Stability of water/toluene interfaces saturated with adsorbed naphthenic acids—A molecular dynamics study

Stability of water/toluene interfaces saturated with adsorbed naphthenic acids with various sizes at room temperature was studied by molecular dynamics simulation. In direct comparison with our recent results on similar systems involving n-heptane as the oil phase, it has been found that toluene significantly enhances the stability of the water/oil interface, as suggested by the considerably reduced diffusion coefficients of the interface-bound naphthenic acids. The oil phase dependence may be rationalized in terms of the oil–naphthenic acid interactions, both electronically and sterically, and the intermolecular interactions between the molecules in the oil phase. Consistent with our previous studies, mobility of naphthenic acid has been found to be dependent on its size, with naphthenic acid featuring a longer carbon chain in between the carboxylic and cycloaliphatic groups showing lower mobility, leading to a more stable interface. However, such size dependence is much more pronounced for the water/toluene than the water/n-heptane interface. This may be understood in terms of the structural influence of toluene on the compactness of the liquid crystalline mesophase formed by naphthenic acids adsorbed at the interface.