Heats of Mixing in Liquid Alkali Nitrate Systems

The enthalpies of mixing in binary liquid alkali nitrate systems have been measured, and are all negative. It is found that the magnitude of the molar mixing enthalpy (ΔH^M) increases in a regular fashion with increasing difference in size between the two participating cations, according to the following approximate relation $${\mathrm{\Delta H}}^M{\text{≅−X(1−X)U[(d}}_1{\mathrm{-d}}_2{\mathrm{)/(d}}_1{\mathrm{+d}}_2{\mathrm{)]}}^2.$$ Here X and (1—X) are the mole fractions of the two components, U is the lattice energy of the mixture, while d₁ and d₂ are the interionic distances characteristic of the two pure components. The results are in accord with the view that in these systems the principal source of the enthalpy of mixing is the reduction in Coulombic repulsion between the second nearest neighbor cations. For binary systems with negative enthalpies of mixing of the order of RT there is evidence for short‐range order among the cations.