The role of competitive counterion adsorption on the electrolyte induced surface ordering in methyl ester sulfonate surfactants at the air-water interface

Abstract The strong binding of Al 3+ trivalent counterions to the anionic surfactants sodium polyethylene glycol monoalkyl ether sulfate and α-methyl ester sulfonate results in surface multilayer formation at the air-water interface. In contrast the divalent and monovalent counterions Ca 2+ and Na + result only in monolayer adsorption. Competitive counterion adsorption has been extensively studied in the context of surfactant precipitation and re-dissolution, but remains an important feature in understanding this surface ordering and how it can be manipulated. The α-methyl ester sulfonate surfactants are a promising class of anionic surfactants which have much potential for improved performance in many applications, greater tolerance to extreme solvent conditions such as water hardness, biocompatibility and sustainable production. Hence in this study we have used neutron reflectivity to extend previous studies on the surface ordering of the α-methyl ester sulfonate surfactant, sodium tetradecanoic 2-sulfo 1-methyl ester, in the presence of electrolyte to investigate the role of binary mixtures of electrolytes, AlCl 3 /CaCl 2 , and AlCl 3 /MgCl 2. In the mixed electrolytes the evolution of the surface structure, from monolayer to multilayer with increasing AlCl 3 concentration, is observed. It is broadly similar to that reported for the addition of only AlCl 3 . However with increasing CaCl 2 concentration the structural evolution is shifted progressively to higher AlCl 3 concentrations. Similar observations occur for the AlCl 3 /MgCl 2 mixtures. However the presence of the MgCl 2 results in an additional phenomenon; the partial co-adsorption of a more compact lamellar structure which exists until the highest AlCl 3 concentrations. The results demonstrate the importance of the competitive adsorption of different counterions in driving and controlling the formation of surface multilayer structures with anionic surfactants. Furthermore it offers a facile route to the manipulation of these surface structures.