Microscopic structure of liquid dimethyl sulphoxide and its electrolyte solutions: molecular dynamics simulations

Molecular dynamics (MD) simulations of pure dimethyl sulphoxide (DMSO) and solutions of Na+, Ca2+, Cl, NaCl and CaCl2 in DMSO have been performed at 2 98.15 K and 398.15 K in NVT ensembles by using a four-interaction-sitemodel of DMSO and reaction field method for Coulombic interactions. The structur e of solvent, ion-solvation shells and ion-pairs have been analysed by empl oying a concept of coordination centres and characteristic vectors of the s olvent molecule. Results are given for atom-atom (corresponding to DMSO), i on-atom and ion-ion radial distribution functions (RDFs), orientation of th e DMSO molecules and their geometrical arrangements in the first solvation shells of the ions (Na+, Ca2+, Cl). A preferential formation of cyclic dime rs with antiparallel alignment between dipole moments of nearest-neighbour molecules in the pure solvent is found. Geometrical models of the first coo rdination shells of the ions in 'infinitely dilute solutions' are proposed. Ion-ion RDFs in NaCl-DMSO and CaCl2DMSO solutions reveal the presence of b oth solvent separated (SSIP) and contact (CIP) ion pairs. The structures of the solvation shells of such ion pairs are also discussed.