Adsorption of Water as a Means to Remove Bulk Metastability of CsHSO4 and RbH2PO4

Abstract

Differential scanning calorimetry (DSC) and thermogravimetry (TG) are applied to study the conditions required for CsHSO4 and RbH2PO4 to undergo phase transitions at 333 ± 1 K and 352 ± 3 K, respectively. It is demonstrated that water adsorption on the surface of metastable polycrystalline samples triggers their transition to the stable state. It is further shown that the transition of CsHSO4 only occurs if the water vapour pressure is sufficiently high. If the sample is prepared at 293 K this threshold is 6.3 ± 0.7 mm Hg. A correlation is found between the transition temperature and the water vapour pressure. While it has been reported recently that adsorbed molecules can cause phase transitions in the surface layer of a solid matrix, our results are the first well-established examples of a direct correlation between surface adsorption and bulk phenomena. There is a relation to an early idea of Schwab.