Aggregation of sodium dodecylbenzene sulfonate: Weak molecular interactions modulated by imidazolium cation of short alkyl chain length

Highlights

• Aggregation of SDBS is studied in imidazolium IL aqueous solution.

• SDBS aggregation process involves two stages.

• The SDBS aggregate stability is determined by the IL structure.

• The IL changes the size of the SDBS aggregates.

Abstract

Ionic liquids (ILs) can modify cooperative process in aqueous solutions to a large extent, including anionic surfactant aggregation. Here, the micellization of sodium dodecylbenzene sulfonate (SDBS) was evaluated in low concentrations of 1-alkyl-3-methylimidazolium chloride (C_nmimCl, n = 0, 2, and 4) aqueous solutions through fluorescence spectroscopy, isothermal titration calorimetry, dynamic light scattering, and conductometry. The thermodynamic stability of SDBS aggregates strongly depended on the IL structure and concentration, following the order C₄mim⁺ > C₀mim⁺ ≈ C₂mim⁺. At 1.0 mmol L⁻¹ of the ILs, the increase of the hydrophobicity of the imidazolium cation decreased the enthalpic favorableness, changing ${\Delta H}_{mic}^o$ from −3.75 ± 0.07 kJ mol⁻¹, for C₀mim⁺, to −2.69 ± 0.01 kJ mol⁻¹, for C₄mim⁺. On the other hand, the entropic feasibility showed an opposite trend, i.e., the higher hydrophobicity of C₄mim⁺ overcame the kosmotropic effect of IL cations in the bulks. We suggested that the imidazolium cations interact with the SDBS monomers on the micellar surface, mainly through hydrophobic, π-π, and electrostatic interactions for C₄mim⁺ and C₂mim⁺, and through electrostatic interactions and hydrogen bonds for C₀mim⁺.

Introduction

The self-assembly of chemical compounds in aqueous solution is a fundamental physicochemical process, both in theorical and practical aspects. Among several molecules that can aggregate through self-assembly, surfactants have gained considerable attention because of the wide range of their potential applications in fields such as pharmaceutical formulations [1], environmental remediation [2], foam generation [3], personal care products [4], cytotoxicity of metal nanoparticles [5], nanomaterial synthesis [6], reactivity in micellar solutions [7], and drug delivery [8]. Surfactants can form aggregates above a narrow concentration range, known as the critical micellar concentration ($cmc$), at fixed temperature and pressure. [[9], [10], [11]] The $cmc$ depend on several properties, specially the molecular properties of the solvent; [[12], [13], [14]] therefore, it can be modulated by the addition of cosolutes or cosolvents.

The effect of cosolutes and cosolvents, such as methanol, propanol, acetamide, urea, NaBr, and ethylene glycol [[15], [16], [17]] on the surfactant micellization has been widely studied. In the last fifteen years, ionic liquids (ILs), electrolytes with bulky organic cation and inorganic (or organic) anions [18], have attracted attention as potential cosolutes [[19], [20], [21]] because of their ability to simultaneously modify, electrostatic and hydrophobic interactions in aqueous systems, thereby modulating different cooperative processes [[22], [23], [24], [25]]. The ILs containing the imidazolium cation are widely studied due to their negligible vapor pressure, high ionic conductivity, and because their polarity can be modulated by the number of carbons in the lateral chain on the imidazolium cation ring [26].

Moreover, the imidazolium ILs have been extensively used to investigate the thermodynamics of micellization of model ionic surfactants, such as sodium dodecyl sulfate (SDS). In 2009, Behera et al. [27,28] studied the effects of hydrophilic 1-butyl-3-methylimidazolium tetrafluoroborate (C₄mimBF₄) and hydrophobic 1-butyl-3-methylimidazolium hexafluorophosphate (C₄mimPF₆) ILs on SDS micellization, by conducting fluorescence spectroscopy and conductivity. Both these ILs showed two distinct effects: for C₄mimBF₄, the $cmc$ decreases when IL concentration is increased up to 2 % wt, but increases when IL concentration is raised from 2 to 30 % wt; for C₄mimPF₆, the $cmc$ of SDS decreases when IL concentration is increased up to 0.1 % wt and, remains constant thereafter even when the IL concentration is increased up to 30 % wt. These results confirm that ILs act as simple electrolytes at lower concentrations and are partitioned from the bulk to the SDS micelles core at higher concentrations, promoting the micelle growth. In 2009, Smirnova et al. [29] studied the SDS micellization in 1-alkyl-3-methylimidazolium salts + water mixtures through conductivity and calorimetry measurements. The increase in concentration and carbon number in the lateral chain of the imidazolium cation promoted a sharper decrease in the $cmc$ and micellization enthalpy change values (${\Delta H}_{mic}^o$) when compared with those of the simple electrolyte NaCl. This decrease was attributed to the changes in the electrostatic and hydrophobic interactions in the system caused by IL cations. In 2013, Pal et al. [30] examined the behavior of SDS solutions in the presence of 3-methyl-5-methylimidazolium hexafluorophosphate through conductivity and fluorescence spectroscopy. They showed that the IL causes an increase in the $cmc$ values of the SDS due to the presence of solvophobic interactions between the IL and the surfactant chains, facilitating the formation of mixed micelles. In 2015, Ferreira et al. [24] investigated the effect of 1-butyl-3-methylimidazolium halides on the SDS micellization by performing calorimetry, fluorescence, and conductivity and showed that the imidazolium cation introduces its small carbonic chain into the hydrophobic part of the micelle.

Micellar systems based on more complex anionic surfactants, such as sodium dodecylbenzene sulfonate (SDBS), have also been studied in the presence of imidazolium ILs. In 2010, Rai et al. [31] studied the size of SDBS micelles in the presence of the C₄mimPF₆ IL using dynamic light scattering (DLS). They showed that for a SDBS concentration of 300 mmol L⁻¹ and an IL concentration of approximately 100 mmol L⁻¹ (3:1 stoichiometric ratio), the diameter of the micelles increases drastically by approximately 20 nm due to the incorporation of the imidazolium cations in the micellar aggregate. In 2015, Chabba et al. [32] studied the formation of SDBS micelles in aqueous solutions in the presence of C_nmimCl (n = 8, 10 or 12) using tensiometry, fluorescence spectroscopy, and dynamic light scattering. Their results suggested that the formation of mixed micelles and vesicles occur due to the balance between cation-π, hydrophobic, strong electrostatic, and H-bonding interactions.

Despite the extensive information about the effect of imidazole-based ILs on the micellization of model ionic surfactants, systematic thermodynamic studies involving the effect of the hydrophobic/hydrophilic balance of these ILs on the micellization of atypical anionic surfactants, particularly in low concentrations of the IL, are scarce.

Here, we study the effect of low concentrations of 1-alkyl-3-methylimidazolium chlorides (Fig. 1S) on the micellization thermodynamics of SDBS in aqueous solution using isothermal titration calorimetry, fluorescence spectroscopy, electrical conductivity, and dynamic light scattering. The thermodynamic parameters of micellization (${\Delta G}_{mic}^o$, ${\Delta H}_{mic}^o$, and $T{\Delta S}_{mic}^o$) have been evaluated for ILs with distinct concentrations and hydrophobicity.