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

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 ΔEa 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, ΔHm, with a decrease of melt content was observed in this system. The decreased ΔHm 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.