Effect of Pore Size on the Condensation/Evaporation Transition of Confined Water in Equilibrium with Saturated Bulk Water

Abstract

The effect of pore size on the condensation/evaporation transition of confined water upon varying the strength of the water-surface interaction is studied under conditions of equilibrium with saturated bulk. Monte Carlo simulations in the grand canonical ensemble were used to determine water density in spherical pores of radius R(p) = 9, 12, 15, 20, and 25 Å in the temperature range from T = 270 K to the bulk critical temperature. The critical values of the well depth of the water-surface interaction potential, which mark the limits of the metastability of vapor and liquid phases in pores (U(0)(cond) and U(0)(evap), respectively), were determined. U(0)(cond) strongly depends on temperature, practically does not depend on the pore size, and corresponds to some particular density of water vapor near a surface. In contrast, U(0)(evap) only slightly depends on temperature, depends strongly on pore size, and corresponds to the density in the pore interior by about 2% below the bulk value. The critical water-pore interaction U(0)(c), which separates regimes of capillary condensation and capillary evaporation, is found to be changed from -1.75 to -0.94 kcal/mol when the pore radius R(p) increases from 9 to 25 Å. The size dependence of U(0)(c) is attributed to the change of the contact angle due to the line tension effect. Extrapolation of the dependence U(0)(c)(R(p)) to the flat surface gives the critical value U(0)(c)(∞) ≈ -0.61 kcal/mol.