Adsorption, Desorption, and Crystallization of Aqueous Solutions in Nanopores

Probing nanoconfined solutions in tortuous, mesoporous media is challenging because of pore size, complex pore connectivity, and the coexistence of multiple components and phases. Here, we use optical reflectance to experimentally investigate the wetting and drying of a mesoporous medium with ∼3-nm-diameter pores containing aqueous solutions of sodium chloride and lithium chloride. We show that the vapor activities (i.e., relative humidities) that correspond to optical features in the isotherms for solutions can be used to deduce the thermodynamic state of a nanoscopic solution that undergoes evaporation and crystallization upon drying and condensation and deliquescence when increasing the relative humidity. We emphasize specific equilibrium states of the system: the onset of draining during desorption and the end of filling during adsorption as well as percolation-induced scattering and crystallization. We find that theoretical arguments involving classical thermodynamics (a modified Kelvin-Laplace equation and classical nucleation theory) explain quantitatively the evolution of the optical features and thereby the state of the solution as a function of imposed vapor activity and solute concentration.