Enthalpies of Mixing in Some Binary Liquid Halide Mixtures

The enthalpies of mixing in the binary liquid alkali chloride, bromide, and iodide mixtures in the mixtures of silver chloride with the alkali chlorides and thallium chloride and in the mixture of silver bromide with thallium bromide have been measured by means of high-temperature reaction calorimetry. The results are discussed in terms of the conformal solution theory for fused-salt mixtures of Reiss, Katz, and Kleppa and the recent perturbation theory of Davis and Rice. For the binary alkali halides, the molar enthalpies of mixing are represented by the following approximate relation: HM=N1N2(U0++−340δ122) kcal/mole. Here N1 and N2 are mole fractions and δ12=(d1−d2)/d1d2, where d1 and d2 are the sums of the ionic radii for the two salts. The positive term U0+ + differs from system to system. It represents a theoretical estimate of the contribution to the mixing enthalpy arising from the dispersion energy between next-nearest-neighbor cations, calculated by the method of Mayer. The above relation does not hold for the silver halide mixtures, for which the corresponding semiempirical relation is: HM=N1N2(U0+++40δ12−340δ122) kcal/mole. This expression is of the form predicted by the perturbation theory of Davis and Rice. However, a comparison of the silver chloride systems with the previously explored silver nitrate—alkali nitrate mixtures indicates that it is difficult to account for the results in terms of ionic and dispersion interactions alone without invoking also a small degree of covalent bonding in the silver salts.