Solvation of amide group by water and alcohols investigated using the Kirkwood–Buff theory of solutions

Abstract

The Kirkwood–Buff theory of solutions has been used to investigate the preferential solvation of N-methylformamide by water and alcohol. Binary mixtures (N-methylformamide–A), where A = water, methanol or ethanol and ternary mixtures (N-methylformamide–alcohol–water), were studied at 313.15 K. The results are compared with those obtained previously for analogous mixtures containing N,N-dimethylformamide at the same temperature. It was found that the differences between the local mole fractions of water or alcohol in the immediate vicinity of the amide molecule and the bulk ones can be mainly explained by the differences in molecular sizes, the deviations are only a few per cent. From the calculation of the entropy change resulting from the orientation effects it follows that for amide–alcohol mixtures, for the investigated amides, this entropy change is positive, increasing systematically in the order: methanol<ethanol<propanol. For the N,N-dimethylformamide–water mixture the orientational entropy is strongly negative, and for N-methylformamide–water this value is expected also to be strongly negative. For ternary mixtures containing amide, alcohol and water, preferential hydration is weak and it is observed in the alcohol-rich region only. Both the amides are solvated in the mixture in a very similar manner, which means that replacing a methyl group by hydrogen and exposing the lone electron pair on the nitrogen atom to interactions with water or alcohol molecules shows insignificant influence on the local mole fractions of the components around the amide molecule and on the solvation process.