Glass-Forming Composition Regions and Glass Transition Temperatures for Aqueous Electrolyte Solutions

Abstract

As a first stage in a study of the low‐temperature‐region behavior of aqueous solutions, the composition regions of a large number of binary salt‐water systems in which macroscopic samples can be obtained in the vitreous state have been determined. The glass transition temperatures $T_g$ within these regions have also been measured. Correlations of the magnitude of $T_g$ at a given composition with the formal charge on the cation (for salts with a common anion) and with the viscosity “$B$ ” coefficient of the anion (for salts with a common cation) are presented. In addition, an important correlation is found with the basicity of the anion which is interpreted in terms of the bonding of anions to the protons of the water molecules in the cationic hydration sheath. Thermodynamic relations for the composition dependence of $T_g$ are presented. From the breaks in the otherwise monotonic composition dependence of $T_g$ in the chloride systems, the existence of low temperature liquid–liquid immiscibility regions is proposed and is utilized in discussion of “total” hydration numbers. The $T_g$ for pure water, obtained by extrapolation of the data, is found to be in agreement with the value obtained for vapor‐deposited vitreous ice but to be incompatible with the value predicted from thermodynamic data; an interpretation in terms of different short‐range order possibilities for water molecules is offered.