MD Simulation Studies of Selective Solvation in Methanol−Water Mixtures:  An Effect of the Charge Density of a Solute

Abstract

MD simulations have been performed for NaCl, NaI, and fictitious solutions of discharged counterparts of Na + and Clions to investigate an effect of the charge density of a solute on its solvation in methanol-water mixtures. Solvent-solvent interactions have been described in terms of flexible models, whereas solutes have been considered as charged or uncharged Lennard-Jones spheres. An analysis of solvation shells has been based on radial distribution functions, angular distributions, coordination numbers, and residence times of solvent molecules. Preferential solvation of anions by methanol molecules becomes less pronounced with decreasing charge density of a solute and vanishes for the discharged chloride ion. In contrast to preferential hydration of Na + in water deficit solvents, its uncharged counterpart Na 0 is preferentially solvated by methanol molecules over the whole range of solvent composition. Results for NaCl solution have been compared with those obtained with ab initio ion-solvent potentials and the same model of solvent molecules. The type of ion-solvent potential has small effect on the structure and composition of ionic shells, but its influence on the persistence of the coordination shells is more noticeable.

MD Simulation of the Ion Solvation in Methanol-Water Mixtures

“…These potentials overestimate, however, the number of the solvent www.intechopen.com molecules in the ion shells (Hawlicka & Swiatla-Wojcik, 2002) and underestimate a stability of the ion shells (Hawlicka & Swiatla-Wojcik, 2002, Bujnicka & Hawlicka, 2006. Moreover such potentials are inconsistent with flexible models of the solvent molecules.…”

Ratiometric detection of Raman hydration shell spectra

“…[73] Figure 5a shows the RDF between the oxygen atom of water molecules and the nitrogen atom of NO 3 À . The RDF shows a minimum at 4.45 Å.…”

Influence of CaCl2 on microheterogeneity of methanol–water mixtures

“…In methanol-water mixtures the Ca 2+ ions are preferentially hydrated and these water molecules prefer the antidipole orientation towards the cation spheres, what favours them as H-donors [17]. Preferential solvation of the Cl À ion by the methanol molecules favours them as H-acceptors [16]. In consequence the selective solvation of both ions preserves the H-bonds between the H-acceptor methanol molecule and the H-donor water molecule.…”

“…Parameters A ij , B ij and C ij , employed to describe non-Coulomb interactions have been presented previously [24]. The parameters do not have any physical meaning, but they reproduce reasonably the solvation of calcium [17] and chloride [16] ions in the methanol-water mixture. The starting configurations were obtained by a random displacement of particles in the cube.…”

“…Thus one may expect that electrolytes may also enhance a formation of methanol and water clusters. It is worthy to notice that in the methanol-water mixtures Ca 2+ and Cl À ions are preferentially solvated [16,17]. This solvation is selective and opposite, i.e.…”