Aggregation of 1-alkyl-3-methylimidazolium octylsulphate ionic liquids and their interaction with Triton X-100 micelles

Abstract

Halogen-free surface active and biamphiphilic ionic liquids 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄, n = 4, 6, 8, 10) were synthesized and their aqueous solution behaviour was studied using surface tension, conductance, ¹H nuclear magnetic resonance (¹H-NMR), and small angle neutron scattering (SANS). These catanionic type surfactants demonstrate high surface activity. SANS data revealed that C₄mim C₈SO₄ and C₆mim C₈SO₄ form spherical and ellipsoidal micelles in water, while those with long chain imidazolium cations (C₈mim C₈SO₄ and C₁₀mim C₈SO₄) depict limited aqueous solubility and form vesicles. The effect of addition of these ionic liquids on the phase and aggregation behaviour of non-ionic surfactant Triton X-100 (TX-100) solution was investigated using cloud point (CP), ¹H-NMR, zeta potential and scattering (DLS and SANS) techniques. These C_nmim C₈SO₄ ionic liquids show remarkable changes in the CP of TX-100 aqueous solutions and micelle size depending on the alkyl chain of the cation and anion. The site of C_nmim C₈SO₄ in TX-100 micelles was investigated by NMR. The SANS data for the size/shape of aqueous TX-100 with C₄mim C₈SO₄ and C₆mim C₈SO₄ systems showed an ellipsoidal to smaller ellipsoidal micelle transition. Addition of C₈mim C₈SO₄ and C₁₀mim C₈SO₄ in TX-100 showed a micelle transition of ellipsoidal to smaller ellipsoidal at lower concentration and ellipsoidal to extended ellipsoidal at higher concentration, while addition of 100 mM C₁₀mim C₈SO₄ in TX-100 formed vesicle structures. The obtained results are discussed in terms of the formation of charged mixed micelles of TX-100 with C_nmim C₈SO₄ and TX-100 micelles desirably tailored by adding appropriate concentrations of ionic liquids.

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Introduction

Ionic liquids (ILs) have emerged as green solvents for organic synthesis/separation, catalysis and electrochemistry.^1,2 ILs are often derived from imidazolium, pyridinium, pyrrolidinium, morpholinium and phosphonium cations with several hydrophilic, hydrophobic and amphiphilic anions like halides (X⁻), tetrafluoroborate (BF₄⁻), hexaflurophosphate (PF₆⁻), trifluoromethanesulfonate (CF₃SO₃⁻), triflate (Tf⁻), octylsuphate (C₈SO₄⁻), dodecylsulphate (C₁₂SO₄⁻), benzoate (Bz⁻) etc. Chemical and physical properties of ILs can be tuned by suitable combination of an organic cation, inorganic or organic anions and substituents due to their amphiphilic and non-amphiphilic nature.^3–5 Therefore, ILs gained attention in chemical reactions, electrochemistry, as gas adsorbents, for catalytic and oil recovery processes and many other fields.^6–8

ILs with alkyl substituted heterocyclic cations with shorter alkyl chain (6) and hydrophilic anion behave as room temperature ionic liquids (RTIL), but with >C₆ chain in cations make them behave like cationic surface active ionic liquids (SAILs) and alkyl chain >C₆ in anion behave as anionic surface active ionic liquids. Interfacial activity and aggregation behavior of SAILs has been examined in water and the effect of different cations and anions is described in such studies.^9–19 The effect of electrolytes, nonelectrolytes and additives on surface activity and micelle formation of SAILs has been investigated from variety of techniques.^20–23

The influence of RTILs and SAILs on the interfacial and bulk properties of different types of surfactants has also been investigated by many researchers^24–28 employing physical methods, spectral and scattering techniques. Rao et al.²⁹ have observed micelle–vesicle–micelle transitions in the mixture of 1-butyl-3-methylimidazolium octylsulphate and 3-methyl-1-octylimidazolium chloride at different mole fractions. The solvent and rotational relaxation of coumarin-153 in 1-butyl-3-methylimidazolium octylsulphate micelle was investigated by Sarkar and coworkers.³⁰ They have also investigated temperature effect on the dynamics of photoinduced electron transfer between different coumarin dyes and N,N-dimethyl aniline in 1-butyl-3-methylimidazolium octylsulphate micelle.³¹

Biamphiphilic ionic liquids (BAILs) are halogen free ILs containing long alkyl chain on both cation and anion which in aqueous solution behave like catanionic surfactants and aggregate to form micelles, vesicles or other aggregates depending on the relative hydrophobicity of anion and cation. Most applicable and studied are the alkyl sulphates, alkane sulphonates and alkanoates. However, there exist only a few reports on surface activity and aggregation behavior of BAILs^32,33 in aqueous media. Structural changes and stability of cellulose and bovine serum albumin in aqueous BAIL solutions have been studied by Bharmoria et al.^34,35 The self-assembly of BAILs and synthesis of nano gold are studied by Rao et al.³² Complexation and dimerization of the dye methylene blue³⁶ and supramolecular assemblies of β-cyclodextrin³⁷ in aqueous BAILs have also been reported.

TX-100 is a polyoxyethylene based nonionic surfactant and is commonly used for biological application, household products and industrial formulations such as in paints, paper, metal working fluids, laundry, textile, oilfield chemicals, agrochemicals etc.³⁸ Micellar and phase behavior of TX-100 has been extensively examined.^39–43 Bahadur and coworkers have earlier reported the effect of various additives and surfactants on TX-100 micelles.^44–48 The aqueous solution behavior of nonionic surfactants can be conveniently modulated by the presence of ILs. Non-amphiphilic as well as amphiphilic ionic liquids have shown profound effect on micellar characteristic and phase behavior of nonionic surfactants such as TX-100 ^49–54 and PEO–PPO–PEO.^33,55–60 There are also reports on the micelles of TX-100 in ionic liquids.^61,62 Evans and coworkers have reported the aggregation behavior of TX-100 in ethyl ammonium nitrate (EAN).⁶³ The aggregate formation of TX-100 in ethylmethylimidazolium NTf₂ was observed on the basis of response of solvatochromic probes.⁶⁴ Rao et al.⁶² investigated solvation dynamics of EAN on micellar solution of TX-100 in Bmim PF₆. Gao and coworkers^65–69 have examined swollen micelle of Triton X-100/IL/solvent.

Considering the literature survey and to the best of our knowledge, we believe that there is no report on the effect of such ionic liquids on solution behavior of TX-100. We have investigated the solution behavior of 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄) based SAILs (n = 4, 6) and BAILs (n = 8, 10) in aqueous media and their modulating effect on TX-100 micelle using array of techniques.

Experimental

Material

Triton X-100 (TX-100), sodium octylsulfate (Na C₈SO₄), 1-butyl-3-methylimidazolium chloride (C₄mim Cl), 1-hexyl-3-methylimidazolium chloride (C₆mim Cl), 1-methyl-3-octylimidazolium chloride (C₈mim Cl) and 1-decyl-3-methylimidazolium chloride (C₁₀mim Cl) were purchased from Sigma Aldrich and used without further purification. ILs based on 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄, n = 4, 6, 8, 10) were synthesized using reported method. Deionized water from a Millipore Milli-Q system was used to prepare all aqueous solutions. Solutions for SANS and NMR measurements were prepared in D₂O.

Synthesis of 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄)

C_nmim C₈SO₄ were synthesized by metathesis and purified as per the reported procedures.^32,36,70 This involved equimolar mixing of 1-alkyl-3-methylimidazolium chlorides (C_nmim Cl) and sodium octylsulphate (Na–C₈H₁₇OSO₃) dissolved in water and mixed with dichloromethane (CH₂Cl₂) in a round bottom flask kept stirring for about 5–6 hours. The product was filtered and washed several times with distilled water to make product chlorine free (checked with acidic AgNO₃ solution). Solvent was evaporated under vacuum and solvent free product obtained which were dried at 70 °C. The purity of the products was ascertained by ¹H NMR measurements.

Methods

Surface tension and electrical conductance. Surface tensiometer (Kruss Model K9, Germany) was employed for the surface tension of aqueous solution of SAILs measured at 30 °C. The measurements were repeated twice-thrice to ascertain the reproducibility. Calibration of instrument was checked using the surface tension of Milli-Q water (ST = 71.4 ± 0.1 mN m⁻¹).

Conductivity measurements were carried out using a digital conductometer EUTECH, PC 2700 at 30 °C maintained by LEQUITRON stirred water bath. Calibration was done using standard KCl solutions. All glassware and probe were carefully cleaned before use.

Cloud point (CP). The cloud points were determined for aqueous 5% TX-100 solution with different concentration of ionic liquids by gently heating in thin glass tubes immersed in a water containing beaker under constant stirring by using magnetic stirrer equipped with hot plate (heating rate 1 °C min⁻¹). The CP of solutions was measured visually by noting the turbid solution at certain temperature. Measurements were repeated trice and were reproducible up to ±0.5 °C.

Dynamic light scattering (DLS) and zeta potential. Malvern ZetasizerNano-ZS 4800 apparatus with He–Ne laser light source at a wavelength λ = 633 nm was used to measure the apparent hydrodynamic diameter (D_h) of the micelles. Each measurement was repeated 3–4 times and average size was considered. An uncertainty in size is within ±0.3 nm. The detailed procedure is similar to reported earlier.⁴⁷

The zeta potential of C_nmim C₈SO₄ mixture with TX-100 were performed using a polystyrene zetasizer cell. The cell was filled with ∼1 mL of the sample solution and placed inside after removing bubbles by gently tapping. Zeta potential of systems was obtained by electrophoretic mobility using Henry equation.

Nuclear magnetic resonance (NMR). ¹H NMR experiments were performed on a Bruker, Avance II 500 MHz spectrometer at 30 °C at CSMCRI, Bhavnagar India. Samples were prepared in D₂O.

Small angle neutron scattering (SANS). The SANS measurements were carried out for structural analysis of ILs and their mixtures with 5% TX-100 solution at Dhruva reactor, Bhabha Atomic Research Center (BARC), Mumbai, India. The sample solutions were prepared in D₂O and 5 mm thick quartz cuvette with Teflon cap was used as sample holder and temperature was maintained 30 °C for all measurements. The scattering data were measured in the range of wave vector transfer Q 0.017 to 0.35 Å⁻¹ (Q = 4π sin(θ/2)/λ, where θ is the scattering angle and λ is the incident neutron wavelength). The scattering intensities of neutrons were corrected using standard protocols for the background, empty cell contribution, sample transmission and normalized to absolute cross-sectional. The differential scattering cross section, dΣ/dΩ per unit volume of solution for micelles is expressed bywhere, n denotes the number density of particles, V is volume fraction, (ρ_P − ρ_S)² is composition (ρ_P and ρ_S are the scattering length densities of particle and solvent, respectively), P(Q) is the intra-particle structure factor depends on shape and size of the particles, S(Q) is the inter-particle structure factor depends on ordering particles and inter-particle interaction. B is a constant term representing incoherent background, which is mainly due to the hydrogen present in the sample.

The SASFIT program was used for analysis of SANS data which was developed by Kohlbrecher and Bressler.⁷¹

Results and discussion

Aqueous solution behaviour of 1-alkyl-3-methylimidazolium octylsulphates

The aqueous solutions behavior of 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄, n = 4, 6, 8, 10) were examined by surface tension, conductance, NMR and SANS. The CMCs of SAILs C_nmim C₈SO₄ (n = 4, 6) obtained from surface tension and electrical conductance are shown in Fig. 1 and values are reported in Table 1 including comparison with data noted in literature. The degree of counterion dissociation (α) is calculated by the ratio of two straight lines intersecting at different slopes as obtained by conductance and area/molecule values obtained by surface tension plots are shown in Table 1. The CMC values of SAILs reduce with increasing alkyl chain of imidazolium cations as assisted by increasing hydrophobicity. The decrease in counterion dissociation confirms that anion and cation come closer to each other. However, BAILs C_nmim C₈SO₄ (n = 8, 10) form little turbid solution due to limited aqueous solubility. Kumar and coworkers^32,72 have studied aqueous solution behavior of C₄mim C₈SO₄ and observed that octylsulphate anions form aggregate and imidazolium cation behave as counterion. They have also reported the values of CACs (mM) for C_nmim C₈SO₄ (n = 4, 6, 8) were 34.9, 14.2 and 4.1 as calculated by surface tensiometry and 35.8, 22.2 and 5.6 according to conductivity measurement in aqueous solution. The degree of counterion binding increases by increase in the alkyl chain due to enhanced hydrophobicity.

Fig. 1 (a) Surface tension (b) specific conductance versus concentration plots of C₄mim C₈SO₄ (), C₆mim C₈SO₄ () at 30 °C.

Table 1 CMCs, surface tension at CMC (γ_CMC), area/molecule (A_min), degree of counterion dissociation (α) and SANS parameters for C_nmim C₈SO₄ at 30 °C

ILs	ST			Cond.		SANS
	CMC, mM	γCMC	Amin, Å^2	CMC, mM	α	Size, Å	Nagg
a CMClit 31 mM,^73 35 mM,^74 (34.9, 35.8),^32 R = radius, t = thickness.
C4mim C8SO4^a	31.9	32.8	69	32.4	0.31	R = 13.5	27
C6mim C8SO4	18.6	30.3	66	19.5	0.22	a = 41.8, b = c = 13.8	77
C8mim C8SO4	Vesicle					R = 220, t = 21	—
C10mim C8SO4	Vesicle					R = 230, t = 21	—

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To evaluate the aggregate size and shape of C_nmim C₈SO₄ in water, SANS measurements were carried out for 50 mM solutions and are shown in Fig. 2. The results were analyzed using SANS model and obtained parameters are reported in Table 1. The scattering intensity increases with increase in the hydrophobicity of imidazolium cation. C₄mim C₈SO₄ shows very low scattering and suggests low aggregation and micellar density at measured concentration, moderate increase in scattering was noticed for C₆mim C₈SO₄, C₈mim C₈SO₄ and C₁₀mim C₈SO₄. The SANS parameters (Table 1) show that C₄mim C₈SO₄ forms spherical micelle in aqueous solution with 13.5 Å radius while C₆mim C₈SO₄ depicts ellipsoidal micelle in aqueous solution with higher aggregation number. Strong scattering in the low Q-region is observed for C₈mim C₈SO₄ and C₁₀mim C₈SO₄, clearly depict the micellar growth. The measured slope value on log–log scale is near to −1 indicating formation of vesicular structures. Formation of vesicles in BAILs is also confirmed by NMR.

Fig. 2 SANS profiles for C₄mim C₈SO₄ (), C₆mim C₈SO₄ (), C₈mim C₈SO₄ () and C₁₀mim C₈SO₄ () at 30 °C.

¹H NMR spectra were recorded for 50 mM aqueous solution of ILs (Fig. 3). The protons of the cation and anion of ILs are labeled as C and A, as demonstrated in Scheme 1. For C₄mim C₈SO₄ and C₆mim C₈SO₄, down field chemical shift is observed in the region at ∼7.5 and ∼7.4 ppm for imidazolium ring protons C₄ and C₅, respectively. Methyl (C₆) and methylene (C₇) protons attached to imidazlolium ring show resonance at ∼3.9 and ∼4.1 ppm, respectively, while alkyl chain (C₈, C₉ and C₁₀) attached to imidazolium ring resonates at ∼1.8, ∼1.2 and ∼0.8 ppm, respectively. The methylene proton attached to anionic sulphate group (A₁) gives resonance at ∼4.0 ppm. Other alkyl chain protons of octylsulphate anion (A₂, A₃ and A₄) show chemical shifts in upfield region at ∼1.9, ∼1.2 and ∼0.8, respectively. The anionic/cationic alkyl chain (–CH₂–) protons (A₃ and C₉) give resonance peak at the same ppm value at ∼1.2 and give broad split peak. Terminal proton of –CH₃ group of both chains (A₄, C₁₀) also give chemical shift in same region of the spectrum. The peak broadening was observed with increase in alkyl chain of imidazolium cation from 6C to 8 and 10C, which demonstrates the formation of vesicle.³²

Fig. 3 ¹H NMR profiles for C₄mim C₈SO₄ (a), C₆mim C₈SO₄ (b), C₈mim C₈SO₄ (c) and C₁₀mim C₈SO₄ (d) in D₂O at 30 °C.

Scheme 1 Chemical structures and proton labeling of TX-100 and ILs.

Effect of C_nmim C₈SO₄ on TX-100 micelle

The solution behaviour of TX-100 in aqueous and saline media are well reported in our earlier papers.^48,54 The effect of C_nmim C₈SO₄ on aqueous solution behavior of TX-100 are systematically examined using cloud point, DLS, zeta potential, SANS and NMR techniques.

The cloud points of 5% TX-100 solution in the presence of C_nmim C₈SO₄ are shown in Fig. 4a. The CP of 5% TX-100 aqueous solution is 65 °C, which gradually increases on addition of SAILs (C₄mim C₈SO₄, C₆mim C₈SO₄). Interestingly unusual behavior is observed in case of BAILs (C₈mim C₈SO₄, C₁₀mim C₈SO₄), where the CP initially increases then remains almost constant up to a certain value and finally decreases. In presence of SAILs, octylsulphate anion (C₈SO₄⁻) is likely to penetrate in TX-100 micelles because of sufficient long hydrophobic alkyl chain and replace existing TX-100 monomers to form negatively charged mixed micelle similar to anionic surfactants. Consequently, with increase two carbons in alkyl chain of imidazolium cation (C₆mim C₈SO₄), CP of TX-100-C₆mim C₈SO₄ system are marginally lower compared to C₄mim C₈SO₄ because C₆mim⁺ resides in closer proximity to C₈SO₄⁻ causing screening effect and reduction in electrostatic repulsions among negatively charged head groups. However, in the presence of BAILs both surface active anion and cation incorporate into TX-100 micelle. Initially, added low concentrations of C₈mim C₈SO₄ and C₁₀mim C₈SO₄ into TX-100 micelle causes slight increase the CP of TX-100 due to imidazolium cation and octylsulphate anion penetrating into TX-100 micelle by replacing TX-100 monomers and form charged mixed micelle with marginally smaller size. Another possible reason for increase in CP is hydrogen bonding between the H-atom attached to the imidazolium ring (C₂) and O-atom of the oxyethylene chain of TX-100. With an increase in the concentration of BAILs, more number of octylsulphate anion and octyl/decyl chained imidazolium cations penetrate in TX-100 micelle, which induce electrostatic interactions between oppositely charged head groups and allows swelling of mixed micelles and reduced the CP of TX-100. Above 80 mM concentration of BAILs, more and more anion and cation remain in micelle with few Triton X-100 molecules (as shown in Scheme 3) and the CP of 5% TX-100 becomes less than 65 °C. We also believe that imidazolium cations interact with TX-100 monomers via π–π interactions between the phenyl ring and imidazolium ring,^50,54 while octylsulphate anions interact with TX-100 monomers through ion–dipole interaction (Scheme 2). The obtained results of CP are further supported by DLS, NMR and SANS. The effect of alkyl chain in increasing the cloud point for aqueous 5% TX-100 solution follows the order C₄ > C₆ > C₈ > C₁₀ for C_nmim C₈SO₄.

Fig. 4 (a) Cloud point (b) hydrodynamic diameter (D_h) of 5% TX-100 in presence of varying concentration of C₄mim C₈SO₄ (), C₆mim C₈SO₄ (), C₈mim C₈SO₄ () and C₁₀mim C₈SO₄ ().

Scheme 2 Different types of interaction forces in TX-100-ILs system.

In order to perceive the influence of SAILs and BAILs on hydrodynamic diameter (D_h) of TX-100 micelles, DLS of 5% TX-100 were performed as a function of concentration of ILs. Fig. 4b shows that the trends of D_h are contrast to corresponding CP results. As observed in CP, SAILs are most effective to increase clouding temperature by replacing TX-100 monomers with octylsulphate anion therefore the size of the micelle decrease. On the other side, BAILs initially increase CP due to replacing TX-100 monomer but after certain concentration it reduced CP by favouring micellization therefore at low concentration of BAILs micelle became smaller afterwards the D_h shift at high values due to micellar transition as shown in Scheme 3 (proved by SANS). The growth of the micelle is favoured due to predominant of different interaction forces, such as electrostatic interaction between imidazolium cation and octylsulphate anion, ion–dipole interaction between octylsuphate anion and TX-100, π–π interaction between imidazolium ring of cation and phenyl ring of TX-100 (Scheme 2).

Scheme 3 Mixed micelle of TX-100-BAILs system at different concentration.

The effect of 1-butyl-3-methyl imidazolium octylsulphate (C₄mim C₈SO₄) on CP and D_h of TX-100 is compared with conventional anionic surfactant sodium octylsulphate (Na–C₈SO₄) (Fig. 5). The effect of Na–C₈SO₄ is relatively higher in terms of increasing CP of TX-100 as compared to C₄mim C₈SO₄ (Fig. 5a), which is understood by bigger size and higher hydrophobicity of imidazolium cation than sodium ion.⁷² That causes significant change in degree of counterion dissociation (0.74 and 0.31 for Na–C₈SO₄ and C₄mim C₈SO₄, respectively as measured by conductance). This results showed that in case of SAILs, imidazolium cation behave as counterion and adsorb at aqueous–micelle interface, while surface active octylsulphate anion interact with TX-100 micelles and form mixed micelles with negative charge. The contraction of TX-100 micelle in presence of C₄mim C₈SO₄ and Na–C₈SO₄ is shown in Fig. 5b. C₄mim C₈SO₄ is less effective in reducing the size of TX-100 micelle as compared to Na–C₈SO₄ due to larger size and higher hydrophobicity of imidazolium cation than sodium ion. The obtained results confirm that octylsulphate anions incorporate in TX-100 micelle and imidazolium cation act as counterion by stronger interactions with the micelle than sodium ion.

Fig. 5 (a) Cloud point, (b) hydrodynamic diameter (D_h) of 5% TX-100 in presence of varying concentration of Na C₈SO₄ and C₄mim C₈SO₄.

The values of zeta potential for 5% TX-100 micelles in the presence of 50 mM C_nmim C₈SO₄ (n = 4, 6, 8, 10) are shown in Table 2. With an increasing alkyl chain from C₄ to C₁₀, the zeta potential shows charge reversal from negative to positive value. The result for C₁₀mim C₈SO₄ is described in terms of higher surface activity of decyl chain in C₁₀mim⁺ cation than octyl chain of C₈SO₄⁻ anion.

Table 2 Micellar parameters for 5% TX-100 in presence of ILs at 30 °C

ILs	Semi major axis (a), Å	Semi minor axis (b), Å	Axial ratio (a/b)	Nagg TX-100	Nagg ILs	ς, mV
a Vesicles R = radius, t = thickness.
0 mM	63.5	19.8	3.2	275	—	−6.9 ± 2.4
50 mM C4mim C8SO4	28.4	17.4	1.6	60	35	−7.8 ± 1.2
50 mM C6mim C8SO4	32.6	17.3	2.0	64	38	−9.1 ± 2.7
50 mM C8mim C8SO4	66.5	17.2	3.9	124	73	−2.5 ± 0.5
50 mM C10mim C8SO4	106.2	17.6	6.0	198	116	+5.4 ± 0.2
25 mM C8mim C8SO4	55.9	18.4	3.0	161	38	—
25 mM C10mim C8SO4	80.5	19.8	4.1	246	72	—
50 mM C10mim C8SO4	106.2	17.6	6.0	198	116	—
75 mM C10mim C8SO4	132.0	17.4	7.6	196	173	—
100 mM C10mim C8SO4^a	R = 200	t = 22.2	—	—	—	—

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The SANS measurements were followed to study microstructural changes in TX-100 micelles. Fig. 6 represents SANS results on 5% TX-100 solution in absence and the presence of C_nmim C₈SO₄.

Fig. 6 SANS profiles for 5% TX-100 () and in the presence of 50 mM C₄mim C₈SO₄ (), C₆mim C₈SO₄ (), C₈mim C₈SO₄ (), C₁₀mim C₈SO₄ () 30 °C.

5% TX-100 forms ellipsoidal micelles in water with 63.5 Å and 19.8 Å as the dimension of semi major and semi minor axis, respectively.^48,54 In the presence of SAILs, the scattering intensity of TX-100 decreases and correlation peak shifts towards high Q-region, which clearly indicates the contraction of TX-100 micelles. The appearance of correlation peak in the SANS spectra indicates that C₄mim C₈SO₄ and C₆mim C₈SO₄ form charged micelle. The fitting parameters (Table 2) demonstrate the ellipsoidal shaped micelle of TX-100 transforming to smaller ellipsoidal on addition of C₄mim C₈SO₄ and C₆mim C₈SO₄. With the addition of C₆mim C₈SO₄, correlation peak depicts minor shifts towards low Q-value compared to C₄mim C₈SO₄, which is an indication of formation of bigger size micelle (Q_max ∝ 1/D) in presence of C₆mim C₈SO₄ than to C₄mim C₈SO₄ as confirmed by the higher aggregation number (N_agg) for C₆mim C₈SO₄. In the presence of BAILs (C₈mim C₈SO₄ and C₁₀mim C₈SO₄), the scattering intensity of TX-100 solution became stronger revealing increase in size of TX-100 micelle. It is because of the same reason discussed in CP section that incorporation of both long chain of cation and anion into TX-100 micelles induces electrostatic interactions between head groups, ultimately favors micellar growth. It clearly validates a link with CP and DLS results. The fitted parameters data are displayed in Table 2, which is clearly portraying ellipsoidal micelle of TX-100 is transforming to extended ellipsoidal micelles on addition of BAILs.

It is summarized that added SAILs favor demicellization and shows smaller axial ratio and aggregation number of TX-100, while BAILs favor micellar growth of TX-100. C₁₀mim C₈SO₄ is most efficient in influencing micellar size.

Microstructure changes in TX-100 micelles in presence of different concentration of BAILs were also investigated by SANS measurements. SANS profiles for 5% TX-100 solutions at varying concentration of C₈mim C₈SO₄ and C₁₀mim C₈SO₄ are shown in Fig. 7a and b, respectively. With progressive addition of BAILs, a strong build-up of scattering intensity in the low-Q region is observed, clearly indicating the micellar growth. C₁₀mim C₈SO₄ shows stronger effect on increasing TX-100 micelle size with the function of concentration and at 100 mM concentration of C₁₀mim C₈SO₄ ellipsoidal TX-100 micelle transform to vesicular structure.

Fig. 7 SANS profiles for 5% TX-100 () in the presence of different concentration of (a) C₈mim C₈SO₄, (b) C₁₀mim C₈SO₄ at 30 °C.

The ¹H NMR experiments could give better understanding on the change in microenvironment of TX-100 protons. ¹H NMR spectra for 5% TX-100 in the absence and presence of these ILs are shown in Fig. 8 and 9. For TX-100, hydrocarbon isooctyl chain protons T₁, T₂, T₃ absorbed in upfield region of the spectrum which makes up hydrophobic core of TX-100 micelles. Chemical shifts at ∼0.6 and ∼1.6 ppm are due to terminal and internal –CH₃ protons (T₁ and T₃), respectively and the T₂ protons (–CH₂) of isooctyl chain give intense resonance at ∼1.2 ppm. The aromatic ring protons (T₅ and T₄) show chemical shifts at ∼6.8 and ∼7.2 ppm, respectively. Protons of ethylene oxide chain (T₈, T₇ and T₆) constitute hydrophilic shell of TX-100 micelles. These protons are deshielded and show resonance comparatively in downfield region at ∼3.6, ∼3.8 and ∼4 ppm, respectively.

Fig. 8 ¹H NMR profiles for pure 5% TX-100 (a) and with addition of C₆mim C₈SO₄ (b), C₈mim C₈SO₄ (c) and C₁₀mim C₈SO₄ (d) at 30 °C.

Fig. 9 ¹H NMR profiles for pure 5% TX-100 (a) and with addition of C₆mim C₈SO₄ (b), C₈mim C₈SO₄ (c) and C₁₀mim C₈SO₄ (d) at 30 °C (expanded).

The core protons of TX-100 (T₁, T₂ and T₃) are little shifted to lower ppm values in the presence of these ILs. It indicates that these protons experience more hydrophobic environment due to incorporation of hydrophobic chains of ILs in the core of TX-100 micelles. The resonance of T₂ and T₃ protons of TX-100 at ∼1.2 ppm and ∼1.6 ppm show broadening and merging due to strong interaction of anion and cation methylene protons (A₃, C₉) of ILs with T₂ proton and C₈ proton of imidazolium cation with T₃ proton and two addition peaks observed in upfield ∼0.8 ppm (A₄, C₁₀) and ∼1.8 ppm (A₂ proton). It was inferred that both alkyl chain of BAILs resides near core of TX-100.

The resonance peaks in between 3.6–4.2 ppm for shell protons (T₆, T₇ and T₈) shift toward high ppm values and new chemical shifts are observed in this region, which are due to involvement of positively charged imidazolium ring and negatively charged octylsulphate providing hydrophilic environment (Fig. 9). It was informed that imidazolium ring and sulphate group resides in the shell region near aromatic ring of TX-100 micelles via ion–dipole interaction. The splitting in T₄ and T₅ peak became single broad peak, which is indicate the environment near phenyl ring is changed.

The chemical shift for C₂ protons is observed at ∼8.7 ppm for all ILs, which gradual shift to downfield and appeared at ∼8.76, ∼8.82, ∼8.86 ppm respectively for C₆mim C₈SO₄, C₈mim C₈SO₄, C₁₀mim C₈SO₄ with TX-100. These suggest deshielding effect due to the formation of mixed micelles.⁷⁵ On basis of ¹H NMR study, it was suspected that hydrophobic alkyl chain of ILs resides in the core, while imidazolium ring and octylsulphate situated near the shell part of TX-100 micelles.

Conclusion

The aqueous solution characteristics of 1-alkyl-3-methylimidazolium octylsulphates (C_nmim C₈SO₄, n = 4, 6, 8, 10) and their effect on micellar behavior of TX-100 is systematically investigated using variety of analytical and physical techniques. The short alkyl chained C₄mim C₈SO₄ and C₆mim C₈SO₄ (SAILs) form negatively charged spherical and ellipsoidal micelles in aqueous solution respectively, while longer alkyl chained C_nmim C₈SO₄, n = 8, 10 (BAILs) form vesicles due to limited aqueous solubility. Effect of these ILs on TX-100 micelle depends on alkyl chain of imidazolium cation and also on octylsulphate anion. Results demonstrate that in presence of SAILs, octylsulphate anion incorporate into TX-100 micelle and form mixed micelle similar to conventional anionic surfactants and imidazolium cation acts as counterion. The negative charge of formed mixed micelles is confirmed by zeta potential. On the other side in presence of BAILs, amphiphilic anion and cation penetrate into TX-100 micelles with such a molecular arrangement that induced electrostatic interaction between polar heads and show micellar growth. NMR shows that alkyl chain of cation and anion interacts with isooctyl chain of TX-100, while imidazolium cation and anionic sulphate groups align in shell part of TX-100 micelle. SANS data show that morphology of TX-100 micelles transforms ellipsoidal to smaller ellipsoidal in presence of SAILs and at lower concentration of BAILs, while at higher concentration of BAILs extended ellipsoidal micelles were formed. Vesicle structures are formed on addition of 100 mM C₁₀mim C₈SO₄ to TX-100 aqueous solutions. From the overall study we conclude that five types of different forces are predominant in studied TX-100-ILs systems: (i) hydrophobic interaction between alkyl chains, (ii) hydrogen bonding between imidazolium ring proton and oxygen atom of TX-100, (iii) π–π interaction between imidazolium ring and phenyl ring (iv) electrostatic interaction between imidazolium cation and octylsulphate anion and (v) ion–dipole interaction between octylsulphate anion and TX-100.

Acknowledgements

P. B. gratefully acknowledges financial assistance from CSIR project (No. 01/2763/13(EMR-II)). B. B. thanks UGC New Delhi for project no. MRP-MAJOR-CHEM-2013-38176.

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