Condensation/Evaporation Transition of Water in Spherical Pores in Equilibrium with Saturated Bulk Water

Abstract

Liquid−vapor phase transition of water in spherical pores with various strengths of water−surface interaction is studied under conditions of equilibrium with the saturated bulk water. The excess chemical potential of bulk liquid water along the liquid−vapor coexistence curve was determined by the overlapping distribution method. The adsorption and desorption of water in pores were studied by Monte Carlo simulations in the grand canonical ensemble in the temperature range T = 270−580 K with a step 10 K. The well depth U(0) of the water−surface potential was varied from ≈−0.6 kcal/mol (hydrophobic pore surface) to ≈−7.1 kcal/mol (hydrophilic pore surface). The diagram showing the areas of the stable, metastable, and unstable liquid and vapor phases of confined water in the T − U(0) plane has been constructed. Water vapor only exists in pores with U(0) > −1.2 kcal/mol, whereas liquid water only exists in pores with U(0) < −2.8 kcal/mol. The interval ΔU(0), where both stable and metastable states of confined water are possible, shrinks upon heating almost linearly and disappears at the hysteresis pore critical temperature T(p)(h). Above T(p)(h), there is a particular value U(0)(c) ≈ −1.35 kcal/mol that divides the regimes of capillary condensation and capillary evaporation up to the pore critical temperature T(p)(c). The obtained results can be used for the optimization of porous materials for applications that require controlled adsorption and desorption of water.