Molecular dynamics study of the effect of sodium and chloride ions on water-surfactant-hydrocarbon interfaces

Abstract

Molecular dynamic simulations have been performed to study the effect of electrolyte concentrations and temperature on the structural, dynamic, and thermal properties of cationic surfactants at the water/ n -decane interface. Results show that optimal salinity (where the interfacial tension (IFT) passes through a minimum) could be reached for erucylbis-(hydroxyethyl)-methylammonium chloride (EHAC). In contrast, the IFT of cetyltrimethylammonium chloride (CTAC) solution increased for all NaCl concentrations tested. It was demonstrated that above optimum salinity, EHAC molecules were repelled from the interface due to Na + diffusing closer to the Stern layer and contributing to the positive interfacial charge, resulting in micelle formation in the bulk phase. However, CTAC molecules moved further inside the hydrocarbon phase upon salt addition. These findings provide an important basis for studying surfactant interfacial behavior as a function of salinity, so that better formulations can be developed for applications where salinity is non-zero, e.g. in pharmacy or enhanced oil recovery.