Surfactant adsorption at the surface of mixed ionic liquids and ionic liquid water mixtures

Surface tensiometry and neutron reflectivity have been used to elucidate the structure of the adsorbed layer of nonionic surfactant tetraethylene glycol tetradecyl ether (C₁₄E₄) at the free surface of the ionic liquids ethylammonium nitrate (EAN) and ethanolammonium nitrate (EtAN) and their binary mixtures with each other and with water. Surface tensions reveal that the critical micelle concentration (cmc) depends strongly on solvent composition. The adsorbed surfactant structure elucidated by neutron reflectivity shows that the level of solvation of the ethylene oxide groups varies for both the pure and mixed solvents. This is attributed to solvent–solvent interactions dominating solvent–surfactant interactions.