Computational Investigation of the n-Alkane/Water Interface with Many-Body Potentials: The Effect of Chain Length and Ion Distributions

Molecular dynamics simulations with polarizable potentials were carried out to investigate the n-alkane/water interface, including sodium-halide ion distributions. A new polarizable force field was developed for n-alkanes, which gave good agreement with experiment for liquid densities and heats of vaporization for different n-alkanes at different temperatures. Also, good agreement with experiment was found for alkane/water interfacial tensions for a variety of alkanes. Alkane/water interfaces with linear alkanes of different chain lengths had fairly similar properties, except that with longer alkanes, the interfacial width decreased. Water induced dipoles were reduced at the alkane/water interface in comparison with the bulk, but their induced dipoles were greater at the alkane/water interface than at the air/water interface. Furthermore, the water structure expanded at the alkane/water interface in comparison with the bulk, but this expansion was not as significant as at the air/water interface. Sodium-halide concentrated solutions at the n-octane/water interface were simulated and compared with the air/water interface. Iodide had a similar interfacial concentration at both interfaces, while bromide and chloride showed significantly reduced interfacial concentration at the alkane/water interface. Iodide's differing behavior was linked to the fact that it has favorable hydrophobic interactions with alkanes that were not as strong for bromide and chloride with the alkanes.