Structure of concentrated aqueous NaCl solution: A Monte Carlo study

Abstract

The structure of aqueous NaCl 1 M solution was studied by Monte Carlo simulation data obtained with 2117 rigid water molecules 40 Na+ and 40 Cl− at 293 K. The structural features were determined from the usual radial distribution functions, angular-radial distribution functions, angular correlations and the mean force potential. The ion hydration structure was obtained as such as the structure of ions associations. The first hydration shell structure is almost not affected by the finite concentration. The second shell is concentration dependent mainly with respect to the tilt angle. The cation hydration is less affected than the anion hydrated shell. Anion–cation, anion–anion and cation–cation pairs were detected. A fraction 0.46 of the cations and 0.32 of the anions are associated in pairs formed by two ions sharing, at least, two water molecules in their solvation shells. The idealized structures of all pairs include two to four water molecules located in such a way that the attractive energies are increased and the repulsive ones decreased. The structure of 1 M NaCl is also determined by recovering the water tetrahedral structure around the solute ions and by the stabilizing influence of water molecules that are shared by the ions. The last conclusion is that about 20% of the water molecules lose some degree of freedom because they solvate directly the solute. Almost all the water molecules are involved to some degree in the ion solvation.