MIXED ALKALI EFFECT IN HYDRATE MELTS

Electrical conductivities, densities, and glass transition temperatures have been measured as a function of composition in the hydrate melt systems XNaNO3-(0.2 - X) MNO3-0.8Ca(NO3)2 • 4.09H2O for M = K and Tl. Unlike mixed monovalent cation fused salts and network oxide glasses and melts, in the experimental range of -5° to 70'C no minima were observed in the isotherms of equivalent conductance A for the Na/K hydrate melts and weak minima (~3%) only at the lowest temperature for the Na/Tl hydrate melts. A review of data and theories currently in the literature leads to the conclusion that a strong mixed alkali effect, i.e. minimum in the A isotherm, should be observed only if the time scale for rearrangement of the local liquid structure is very long compared to the time scale for local ionic transport. This condition is not met by the hydrate melts in the experimental temperature range. It should, however, be met at lower temperatures, and extrapolation of the data indicates that the hydrate melt A isotherms develop appreciable minima at such lower temperatures. © 1979, The Electrochemical Society, Inc. All rights reserved.