Preferential solvation of N-methylformamide, N,N-dimethylformamide and N-methylacetamide by water and alcohols in the binary and ternary mixtures

Using the Kirkwood-Buff theory of solutions, the preferential solvation of the N-methylacetamide molecule has been investigated in the binary and ternary mixtures containing N-methylacetamide (NMA), aliphatic alcohol and water. The results are compared with those obtained previously for N-methylformamide (NMF) and N,N-dimethylformamide (DMF). The thermodynamic investigations, based on the Kirkwood-Buff theory of solutions, lead to the unexpected conclusion that both NMA and DMF are solvated in the investigated binary and ternary mixtures in a very similar manner, but solvation of NMF differs from other amides. For all the investigated amides, the local mole fractions differ only slightly from the bulk ones-the deviations are only a few per cent or less. Moreover, for the {amide+methanol} binary mixtures, where amide=NMF, DMF and NMA, molecular dynamics calculations at x(amide)=0.518 were performed. From the obtained molecule-molecule radial distribution functions (rdf) and atom-atom rdf, it was possible to estimate the local mole fractions around the amide molecule, the orientation effects of molecules within the solvation shell, and a possibility of the formation of complexes. The general picture obtained from analysis of the molecular dynamics results is consistent with the deductions derived from thermodynamic data.