Structure and transport properties of liquid alkanes in the vicinity of α-quartz surfaces

Structure and mass transport properties of several liquid n-alkanes, methane, decane and tetracosane, in the vicinity of α-quartz surfaces of three crystal planes have been investigated by using molecular dynamics simulations. Solid α-quartz surfaces were terminated with -H and -OH groups to create hydrophobic and hydrophilic surfaces, respectively. It was observed that adsorption of the liquid alkane molecules is more noticeable near the (0 0 1) crystal plane when compared with other two planes (0 1 1) and (1 0 0). For a given alkane, the number of molecules adsorbed near a α-quartz wall marginally depends upon the crystal plane and type of surface termination. Ordering parameter and radius of gyration for liquid molecules were examined in the interface region to gain the knowledge about molecular alignment and chain configuration in the interface region. Liquid molecules in the first adsorption layer near the hydrophilic and hydrophobic surfaces are more parallel to the interface in the following order (0 0 1), (0 1 1) and (1 0 0). Portions of decane and tetracosane liquid molecules tend to enter into the H-terminated side with an orientation in the parallel direction to the interface and into the OH-terminated side with an orientation in the perpendicular direction for (1 0 0) crystal plane. Molecules are flattened in the direction perpendicular to the interface and are elongated in the other direction in the interface region for decane and tetracosane. It is observed that the in-plane self-diffusion coefficient of liquids, the self-diffusion coefficient according to the migration of molecules in the directions parallel to the interface, is smaller in the vicinity of the solid surface and approaches to the bulk liquid value away from the interface. For methane, decane and tetracosane, the in-plane diffusion in the first adsorption layer near the smoother silica surface (0 0 1), which has molecularly smoother surface than the other two surfaces (0 1 1) and (1 0 0), for both of the terminations is higher as compared with the other two surfaces (0 1 1) and (1 0 0). It was found that the diffusion coefficients in the two directions of the 2-D in-plane diffusion are different significantly and this anisotropic nature is resulted by the topology of the solid surface. At the same reduced temperature, a tendency was observed that the in-plane diffusion near the H-terminated and OH-terminated surfaces for three crystal planes increases with an increase in the chain length of the liquid alkanes.