Water Exchange around Li+ and Na+ in LiCl(aq) and NaCl(aq) from MD Simulations

Abstract

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 ion−water potentials were used. Most of the exchange processes can be classified in terms of dissociative (D), associative (A) or interchange type mechanisms: ID, I, or IA. Many exceptions are found in the NaCl(aq) solution, where ∼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 ID). 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 ΔV⧧ for the associative process and a positive ΔV⧧ for the dissociative process, supporting a mechanistic interpretation of the sign of ΔV⧧.