Equilibrium and transport properties of Ca(NO₃)₂ + (Li,K)NO₃ hydrate melts: Confirmation of a mixed-alkali effect

Abstract

Glass-transition temperature, density, electrical conductivity and viscosity have been determined as a function of the relative proportions of the two alkali metal nitrates in the hydrate melts

               0.55[Ca(NO₃)₂,RH₂O]+0.45[XLiNO₃+(1-X)KNO₃]

where X is the mole fraction of LiNO₃ relative to total alkali metal nitrate, with mole ratios (R) of water to Ca(NO₃)₂ of 4.090 and 6.545, at 298.15 K. For both series of melts, molar volume varies linearly with X, i.e. is an additive function of composition. Glass-transition temperature and molar conductivity show negative deviations from additivity, the magnitude of the deviations decreasing with increased water content. Fluidity isotherms show much smaller negative deviations from additivity and the magnitude of the deviations is approximately independent of water content.    

By analogy with the properties of network oxide melts and glasses, the composition variation of the properties investigated for the hydrate melts is interpreted as being indicative of a significant mixed-alkali effect qualitatively similar to the effect which occurs in network oxide media. The hydrate melts are closely similar in their behaviour to fused anhydrous (Na,Tl)NO₃ mixtures, and it is suggested that the observed trends in the properties of the hydrate melts have a similar origin to that which was postulated for the anhydrous melts.