Adsorption, Desorption, and Crystallization of Aqueous Solutions in Nanopores

Abstract

Probing nano-confined solutions in tortuous, mesoporous media is challenging due to pore size, complex pore connectivity, and 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 1 Page 1 of 41 ACS Paragon Plus Environment Langmuir of the system: the onset of draining during desorption and the end of filling during adsorption, percolation-induced scattering and crystallization. We find that theoretical arguments involving classical thermodynamics (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.

Deliquescence of NaCl Confined in Nanoporous Silica

“…In contrast to the behavior of NaCl nanoparticles, increased water uptake of salt-loaded nanoporous materials below the bulk DRH , and, more recently, a shift of the DRH to lower vapor pressures were found in experimental studies of salt crystals in confinement. A systematic investigation of the influence of pore size on the deliquescence behavior of confined crystals has not been carried out as yet.…”

“…The reason for this increase is assumed to be the Kelvin effect, due to which the water vapor pressure in a solution drop formed after deliquescence is increased in comparison to that of a large droplet with the same concentration. 16 In contrast to the behavior of NaCl nanoparticles, increased water uptake of salt-loaded nanoporous materials below the bulk DRH 3,4 and, more recently, a shift of the DRH to lower vapor pressures 17 were found in experimental studies of salt crystals in confinement. A systematic investigation of the influence of pore size on the deliquescence behavior of confined crystals has not been carried out as yet.…”

NMR Investigation of Water in Salt Crusts: Insights from Experiments and Molecular Simulations

“…Indeed, prior to evaporation, the experimental concentrations of the NaCl and Na 2 SO 4 solutions were chosen as 4 M and 1 M, respectively (see Methods section), and during evaporation, the concentration is expected to reach saturation within the pores (at least locally). The salt concentration within nanopores can even reach supersaturation, as has been observed for NaCl solutions confined in silica nanopores [47]. To evaluate the effect of finite salt concentration c s on T 1 in the case of confined liquids, simulations were performed for slit pore systems with size h ≈ 7 nm and c s up to 5 mol/kg in the case of NaCl, and up to 1.5 mol/kg in the case of Na 2 SO 4 (Fig.…”

The Effect of Nanoconfinement on Deliquescence of CuCl2 Is Stronger than on Hydration

“…On the other hand, the limitation of the crystal size by the pore size could influence the hydration and deliquescence transitions through increased surface effects. The effect of confinement on the deliquescence transition was already studied for the simple salts of NaCl and LiCl in mesoporous silica materials in ref and ref . Here, it was shown that the onset of deliquescence for NaCl and LiCl shifts to lower water vapor pressures in isotherms when confined inside mesoporous silicas due to the concave curvatures of the water–gas interface inside these pores.…”