Confined Water: Comparison between a Purely Repulsive Confinement and Hydrophobic Lennard-Jones Confinement

Much of the understanding of bulk liquids has progressed through study of the limiting case in which molecules interact via purely repulsive forces, such as a hard-core potential. In the same spirit, we report progress on the understanding of confined water by examining the behavior of water-like molecules interacting with walls via purely repulsive forces. Specifically, we perform molecular dynamics simulations of 512 water-like molecules which are confined between two smooth plates that are separated by 1.1 nm and are interacting via the TIP5P potential. At this separation, there are either two or three molecular layers of water, depending on density. We systematically compare our results with purely repulsive interactions between molecules and the walls with those obtained for Lennard-Jones (LJ) interactions between the molecules and the walls. We find that the thermodynamic, dynamic and structural properties of the liquid qualitatively match those for a system with a LJ attraction to the wall. In previous studies that include attractions, freezing into monolayer or trilayer ice was seen for this wall separation. Using the same separation as these previous studies, we find that the crystal state is not stable with repulsive walls. However, by carefully adjusting the separation of the plates with repulsive interactions so that the effective space available to the molecules is the same as that for LJ confinement, we find that the same crystal phases are stable. This result emphasizes the importance of comparing systems only using the same effective confinement, which may differ from the geometric separation of the confining surfaces.