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

The hydration of salts has gained particular interest within the frame of thermochemical energy storage. Most salt hydrates expand when absorbing water and shrink when desorbing, which decreases the macroscopic stability of salt particles. In addition, the salt particle stability can be compromised by a transition to an aqueous salt solution, called deliquescence. The deliquescence often leads to a conglomeration of the salt particles, which can block the mass and heat flow through a reactor. One way of macroscopically stabilizing the salt concerning expansion, shrinkage, and conglomeration is the confinement inside a porous material. To study the effect of nanoconfinement, composites of CuCl2 and mesoporous silica (pore size 2.5-11 nm) were prepared. Study of sorption equilibrium showed that the pore size had little or no effect on the onsets of (de)hydration phase transition of the CuCl2 inside the silica gel pores. At the same time, isothermal measurements showed a significant lowering of the deliquescence onset in water vapor pressure. The lowering of the deliquescence onset leads to its overlap with hydration transition for the smallest pores (<3.8 nm). A theoretical consideration of the described effects is given in the framework of nucleation theory.