Can we understand (and model) aqueous solutions without any long range electrostatic interactions?

A computer simulation experiment has been conducted to study the extent to which long range Coulombic interactions are indispensable when modelling aq ueous solutions of electrolytes. A simple molecular model, which accounts e xplicitly for the molecular structure of water but which does not incorpora te any long range Coulombic interactions is employed. The solvent is primit ive water (EPM5-4 model) and the solute molecules are hard spheres interact ing with the interaction sites of the water molecule by means of either rep ulsive (like-charge interaction) or attractive (unlike-charge interaction) short range triangular-well tails. The structural changes (hydrophobic orde ring, structure breaking, and structure enhancement) which take place in an infinitely dilute solution upon 'charging' the solute were studied, in ter ms of the correlation functions and of the orientational distribution funct ions and of the average binding energy of the water molecules around the so lute in terms of their dependence on the solute-water oxygen distance. The main thermodynamic property reflecting these changes is the residual entrop y. This quantity is found to exhibit an asymmetric double maximum, in agree ment with the findings for a realistic counterpart of this simple model tha t employs long range Coulombic interactions.