A molecular dynamics study of the decane/water interface

Several molecular dynamics simulations on the interface between liquid decane and liquid water have been performed with the purpose to study the sensitivity of surface properties to the van der Waals parameters. The models used consisted of 50 decane molecules and 389 water molecules in a two-phase system. By changing the van der Waals parameters, i.e., varying the Lennard-Jones parameters ε and σ between the united CH2 and CH3 atoms of the decane molecules and the O atom of the water molecules, sharper interfaces were obtained. The excess free energy of water in decane and the surface tension between water and decane are presented for the van der Waals parameters used. From comparison to experimental values for the solubility of water in decane and the surface tension, the best parameters within the tested sets are when using the SPC/E water model: εO-CH3 = 0.849 kJ/mol, εO-CH2 = 0.706 kJ/mol, and σO-C = 0.344 nm. When using the SPC water model, the best parameters are εO-CH3 = 0.637 kJ/mol, εO-CH2 = 0.529 kJ/mol, and σO-C = 0.344 nm. We also evaluated differences between the interfacial and bulk liquids. At the interface, water showed an orientational preference, whereas the decane molecules were more laterally oriented with respect to the interface.