Water confined in cylindrical micropores

A model characterizing water-like fluids confined in cylindrical micropores is presented. The equation of state was derived based on perturbation theory assuming that the reference system of hard spheres confined in the cylindrical pores is homogenous. The perturbed state accounts for fluid–fluid, fluid–wall, and hydrogen bonding interactions. Fluid–fluid and fluid–wall interactions are modeled as the pairwise sum of Lennard-Jones potentials. The hydrogen bonding model accounts for the open structure of liquid water, as well as for the fact that the hydrogen bonding capabilities of confined molecules are distorted. This model was used to analyze the dependence of the density of water inside the micropores on the density outside the pores, the pore radius, and the affinity of the pore walls for water molecules. For gas-phase adsorption, the model predicts that the density of water inside the pores depends on the fluid–wall interactions. The state of the adsorbed phase varies from the density of vapor outside the pores (for the hard sphere wall) to a bulk liquid-like density (for a hydrophilic sample). The predicted behavior of confined water in the presence of bulk liquid outside the pores was more interesting. The model predicts that for small pores of hydrophobic materials, the density of fluid inside the pores is much smaller than the bulk liquid density, i.e., vapor-like. However, as the radius and/or hydrophility are increased, the fluid density inside the pores approaches the bulk liquid density very rapidly.