Structure and the conductive behaviour of hydrate melt coexisting with porous solid materials—α-Al2O3 powder/ZnCl2 hydrate melt coexisting system

Abstract

In order to clarify the relationship between the structure and ionic conduction behaviour of hydrate melt near the solid surface, several physicochemical properties; such as electrical conductivity, thermal analysis, EXAFS and Raman spectra, of ZnCl₂·nH₂O melt (n = 4/3 − 6) coexisting with the α-Al₂O₃ fine powder were measured. No transition point of the electrical conductivity was observed around the melting point of ZnCl₂ hydrate melt. The value of ΔE_a increased up to 50–70 kJ mol⁻¹, which is greater by ca. 40 kJ mol⁻¹ than that of the bulk phase. It is suggested that the interaction between the solid and liquid phases influences the phase transition of ZnCl₂ hydrate. The DSC measurement for the system consisting of α-Al₂O₃ powder and ZnCl₂·4/3H₂O showed a shift of the melting point toward lower temperatures down to −5 °C. A decrease of the molar enthalpy of the fusion, ΔH_m, with a decrease of melt content was observed in this system. The decreased ΔH_m is too small to detect the endothermic peak of melting for the system containing powder having a large specific surface area. In this case, a solidification of the hydrate melt was intensively prevented even below the melting point of the bulk system of the solution. However, such a variation was not observed for the system containing α-SiC. It is suggested that the Zn(II) dissolved ionic species are influenced by the solid phase, and also depend on the hydrophilicity of the solid surface. For the system containing ZnCl₂·nH₂O (n = 2.5 − 6), EXAFS spectra show that the dehydration promoted the coexistence of α-Al₂O₃ powder by an intensification of the ionic interaction between Zn(II) and Cl⁻ ions. For the system containing ZnCl₂·4/3H₂O, the structure of the highly aggregated complex was broken as shown in the Raman spectra.