Ion—solvent interactions in acetonitrile + water mixtures

Standard Gibbs energies of transfer, Δ G° t of alkali metal chlorides (M = Li, Na, K, Rb and Cs) and potassium bromide and iodide from water to the aqueous mixtures of 20, 40, 60 and 80 mass per cents of acetonitrile (ACN) have been determined at 25°C from emf measurements performed on the double cell comprising AgAg X and M (Hg) electrodes. The individual ionic contributions to Δ G° t have also been evaluated using the reference electrolyte (RE = Ph 4AsBPh 4) method, the required Δ G° t 's of the RE being obtained from the measured solubilities of salts viz. Ph 4AsPic, KBPh 4 and KPic (Pic = picrate and Ph = phenyl). The observed increasing Δ G° t values of the halide ions X − and their order Cl − ⪢ Br − ⪢ I − furnish the thermodynamic evvidence for the effect of the well known decreased H-bonding and the anion destabilizing propensities of dipolar aprotic cosolvent ACN. But the observed shallow minima in Δ G° t -composition profiles for M + and H + are indicative of the result of the oppositg effects of water structure breaking propensities and the protophobic character of ACN and their relative order is the combined effects of acid—base, Born-type and soft—soft interactions. Moreover, while the distinctly pronounced stabilization of large tetraphenyl ionshas been ascribed as the combined effects of dispersion and cavity-forming interactions, the less pronounced solvation of Pic − has been attributed to the combined effects of increased dispersion interactions of benzene nucleus and of decreased H-bonding interactions of O-atoms of the substituents. These contentions have been further substantiated by comparing Δ G° t ( i) values of some selected ions in ethylene glycol—water mixtures.