Elucidating the Dehydration Pathways of K2CO3·1.5H2O.

Potassium carbonate sesquihydrate has previously been identified as a promising material for thermochemical energy storage. The hydration and cyclic behavior have been extensively studied in the literature, but detailed investigation into the different processes occurring during dehydration is lacking. In this work, a systematic investigation into the different dehydration steps is conducted. It is found that at higher temperatures, dehydration of pristine material occurs as a single process since water removal from the pristine crystals is difficult. After a single cycle, due to morphological changes, dehydration now occurs as two processes, starting at lower temperatures. The morphological changes open new pathways for water removal at the newly generated edges, corners, and steps of the crystal surface. The observations from this work may contribute to material design as they elucidate the relation between material structure and behavior.

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.

Salt crystallization as damage mechanism in porous building materials--a nuclear magnetic resonance study.

Salts can damage building materials by chemical reactions or crystallization, which is a serious threat to cultural heritage. In order to develop better conservation techniques, more knowledge of the crystallization processes is needed. In a porous material, the size of a salt crystal is limited by the sizes of the pores. It has been predicted that as a consequence, the solubility of a salt increases with decreasing pore size. This increase seems to be related to an increase of the stress generated by a crystal on the pore wall. It has been suggested that the resulting stress could become high enough to induce failure. We have studied the crystallization of salts in porous materials with well-defined pore sizes. Samples were saturated at 40 degrees C with saturated Na2SO4 and Na2CO3 solutions. Next we have cooled the samples to 0 degrees C and waited for nucleation. After nucleation occurred, the solubility in the porous material was measured with nuclear magnetic resonance (NMR) as a function of the temperature. The measurements on Na2CO3 indeed show an increase in solubility with a decrease in pore size. For Na2SO4, we did not observe a pore size-dependent solubility. However, we have to remark that these results show a metastable crystal phase. The results can be used to calculate the actual pressure exerted by the crystals onto the pore wall.