Origins of Solvation Forces in Confined Films

Comparative investigations of equilibrium structures, solvation forces, and conformational dynamics of thin confined films of spherical molecules, straight-chain alkanes (n-hexadecane and n-tetracosane), and a branched alkane (squalane) are performed using a newly developed grand canonical ensemble molecular dynamics method for simulations of confined liquids. The method combines constant pressure simulations with a computational cell containing solid surfaces and both bulk and confined liquid regions in equilibrium with each other. For all the molecular liquid systems layered density oscillations in the confined films are found for various widths of the confining gap. The solvation force oscillations as a function of the gap width for the spherical and straight-chain alkane liquids are more pronounced, exhibiting attractive and repulsive regions, while for the branched alkane the solvation forces are mostly repulsive, with the development of shallow local attractive regions for small values of the gap width. Furthermore, the calculated free-energy changes upon increased confinement of these systems show oscillatory attractive values for the spherical and straightchain molecular films and overall repulsive (positive) values for squalane. The energetic and entropic components of the solvation forces are investigated, and it is found that in the spherical and straight-chain molecular films they exhibit regular oscillations, while for the branched alkane their variation is irregular and of a more monotonic nature. These results correlate with structural characteristics of the films, with those of the spherical and straight-chain alkanes exhibiting enhanced layered ordering and in-plane ordered molecular arrangements, as well as with the relatively high tendency for interlayer molecular interdigitation in the squalane films. The nature of the transitions between well-formed layered configurations is different in these systems, with the spherical and straight-alkane films exhibiting solid-like characteristics portrayed by steplike variations in the number of confined segments occurring in response to a small decrease in the gap width, starting from well-layered states of the film. On the other hand, the behavior of the squalane film is liquid-like, exhibiting a monotonic continuous decrease in the number of confined segments as the gap width is decreased. Reduced conformational (trans-guache) transition rates in the confined films, compared to their bulk values, are found, and their oscillatory dependence on the degree of confinement is analyzed, showing smaller transition rates for the well-formed layered states of the films.