Non-ideality in Born-free energy of solvation in alcohol–water and dimethylsulfoxide–acetonitrile mixtures: Solvent size ratio and ion size dependence

Recent extension of mean spherical approximation (MSA) for electrolyte solution has been employed to investigate the non-ideality in Born-free energy of solvation of a rigid, mono-positive ion in binary dipolar mixtures of associating (ethanol–water) and non-associating (dimethylsulfoxide– acetonitrile) solvents. In addition to the dipole moments, the solvent size ratio and ion size have been treated in a consistent manner in this extended MSA theory for the first time. The solvent–solvent size ratio is found to play an important role in determining the non-ideality in these binary mixtures. Smaller ions such as Li^ + and Na ^+ show stronger non-ideality in such mixtures compared to bigger ions (for example, Cs^ + and Bu4N ^+ ). The partial solvent polarization densities around smaller ions in tertiary butanol (TBA)– water mixture is found to be very different from that in other alcohol–water mixtures as well as to that for larger ions in aqueous solutions of TBA. Non-ideality is weaker in mixtures consisting of solvent species possessing nearly equal diameters and dipole moments and is reflected in the mole fraction dependent partial solvent polarization densities.