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

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.