Structure of concentrated aqueous NaCl solution: A Monte Carlo study

The structure of aqueous NaCl 1 M solution was studied by Monte Carlo simul ation data obtained with 2117 rigid water molecules 40 Na+ and 40 Cl- at 29 3 K. The structural features were determined from the usual radial distribu tion 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 struc ture is almost not affected by the finite concentration. The second shell i s concentration dependent mainly with respect to the tilt angle. The cation hydration is less affected than the anion hydrated shell. Anion-cation, an ion-anion and cation-cation pairs were detected. A fraction 0.46 of the cat ions and 0.32 of the anions are associated in pairs formed by two ions shar ing, at least, two water molecules in their solvation shells. The idealized structures of all pairs include two to four water molecules located in suc h a way that the attractive energies are increased and the repulsive ones d ecreased. The structure of 1 M NaCl is also determined by recovering the wa ter tetrahedral structure around the solute ions and by the stabilizing inf luence 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 becaus e they solvate directly the solute. Almost all the water molecules are invo lved to some degree in the ion solvation. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)51206-3].