Unusual "amphiphilic" association of hydrated protons in strong acid solution

The solvation structure of the hydrated excess proton in concentrated aqueous HCl solution is studied using the self-consistent iterative multi-state empirical valence bond method. At 0.43-0.85 M concentrations, hydronium cations are found to form unusual cation pairs. This behavior is consistent with our earlier finding that hydronium cations can have an "amphiphilic" character due in part to the asymmetric nature of their hydrogen bonding to nearby water molecules. The existence of these hydronium amphiphilic pairs is further supported by a Car-Parrinello ab initio molecular dynamics simulation at 1.0 M HCl concentration. It is also found that the hydronium cation pairs are stabilized by a delocalization of the hydrated excess proton charge defects involving additional water molecules. At the higher concentrations of 1.68 and 3.26 M, the abundance of such hydronium pairs decreases, and the analysis of the radial distribution functions indicates the possible formation of an aggregate structure with longer-ranged order.