Adsorption and self-assembly in methyl ester sulfonate surfactants, their eutectic mixtures and the role of electrolyte

Abstract

The α-methyl ester sulfonate, MES, anionic surfactants are a potentially important class of sustainable surfactants for a wide range of applications. The eutectic-like Kraft point minimum in the C₁₆ and C₁₈-MES mixtures is an important feature of that potential. Understanding their individual adsorption properties and the surface mixing of the eutectic mixtures are key to their wider exploitation.

Neutron reflectivity has been used to investigate the adsorption at the air–water interface of the C₁₆ and C₁₈-MES surfactants and the eutectic mixture of C₁₆ and C₁₈-MES, in aqueous solution and in electrolyte. The micelle mixing of the eutectic mixture is investigated using small angle neutron scattering.

The adsorption isotherms for C₁₄ to C₁₈-MES are found to scale with their critical micelle concentration value. The surface and micelle compositions of the C₁₆ and C₁₈-MES eutectic mixture differ from the eutectic composition; with compositions in the limit of high concentrations richer in C₁₆-MES. The mixing properties are described by the pseudo phase approximation with a repulsive interaction between the two surfactants. The impact of the multivalent ions Al³⁺ on the adsorption at the air–water interface results in a transition from monolayer to multilayer adsorption.

Introduction

The major surface active ingredients in most household detergents are anionic surfactants [1], [2]. The increasing demand for improved formulations to provide better detergency, enhanced performance at lower temperatures and greater tolerance to hard water has resulted in the development of a range of new anionic surfactant structures and the optimisation of anionic/non-ionic surfactant mixtures [1], [2], [3], [4]. The alkyl sulfates, especially for the longer alkyl chain lengths, precipitate readily due to the strong binding and complexation with the multivalent ions in hard water [5], [6], [7]; a common problem in limiting hard water tolerance. This can be partially mitigated by co-adsorption and self-assembly with a non-ionic co-surfactant [7]. The development of the alkyl benzene sulfonate, LAS, [2], [8], and the alkyl ethyoxy ether sulfate, SLES, [9], [10], [11] surfactants has greatly improved hard water detergency properties. LAS has for example demonstrated both improved detergency properties and biodegradability [1], [2].

To meet the sustainability agendas of the major detergent manufacturers there is a greater drive towards the use of surfactants prepared from renewable sustainable sources, instead of petroleum based materials [12], for greater biocompatibility and biodegradability, and for efficient operation at lower temperatures. The α-methyl ester sulfonates, MES, anionic surfactants, prepared from renewable palm-oil based sources, have been promoted as attractive alternatives to the petroleum based counterparts [13], [14], [15], [16], [17]. Improved hard water tolerance, greater biodegradability and better cold water detergency have been demonstrated [18], [19], [20], [21], [22]. Hence the synthesis and purification of MES has been extensively studied and reported [15], [16], [23], and the basic physicochemical properties, surface adsorption and self-assembly have been studied [23], [24], [25], [26].

Xu et al. [27] have recently reported the adsorption properties of C₁₄-MES at the air-water interface; evaluated by surface tension, ST, and neutron reflectivity, NR, and using a synthetic route to provide better defined samples. This resulted in an adsorption isotherm and limiting adsorption amounts freer from the impact of impurities, but consistent with surface divalent counterion impurities. Xu et al. [28] have reported the impact of electrolyte on C₁₄-MES adsorption at the air-water interface. In NaCl, CaCl₂ and low concentrations of AlCl₃ slightly enhanced adsorption is observed, but at higher AlCl₃ concentrations surface multilayer formation occurs, ranging from a single bilayer beneath the initial monolayer at the surface to multiple bilayers. This is consistent with the more extensive studies of surface multilayer formation with the addition of multivalent counterions with LAS and SLES at low surfactant concentrations and outside the regime of precipitation [29], [30], [31], [32]. This was demonstrated with the addition of Ca²⁺ for LAS and Al³⁺ for SLES, in which the evolution of the extent of the surface multilayers is controlled by surfactant, and counterion concentration, and the surfactant structure. The surface multilayer formation results in persistent wetting of hydrophobic solid surfaces, substantially enhanced adsorption at interfaces, and the opportunity to deliver and provide an effective surface reservoir for other surface ingredients, such as perfumes [33].

The MES surfactants can exist with a range of alkyl chain lengths, typically from C₁₂ to C₁₈. Xu et al. [27], [28] have focussed on the characterisation of the C₁₄-MES at the air-water interface. In this paper the focus is on the adsorption of the C₁₆ and C₁₈-MES surfactants, and their eutectic mixture. One of the issues associated with the longer alkyl chain lengths is their relatively high Krafft temperatures, 28 and 40 °C for C₁₆-MES and C₁₈-MES respectively; which would suggest that the pure MES solutions would not be very effective detergents, especially at lower temperatures [18], [34]. However the Krafft temperature of the C₁₆/C₁₈-MES mixture goes through a minimum at a temperature ∼15 °C at a composition of 65/35 mol ratio C₁₆/C₁₈-MES [35], and this mixture exhibits good detergency properties. A minimum in the Krafft point was reported in other anionic surfactant mixtures, in sodium dodecyl sulfate/bivalent metal dodecyl sulfate mixtures [36], in the sodium and calcium salts of other anionic surfactants such as the linear alkyl benzene sulfonates [37], in different alkyl benzene sulfonate mixtures [38], [39], and in myristic/palmitic acid mixtures [40]; and has been likened to a eutectic point.

However little is known about the adsorption and self-assembly of such eutectic mixtures. In light of their potential importance in a range of applications the focus of this paper is on the adsorption of the eutectic mixture of C₁₆/C₁₈-MES, the individual component surfactants, and the impact of electrolyte on the adsorption, using primarily neutron reflectivity, NR. Furthermore small angle neutron scattering, SANS, is used to characterise the self-assembly of the C₁₆/C₁₈-MES eutectic mixture to primarily determine the micelle composition. The NR and SANS data are analysed and evaluated using the Pseudo phase approximation, PPA, to model the eutectic mixing behaviour. The combination of the surface and micelle compositions provides a more rigorous examination of the thermodynamics of the mixing in such systems.