Alkali metal chlorides in DMSO–methanol binary mixtures: insights into the structural properties through molecular dynamics simulations

We have performed a series of constrained molecular dynamics simulations to study the ion-pair solvation of alkali metal chlorides (Li+Cl−, Na+Cl−, K+Cl− and Cs+Cl−) in dimethyl sulfoxide (DMSO)–methanol (MeOH) binary mixtures. Structural and dynamical properties of the ionic association are studied using the potentials of mean force (PMFs), spatial density distribution functions, hydrogen bonding, excess coordination number and angular distribution functions. For Li+Cl− and Na+Cl− ion pair, three minima in the PMFs are observed corresponding to contact ion pairs (CIPs), solvent-shared ion pairs (SShIPs) and solvent-separated ion pairs, whereas for K+Cl− and Cs+Cl− ion pair only CIPs are present and instead of a SShIP minima, a plateau-like region is present. The CIPs are found to be most stable in pure DMSO. An increment in the average number of hydrogen bonds with an increase in the mole fraction of methanol is observed in the bulk as well as in the solvation shell of the ions. Structural analysis indicates that all the alkali metal cations are selectively solvated by methanol in all the compositions except that for \(x_{{{\text{methanol}}}}\) = 0.25, whereas Cl− ion is preferentially solvated by methanol irrespective of mixture composition and the cationic counterpart.