Collins’s rule, Hofmeister effects and ionic dispersion interactions

Models based solely on electrostatics cannot explain ion specific properties of electrolyte solutions. We give calculations of dispersion interactions of ions with other ions and with water molecules. These are done via ab initio Symmetry Adapted Perturbation Theory with Density Functional Theory (DFT–SAPT). The calculations establish the substantial, specific contribution of dispersion interactions to ionic interactions. They explain several puzzling properties of electrolyte solutions: the variation in solvation energy among ions of the same size, the small repulsion of iodide from the air–water interface, and the affinity of large ions for each other in water embodied in Collins’s rules.