The densities of aqueous solutions of most alkali halides have been measured at 25° to the sixth decimal place by a float method. In the concentration range 0.03 to 0.5 M, the derived apparent molal volumes can be fitted to the equation Φv = Φv° + 1.86c1/2 + hc The h parameters increase with the size of the cations for all the halides with the exception of the fluorides where the opposite order is observed. The variations in the h parameter and the corresponding deviations from the limiting Debye-Hückel theory for activity coefficients can be interpreted qualitatively with a structural interaction model. Essentially, two solutes will attract each other if their structural influences, or their tendencies to orient water molecules, are compatible with each other; conversely, an incompatibility in these influences or tendencies will result in repulsive forces. This model can also account for most excess thermodynamic functions of tetraalkylammonium halides.
