Molecular simulations of liquid-liquid interfacial properties: water-n-alkane and water-methanol-n-alkane systems.

Direct molecular dynamics simulations of the liquid-liquid interface of water-n-alkane and water-methanol-n-alkane systems have been performed in order to study the interfacial properties of these systems. The simulations were carried out using the NERD revised force field of Nath et al. for the n-alkanes, the simple point charge extended model for water, and the optimized potential for liquid simulations model for methanol. In order to validate the model employed in this work for the n-alkanes we calculated the coexisting densities, surface tension, and thickness of the interface for pure n-pentane. For all the systems studied the interfacial tension and thickness were calculated at 298.15 K. Our results show that, by adjusting the number of molecules to reproduce the liquid densities in the direct simulation method of the liquid-liquid interface in multicomponent systems, we are able to reproduce available experimental data for interfacial tension. The interfacial thickness is underpredicted and a constant negative deviation of approximately 2.5 A from the experimental data is usually observed. We find that methanol acts like surfactant when it is added to the water-n-alkane mixtures, reducing the interfacial tension of the liquid-liquid ternary system. The interfacial tension results agree quantitatively well for the range of concentrations of methanol studied.