Ion-solvent interactions in dimethylformamide + water mixtures

Standard Gibbs energies of transfer, ΔG°_t, of MCl (M = Li, Na, K, Rb and Cs) and K X (X = Br and I) have been determined by use of the double cells comprising M(Hg) and AgXAg electrodes in some aqueous mixtures of DMF at 25°C. These values were dissected into individual ion contributions using TATB reference electrolyte assumption, as obtained by measuring the solubility products of the salts viz. KPic, KBPh₄ and Ph₄AsPic. The observed increasingly positive magnitudes of ΔG°_t of the halide ions and the increased negative magnitudes of the cations including H⁺, reflect the well known anion-destabilization and the larger “basicity” and cationotropism of the aqueous mixtures of this co-solvent. And the observed distinctly flat regions of ΔG°_t curves of cations over an intermediate composition are attributable to strong intercomponent interactions. Comparison of the results with those in DMSO—water are ACN—water mixtures reveals that the solvating characteristics of DMF—water are somewhat similar to that of DMSO—water but different from that of ACN—water mixtures. Moreover, while the observed relative behaviour of Pic^? are found to be guided by the combined effects of dispersion interactions and anion destabilisation of these co-solvents, that of the large-sized tetraphenyl ion is, however, guided by the combined effects of dispersion interactions as well as the “cavity-forming” interactions as estimated in the light of scaled particle theory.