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 C ₁₆ and C ₁₈ -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 C ₁₆ and C ₁₈ -MES surfactants and the eutectic mixture of C ₁₆ and C ₁₈ -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 C ₁₄ to C ₁₈ -MES are found to scale with their critical micelle concentration value. The surface and micelle compositions of the C ₁₆ and C ₁₈ -MES eutectic mixture differ from the eutectic composition; with compositions in the limit of high concentrations richer in C ₁₆ -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 Al ³⁺ on the adsorption at the air-water interface results in a transition from monolayer to multilayer adsorption.
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