The impact of electrolyte on the adsorption of the anionic surfactant methyl ester sulfonate at the air-solution...

The methyl ester sulfonates represent a promising group of anionic surfactants which have the potential for improved performance and biocompatibility in a range of applications. Their solution properties, in particular their tolerance to hard water, suggests that surface ordering may occur in the presence of multi-valent counterion. Understanding their adsorption properties in a range of different circumstances is key to the exploitation of their potential. Neutron reflectivity and surface tension have been used to characterise the adsorption at the air-aqueous solution interface of the anionic surfactant sodium tetradecanoic 2-sulfo 1-methyl ester, C₁₄MES, in the absence of electrolyte and in the presence of mono, di, and tri-valent counterions, Na⁺, Ca²⁺, and Al³⁺. In particular the emphasis has been on exploring the tendency to form layered structures at the interface. In the absence of electrolyte and in the presence of NaCl and CaCl₂ and AlCl₃ at low concentrations monolayer adsorption is observed, and the addition of electrolyte results in enhanced adsorption. In the presence of NaCl and CaCl₂ only monolayer adsorption is observed. However at higher AlCl₃ concentrations surface multilayer formation is observed, in which the number of bilayers at the surface depends upon the surfactant and AlCl₃ concentrations.