Water in nanopores: III. Surface phase transitions of water on hydrophilic surfaces

Water in hydrophilic pores shows a rich phase diagram due to the appearance of the surface phase transitions. By simulation studies of water in various hydrophilic pores, we have analyzed the evolution of the water phase diagram with the strengthening of a water-wall interaction potential U-0. A second-order wetting transition is observed rather close to the liquid-vapor critical temperature when U-0 approximate to -2 kcal/mol. The wetting temperature rapidly decreases with the strengthening of a water-wall interaction. A first-order wetting transition, accompanied by a prewetting transition, is observed when the hydrophilicity of the walls is within the range of -4 <= U-0 <= -3 kcal/mol. At U-0 <= -4 kcal/mol, the temperature of the wetting transition is below the bulk freezing temperature, and one or two layering transitions appear instead of the prewetting transition. Coexistence curves of the first layering transition are well described by the two-dimensional Ising model with critical exponent beta = 0.125. The critical temperature of a layering transition is not sensitive to the pore size and shape. It is always above the bulk freezing temperature and varies from about 400 K at U-0 approximate to -3 kcal/mol to about 330 K for quasi-two-dimensional water.