The nature of dilute solutions of sodium ion in water, methanol, and tetrahydrofuran@f^a@f

Abstract

Monte Carlo statistical mechanics simulations have been carried out for dilute solutions of Na⁺ in water and tetrahydrofuran (THF) at 25 °C and 1 atm. The intermolecular interactions were described by Lennard-Jones and Coulomb terms in the TIPS format including the TIPS2 parameters for water-water interactions. In conjunction with previous simulation results for Na⁺ in methanol, the present study provides detailed insights into the nature of ionic solvation by dipolar protic and aprotic solvents. In agreement with x-ray data, the coordination number of Na⁺ in water is six, identical to the value obtained for Na⁺ in methanol. However, the coordination number of Na⁺ in THF fluctuates between five and six. Consistently, the ion-solvent interaction is less exothermic in THF compared to the other two solvents. However, the heat of solution which is the sum of two large opposing contributions, viz., the ion-solvent energy and the solvent reorganization energy, does not differ much for the three solvents. In all three cases, the formation of the first solvation shell accounts for all the solvent structure breaking. In particular, the number of hydrogen bonds formed by water and by methanol beyond the first shell rapidly attains the value in the pure solvent. The computed radial distribution functions and bonding and dimerization energy distributions provide additional support for this analysis.