SOLVATION OF DMF IN THE N,N-DIMETHYLFORMAMIDE PLUS ALCOHOL PLUS WATER MIXTURES INVESTIGATED BY MEANS OF THE KIRKWOOD-BUFF INTEGRALS

Using Kirkwood and Buffs theory of solutions, the local compositions in the N,N-dimethylformamide solvation microsphere have been estimated. The investigated mixtures are DMF+A+water, where A = methanol, ethanol, and n-propanol, at the temperature 313.15 K. It has been found that in the DMF-rich region, at X(DMF) > 0.8, the solvation process is governed by the entropy term mainly, and deviations of the local mole fractions from the bulk ones are very small. At low DMF mole fractions, below approximately, 0.15 the preferential hydration of DMF molecules is observed. At very low DMF concentrations, lower than 0.001 in mole fraction scale, the effect of preferential hydration is observed in the range 0.2-1.0 alcohol mole fraction; in the water-rich region (above a water mole fraction of 0.8) solvation of the DMF molecules is nearly random. The limit between both these regions is clearly visible for propanol and ethanol and slightly visible for methanol. Since the local mole fractions reflect the influence of two main factors, differences in molecular sizes and differences in the intermolecular interactions, then analyzing these values, we may estimate the second mentioned factor. This allows ordering the water and alcohol molecules according to their increasing acidity toward the oxygen atom of the amide group as water approximate to methanol > ethanol > n-propanol, with the close proximity of water and methanol in this ranking.