Water Vapor Transport between Salt Solutions at Different Salinity and Temperature: Theory and Experimental Validation.

Water vapor transport between saline solutions is commonly investigated in membrane distillation systems operating under temperature differences between 20 and 40 °C. Such studies, conducted under significant temperature differences, do not address equilibrium or the approach toward equilibrium of two solutions in contact via the gas phase. Here, we study water transport through the gas phase between salt solutions of different temperatures and salinities separated by a hydrophobic membrane. By measuring small water fluxes over extended times, we follow the approach toward equilibrium and compare the measurements with theoretical predictions. The observed fluxes are quantitatively described by the difference in water vapor pressure between the solutions, with no need for additional driving forces, for example, an additional dependence on the difference in temperature. These findings contribute to the thermodynamic description of vapor-phase water transport between saline solutions near equilibrium.