Pressure dependence and activation volume for the water exchange mechanism in NaCl(aq) from MD simulations

The influence of an external pressure on the solvent exchange mechanisms around the Na+ ion in a dilute NaCl(aq) solution has been investigated through MD simulations at 0 and 200 MPa. At zero pressure most of the exchange processes (approximate to 75%) can be classified in terms of associative 6-7-6 mechanisms. At 200 MPa the exchange rate is 20% higher and the mechanistic processes become more involved. The activation volume for the 6-7-6 exchange process at zero pressure was calculated from the MD simulation using Voronoi polyhedra, and gives a negative sign for Delta V-#. This supports the prevailing notion that a negative activation volume is indicative of an associative exchange mechanism. (C) 1997 Published by Elsevier Science B.V.