Water in nanopores: II. The liquid-vapour phase transition near hydrophobic surfaces

The liquid-vapour phase transition near a weakly attractive surface is studied by simulations of the coexistence curves of water in hydrophobic pores. There is a pronounced gradual density depletion of the liquid phase near the surface without any trend to the formation of a vapour layer below the bulk critical temperature T-C. The temperature dependence of the order parameter in the surface layer follows the power law (rho(1) - rho(v)) similar to (1 - T/T-C)(beta1) with a value of the exponent l close to the critical exponent beta(1) = 0.82 of the ordinary transition in the Ising model. The order parameter profiles in the subcritical region are consistent with the behaviour of an ordinary transition and their temperature evolution is governed by the bulk correlation length. Density profiles of water at supercritical temperatures are consistent with the behaviour of the normal transition caused by the preferential adsorption of voids. The relation between normal and ordinary transitions in the Ising model and in fluids is discussed.