Water exchange around Li+ and Na+ in LiCl(aq) and NaCl(aq) from MD simulations

The solvent exchange mechanisms around alkali cations in dilute aqueous solution have been investigated from cation-oxygen distance vs time curves, molecular animations, activation volumes, and lifetime statistics. Constant pressure (0 atm)-constant temperature (300 K) MD simulations were performed for two LiCl(aq) solutions and one NaCl(aq) solution. The two LiCl(aq) solutions had different ionic charges, +/-1.0 and +/-0.8. The TIP4P water-water potential combined with Bounds's icon-water potentials were used. Most of the exchange processes can be classified in terms of dissociative (D), associative (A) or interchange type mechanisms: I-D, I, or I-A Many exceptions are found in the NaCl(aq) solution, where similar to 10% of the exchanges are of a more elaborate nature, although the majority are associative. Around the lithium ion, the exchanges are dissociative (D and I-D). The exchange rate in NaCl(aq) is much higher than in LiCl(aq). The activation volumes for the 6-5-6 [LiCl(aq)] and 6-7-6 [NaCl(aq)] exchange processes were calculated by two methods: from cation-H radial density distribution curves for the 5-, 6-, and 7-complexes, and from the volumes of Voronoi polyhedra. Both methods give a negative Delta V double dagger for the associative process and a positive Delta V double dagger for the dissociative process, supporting a mechanistic interpretation of the sign of Delta V double dagger.