Solvation of solvophilic and solvophobic ions in dimethyl sulfoxide: microscopic structure by molecular dynamics simulations

Abstract

Molecular dynamics (MD) simulations of dimethyl sulfoxide (DMSO) solutions of Li⁺, Me₄N⁺, BPh₄⁻, big spherical ions of the same size but different charge, such as S⁺, S⁻, S⁰ have been performed at 298 K in NVT ensembles by using a four-interacting-sites model of DMSO and reaction field method for Coulombic interactions. Similar simulations were also performed on neat DMSO in which one DMSO molecule acted as a solute. The microscopic structures of ion-solvation shells have been analysed by employing a concept of co-ordination centres and characteristic vectors of the solvent molecule. Results are given for the atom–atom and ion–atom radial distribution functions (RDFs), orientation of the DMSO molecules and their geometrical arrangements in the first solvation shells of the ions. For the solvophilic Li⁺, a highly symmetric and well-pronounced first solvation shell (FSS) with fixed co-ordination number is observed. The co-ordination number and geometry of the FSS of lithium ion is strongly defined by the short-range non-Coulombic interactions between the ion and the surrounding DMSO molecules. The results show the importance of charge distribution in the solvent molecule and consequently the sign of ionic charge in creating local order around the solvated ion. It is found that the DMSO solvates S⁺ better than S⁻, which is better solvated than S⁰. The ‘solvophobic’ nature of the big multiatomic ions in non-aqueous media creates the possibility of the solvent molecules penetrating into the solute that is typically observed from our simulations not only for the charged species like Me₄N⁺ and BPh₄⁻, but also for the neutral solute represented by the DMSO molecule in neat DMSO.