Molecular insights in the temperature effect on adsorption of cationic surfactants at liquid/liquid interfaces

Surfactant adsorption at the oil-water interface plays a significant role in many industrial processes and natural phenomena. Electrolyte concentration and temperature are considered as controlling factors, which determine the surfactant adsorption, and, as a consequence, the associated interfacial tension. In this study, the influence of temperature on the interfacial tension of oil-water-surfactant (erucyl bis-(hydroxyethyl)-methylammonium chloride and cetyltrimethylammonium chloride) systems is examined by molecular dynamics simulations. The focus is on the fundamental difference in the molecular arrangements between surfactant molecules and their counterions at the water/n-decane interface when the temperature is increased. In addition, surfactant diffusion into the aqueous and oleic phases at different temperatures is also addressed. Clearly, as temperature increases, hydrogen bonding between water molecules and surfactant heads decreases, and thus surfactant molecule solubility in water also decreases. Results demonstrate that interfacial tension between water/n-decane in the presence of hydroxyl group containing surfactant differs from that predicted by Gibbs theory, and passes through minimum with temperature increase. This observation provides an important basis for studying the interfacial tension as a function of surfactant molecular structure at high temperatures, and can be used for primary evaluation of surfactants for their implementation in the oil recovery methods. (C) 2019 Published by Elsevier B.V.