the emission spectra were scanned from 350 nm to 500 nm with an interval of 1 nm at 298.15 ± 0.1 K. 2.2.3. Surface tension measurements The surface tensions of BmimTOS aqueous solutions at a series of concentrations were measured using the hanging platinum plate method by a tensionmeter supplied by Shanghai Hengping Instrument Company (Shanghai, China) with an accuracy of ± 0.1 mN·m−1 [34,35]. The temperature was controlled by a water circulating bath at 298.15 ± 0.1 K. The tensionmeter was calibrated with pure water, and the plate was washed and burned over flame before each measurement to ensure its cleanness. Typically, it took 0.5–1 h to reach equilibrium before the measurement.
2. Experimental section 2.1. Chemicals 1-alkyl-3-methylimidazolium p-toluenesulfonate (alkyl- being ethyl-, butyl-, hexyl-, and octyl-) (purity ≥ 99%) was purchased from ChengJie (Shanghai, China). The chemical structure of 1-butyl-3-methylimidazolium p-toluenesulfonate is shown in Scheme 1. Water used in this work was from a Leading Ultra-pure water system. All chemicals were used without further purification.
2.2.4. Conductivity measurements Conductivity measurements were performed with a digital conduct meter supplied by Leici Co. (Shanghai, China) using a titration method [34]. The conduct meter was initially calibrated by a standard KCl solution with the concentration of 0.01 mol.L−1. The concentrated BmimTOS aqueous solution was prepared and transferred into a cell, which was placed in a water bath with temperature being controlled at 298.15 ± 0.1 K. A certain amount of water was titrated into the cell by a micro syringe. The cell was shaken after each titration and kept undisturbed to reach thermal equilibrium before measurement. The uncertainty in conductivity measurement was estimated to be 2%.
2.2. Methods 2.2.1. Solubility measurements Water and excess amount of BmimTOS were weighed and mixed together, followed by rigorous stirring at 298.15 K for at least 24 h to reach equilibrium. The sample then was centrifuged at the same temperature for 20 min at 6000 rpm to separate the undissolved solid and obtain a BmimTOS saturated solution. The solid was dried and weighed for calculation of the solubility of BmimTOS in water. This measurement was repeated for three times and an average value of the solubility of BmimTOS in water at 298.15 K was determined to be 3.85 ± 0.05 M. In order to determine the enhancement of the thymol blue (TB) solubility in water due to addition of BmimTOS, a series of TB saturated
2.2.5. Resonance light scattering measurements The resonance light scattering (RLS) measurements were conducted by a fluorimeter (Model FLS 920) supplied by Edinburgh Instrument with a 450 mW Xe arc lamp as a light source and a PMT detector (R928 P Hamamatsu) [36]. In a typical measurement, the sample was filled in a quartz cuvette with a path length of 1 cm and the temperature in the cuvette was controlled by a water-circulating bath with a stability of ± 0.1 K and the RLS spectrum was recorded by synchronously scanning with λem=λex in the wavelength range from 200 nm to 700 nm. 2.2.6. Dynamic light scattering Hydrodynamic radius of the aggregates in aqueous solutions of BmimTOS were measured by the dynamic light scattering using
Scheme 1. Chemical structure of 1-butyl-3-methylimidazolium p-toluenesulfonate. 96
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 2. Plot of conductivity κ against concentration of BmimTOS; the inset c versus concentration, from which the inflect points are shows the plot of determined.
Fig. 1. Plot of I1/I3 against the concentration of BmimTOS.
instrument supplied by ALV (model ALV-CGS-5022 F) at a fixed angle of 90° and a laser of 633 nm wavelength. Measurements were taken in a batch mode at 298.15 K using a quartz cuvette with a path length of 1 cm. 3. Results and discussion The fluorescence probe method was widely used to detect the change of microscopic microenvironment in surfactant solution. Herein, using pyrene as a probe, we studied the change of ratio (I1/I3) with the concentration of BmimTOS, which was shown in Fig. 1. It is clearly shown that the value of I1/I3 decreases with the concentration of BmimTOS, suggesting the formation of hydrophobic micro-structure in aqueous solution of BmimTOS. However, two plateaus were presented with concentrations of inflect points being C1 = 0.64 M and C2 = 1.41 M, respectively, which are summarized in Table 1. Thus, we may speculate that only small aggregates form at low concentration and experience structure change to form larger and more compact aggregate at high BmimTOS concentration. However, it should be mentioned that the largest change of I1/I3 value in the studied concentration range was only about 0.2, much smaller than those in traditional surfactant solutions, which indicates that the aggregate is not a typical micelle-like structure. In order to shed more light on the detailed information of the structure changes at these two critical concentrations, conductivity and surface tension measurements were carried out and the results are shown in Figs. 2, 3, respectively. Two inflection points are clearly shown in Fig. 2 with the concentrations being C1 = 0.54 M and C2 = 1.13 M, which are in reasonable agreements with those determined from the fluorescence measurements. As shown in Fig. 2, the growth of conductivity with the BmimTOS concentration after C2 declines more obviously than that after C1, which is reflected by the ratio of slopes before and after the inflection point, i.e. 2.8 and 4.3 for C1 and C2. This may be due to that small aggregate formed after C1 thus still a great number of ions can move freely instead of being bound to the aggregate, while more ions take part in and bound to larger aggregate after C2. It should be
Fig. 3. Plot of surface tension σ against concentration of BmimTOS.
mentioned that the viscosity of the solution change slightly in the studied concentration range (from 1.4 cp to 4.8 cp), which contributes little to the change of conductivity. Different from that shown in Fig. 2, only one inflection point C1 = 0.57 M is suggested in Fig. 3, which reflects the formation of small aggregate. The structure change of aggregate occurs at C2 possibly makes negligible influence on the distribution of BmimTOS between the aggregate phase and the air-solution interface, hence does not result in the second inflect point on the plot of surface tension against concentration of BmimTOS. The aggregation and the structure change may further suggested from resonance light scattering measurements, which is an effective and sensitive way to detect aggregate in solution [37]. A typical spectrum of RLS of BmimTOS aqueous solution is shown in Fig. 4(a), where maximum scattering wavelength appears at 330 nm and the corresponding light intensity is recorded as IRLS. A plot of IRLS against the concentration of BmimTOS is presented in Fig. 4(b), indicating two
Table 1 Critical concentrations C1 and C2 obtained from different methods.
C1 /M C2 / M a
Fluorescence method
Conductivity
Surface tension
Resonance light scattering
Average Value a
0.64 ± 0.02 1.41 ± 0.02
0.54 ± 0.06 1.13 ± 0.04
0.57 ± 0.03 /
0.66 ± 0.08 1.06 ± 0.08
0.60 ± 0.05 1.22 ± 0.04
the average values are also shown in Figs. 1–4. 97
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 4. (a) Typical RLS spectrum of BmimTOS aqueous solution; (b) plot of IRLS against BmimTOS concentration.
mentioned that Coutinho et. al. didn’t find the presence of aggregate by dynamic light scattering in their work [22]. It may be due to that the concentration of BmimTOS used in their experiment was 0.64 mol.kg−1 which is in the close vicinity of the C1 value, thus the aggregates are hard to be detected. In order to shed deeper insight on the two-step aggregation process, the influence of temperature and the alkyl length on imidazolium cation were investigated by means of RLS measurements. The results at different temperatures and various alkyl lengths are shown in Figs. 6 and 7, respectively. It is indicated that the two-step aggregation process is presented in the temperature region studied herein, while such aggregation behavior is not observed in 1-alkyl-3-methylimidazolium p-toluenesulfonate with alkyl being ethyl-, hexyl- or octyl-. Finally, we determined the solubility enhancement of a model hydrotropic dye thymol blue (TB), quantified by A/A0, in BmimTOS aqueous solutions with different concentrations, which is shown in Fig. 8. It is clearly indicated that when the concentration of BmimTOS is above 0.7 M (close to the average value of C1), the solubility of TB increases rapidly till C = 1.8 M, where the maximal enhancement reaches 80-fold. This result suggests that the formation of aggregate greatly favors the solubilisation of hydrophobic TB. As demonstrated by Kunz et. al. that for a very hydrophobic solute, the presence of a prestructure in the aqueous solution of a hydrotrope may be propitious to the solubilisation of the hydrophobic solute [12].Our results further support this conclusion.
Fig. 5. Size distributions of BmimTOS aqueous solutions with concentrations being (a) 0.8 M and (b) 1.6 M.
peaks at C1 = 0.66 M and C2 = 1.06 M, which is close to those observed above. It is clearly shown in Fig. 4(b) that the first peak is much smaller than the second one, which is similar to those reported by Shi et. al [37]. for sodium dodecyl sulfate (SDS) aqueous solution that the peak corresponding to the formation of micelle is significantly larger than that of pre-micelle (small aggregate). In order to show the size of the aggregates in the BmimTOS solutions with concentrations being 0.8 M and 1.6 M, DLS measurements were conducted. The results of DLS measurements were shown in Fig. 5, which indicates that small aggregates are formed when BmimTOS concentration is above C1 and the size of the aggregates becomes a little larger as the concentration is above C2. However, it should be
4. Conclusion In this work, we systematically investigated the aggregation behavior of BmimTOS, a new type of cationic hydrotrope, by methods of fluorescence, conductivity, surface tension, resonance light scattering (RLS), and dynamic light scattering measurements. Small aggregate is
Fig. 6. Plots of IRLS against BmimTOS concentration at 288.15 K (a), 308.15 K (b), and 318.15 K (c). 98
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al.
Fig. 7. Plots of IRLS against 1-alkyl-3-methylimidazolium p-toluenesulfonate concentration with alkyl being (a) ethyl-, (b) hexyl-, and (c) octyl- at 298.15 K. [6] S.E. Friberg, C. Brancewicz, O/W microemulsion and hydrotropes: the coupling action of hydrotropes, Langmuir 10 (1994) 2945–2949. [7] V. Srinivas, G.A. Rodley, K. Ravikumar, W.T. Robinson, M.M. Turnbull, D. Balasubramanian, Molecular organization in hydrotrope assemblies, Langmuir 13 (12) (1997) 3235–3239. [8] D. Subramanian, M.A. Anisimov, Phase behavior and mesoscale solubilization in aqueous solutions of hydrotropes, Fluid. Phase Equilib. 362 (2014) 170–176. [9] M. Sedlak, D. Rak, On the origin of mesoscale structures in aqueous solutions of tertiary butyl alcohol: The mystery resolved, J. Phys. Chem. B 118 (10) (2014) 2726–2737. [10] D. Subramanian, J.B. Klauda, P.J. Collings, M.A. Anisimov, Mesoscale phenomena in ternary solutions of tertiary butyl alcohol,Water, and propylene oxide, J. Phys. Chem. B 118 (20) (2014) 5994–6006. [11] A.E. Robertson, D.H. Phan, J.E. Macaluso, V.N. Kuryakov, E.V. Jouravleva, C.E. Bertrand, I.K. Yudin, M.A. Anisimov, Mesoscale solubilization and critical phenomena in binary and quasi binary solutions of hydrotropes, Fluid. Phase Equilib. 407 (2016) 243–254. [12] T. Buchecker, S. Krick, R. Winkler, I. Grillo, P. Bauduin, D. Touraud, A. Pfitzner, W. Kunz, The impact of the structuring of hydrotropes in Water on the mesoscale solubilisation of a Third hydrophobic component, Phys. Chem. Chem. Phys. 19 (3) (2017) 1806–1816. [13] T. Lopian, S. Schöttl, S. Prévost, S. Pellet-Rostaing, D. Horineek, W. Kunz, T. Zemb, Morphologies observed in ultraflexible microemulsions with and without the presence of a strong acid, ACS Cent. Sci. 2 (2016) 467–475. [14] S. Schöttl, D. Horinek, Aggregation in detergent-Free ternary mixtures with microemulsion-like properties, Curr. Opn. Colloid Interf. Sci. 22 (2016) 8–13. [15] J. Marcus, D. Touraud, S. Prévost, O. Diat, T. Zemb, W. Kunz, Influence of additives on the structure of surfactant-Free microemulsions, Phys. Chem. Chem. Phys. 17 (2015) 32528–32538. [16] V. Fisher, J. Marcus, D. Touraud, O. Diat, W. Kunz, Toward surfactant-Free and Water-Free microemulsions, J. Colloid Interf. Sci. 453 (2015) 186–193. [17] S. Schöttl, J. Marcus, O. Diat, D. Touraud, W. Kunz, T. Zemb, D. Horinek, Emergence of surfactant-ffree micelles from ternary solutions, Chem. Sci. 5 (2014) 2949–2954. [18] W.G. Hou, J. Xu, Surfactant-Free microemulsions, Curr. Opn. Colloid Interf. Sci. 25 (2016) 67–74. [19] M.H. Hatzopoulos, J. Eastoe, P.J. Dowding, S.E. Rogers, R. Heenan, R. Dyer, Are hydrotropes distinct from surfactants? Langmuir 27 (2011) 12346–12353. [20] J.J. Booth, M. Omar, S. Abbott, S. Shimizu, hydrotrope accumulation around the drug: the driving force for solubilization and minimum hydrotrope concentration for nicotinamide and urea, Phys. Chem. Chem. Phys. 17 (2015) 8028–8037. [21] B.K. Roy, S.P. Moulik, Effect of hydrotropes on solution behavior of amphiphiles, Curr. Sci. 85 (8) (2003) 1148–1155. [22] A.F.M. Cláudio, M.C. Neves, K. Shimizu, J.N.C. Lopes, M.G. Freire, J.A.P. Coutinho, The magic of aqueous solutions of ionic liquids: ionic liquids as a powerful class of catanionic hydrotropes, Green. Chem. 17 (7) (2015) 3948–3963. [23] R. Kamboj, S. Singh, A. Bhadani, H. Kataria, G. Kar, Gemini imidazolium surfactants: synthesis and their biophysiochemical study, Langmuir 28 (33) (2012) 11969–11978. [24] C. Cao, T. Huang, L. Zhang, F.P. Du, Interfacial rheological behavior of ionic liquid type imidazolium surfactant, Colloids Surf. A 436 (2013) 557–562. [25] A. Klee, S. Prevost, M. Gradzielski, Self-assembly of imidazolium-based surfactants in magnetic room-temperature ionic liquids: binary mixtures, Chemphyschem 15 (18) (2014) 4032–4041. [26] T.X. Yin, J.Q. Wu, S.Z. Wang, W.G. Shen, Structural rearrangement in aqueous solution of surface active ionic liquid 1-butyl-3-methylimidazolium bis(2-ethylhexyl)sulfosuccinate, Small 11 (23) (2015) 4717–4722. [27] A. Bhadani, T. Misono, S. Singh, K. Sakai, H. Sakai, M. Abe, Structural diversity, physicochemical properties and application of Imidazolium surfactants: recent advances, Adv. Colloid Interface Sci. 231 (2016) 36–58. [28] S.B. Wang, X.L. Yin, Y. Yan, Z.Y. Xiang, P. Liu, Y. Chen, X. Xin, Y.Z. Yang, Gold extraction through vesicles self-assembled by cationic gemini surfactant and sodium deoxycholate, Ind. Eng. Chem. Res. 55 (29) (2016) 8207–8214. [29] X.H. Zhao, D. An, Z.W. Ye, Adsorption and thermodynamic properties of dissymmetric gemini imidazolium surfactants with different spacer length, J. Disper. Sci. Technol. 38 (2) (2017) 296–302.
Fig. 8. Effect of BmimTOS concentration C on the solubility of thymol blue quantified by (A/A0) at 298.15 K (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
formed after C1, which results in the decrease of conductivity and the micropolarity of the microstructure where probe pyrene located. Besides, the surface tension decreased much slower thereafter. When the concentration of BmimTOS is above C2, the small aggregate may experience structure change and get together to form larger aggregate. Thus, the conductivity increased much slower and the micropolarity of the microstructure showed further decrease. To sum up, small aggregate was formed in BmimTOS aqueous solution at low concentration and structure change of these aggregates were presented to form larger aggregates at high concentration. However, the micro-polarity measurements suggested that these aggregates were not typical micelle-like ones. Besides, it is showed that the temperature effect on such two-step aggregation behavior is not significant in the region from 288.15 K to 318.15 K, while change of alkyl length on imidazolium cation diminished this phenomenon. Acknowledgement This work was supported by the National Natural Science Foundation of China (Projects 21,773,063). References [1] C. Neuberg, Hydrotropic phenomena. I, Biochem. Z. 76 (1916) 107–176. [2] J. Eastoe, M.H. Hatzopoulas, P.J. Dowding, Action of hydrotropes and alkyl-hydrotropes, Soft. Matt. 7 (13) (2011) 5917–5925. [3] W. Kunz, K. Holmberg, T. Zemb, Hydrotropes, Curr. Opn. Colloid Interf. Sci. 22 (2016) 99–107. [4] S.E. Friberg, R.V. Lochhead, I. Blute, T. Wärnheim, Hydrotropes performance chemicals, J. Disp. Sci. Tech. 25 (3) (2004) 243–251. [5] T.K. Hogdgon, E.W. Kaler, Hydrotropic solutions, Curr. Opn. Colloid Interf. Sci. 12 (3) (2007) 121–128.
99
Colloids and Surfaces A 564 (2019) 95–100
T. Yin et al. [30] P.P. Sun, S.S. Zhang, J.Y. Pang, Y.B. Tan, D. Sun, C.X. Xia, X.H. Cheng, X. Xin, Selfassembly of ionic liquid type imidazolium gemini surfactant with polyoxometalates into supramolecular architectures for photocatalytic degradation of dye, J. Mol. Liq. 272 (2018) 180–187. [31] J. Bowers, C.P. Butts, P.J. Martin, M.C. Vergara-Gutierrez, R.K. Heenan, Aggregation behavior of aqueous solutions of ionic liquids, Langmuir 20 (2004) 2191–2198. [32] J.J. Wang, H.Y. Wang, S.L. Zhang, H.C. Zhang, Y. Zhao, Conductivities, volumes, fluorescence, and aggregation behavior of ionic liquids [C4mim][BF4] and [cnmim]Br (n=4,6,8,10,12) in aqueous solutions, J. Phys. Chem. B 111 (2007) 6181–6188. [33] S. Dorbritz, W. Ruth, U. Kragl, Investigation on aggregate formation of ionic liquids, Adv. Synth. Catal. 347 (2005) 1273–1279. [34] S.Z. Wang, T.X. Yin, W.G. Shen, Comparative investigations on mixing behaviors of
cationic gemini surfactant with surface active ionic liquid in Water and in ethylammonium nitrate, Ind. Eng. Chem. Res. 53 (2014) 18202–18208. [35] C.F. Du, D.X. Cai, M. Qin, P.Z. Zheng, Z.G. Hao, T.X. Yin, J.H. Zhao, W.G. Shen, Thermodynamics of mixed surfactant solutions of N,N’-bis(dimethyldodecyl) -1,2ethanediammonium dibromide with 1-dodecyl- 3-methylimidazolium bromide, J. Phys. Chem. B 118 (2014) 1168–1179. [36] T.X. Yin, M. Qin, Y. Yang, P.Z. Zheng, D.S. Fan, W.G. Shen, The interactions of lcarrageenan with cationic surfactants in aqueous solutions, Soft. Matt. 10 (2014) 4126–4136. [37] Y. Shi, H.Q. Luo, N.B. Li, Determination of the critical premicelle concentration, first critical micelle concentration and second critical micelle concentration of surfactants by resonance Rayleigh scattering method without any probe, Spect. Acta Part. A 78 (2011) 1403–1407.
100