Position-dependent energy of molecules in nano-confined water.

Real time decrease in the energy (or enthalpy) measured during confinement of controlled amounts of water in 2 nm radius pores of Vycor shows that exothermic transfer of bulk water to nanopores via the vapour-phase occurred in two stages. In the first stage, at saturation pressure, H2O molecules from the vapour rapidly accumulated in the nanopore channels near the Vycor surface. In the second, at vapour pressure below saturation, the accumulation rate abruptly decreased and water (slowly) diffused and redistributed in the nanopore channels until the vapour pressure equilibrium was attained. The energy decrease per H2O molecule was highest, 14.5 kJ mol(-1), at low amounts when the pore-wall was incompletely covered by H2O. This value approached zero at higher amounts when pores were gradually filled. The results show that the vibrational and configurational contributions to the energy of H2O molecules depend upon their position in the nanopore and these contributions approach their bulk water values at high water concentration, but do not attain those values for completely filled pores.