Preferential solvation in {amide + alcohol} binary mixtures. Part 2. The N-methylpyrrolidinone + methanol mixture at T = 313.15 K: thermodynamic results and molecular dynamics calculations

Abstract

The preferential solvation of N-methylpyrrolidinone by methanol has been investigated by two complementary methods. First, the thermodynamic measurements of excess Gibbs energies and excess volumes of mixing are reported for the {N-methylpyrrolidinone + methanol} binary mixture at T = 313.15 K. The Kirkwood–Buff theory of solutions was used to interpret the thermodynamic data, and the results are compared with those for other amides: N-methylformamide, N,N-dimethylformamide, N-methylacetamide, N,N-dimethylacetamide and 2-pyrrolidinone. It was found that the presence of amide hydrogen (in the N–H group) scarcely influences the local mole fractions. Moreover, even the large hydrocarbon part in pyrrolidinones changes values of the local mole fractions only slightly. It was found also that the general picture of solvation of amide by alcohol is nearly the same for methanol and for ethanol. Second, the molecular dynamics calculations for the {N-methylpyrrolidinone + methanol} binary mixture was performed, and results of calculations were compared with the thermodynamic data. From calculated radial distribution functions the solvation shell radius was estimated and values of the Kirkwood–Buff integrals were evaluated. Moreover, the formation of hydrogen-bonded complexes was investigated, the lifetime of created N-methylpyrrolidinone–methanol complexes was estimated and compared with literature data. From the obtained results we deduce that alcohol forms a dynamic cage around the amide molecule. The general picture obtained from these calculations is fully consistent with the thermodynamic results, and complements the “thermodynamic” point of view on the solvation of amides by alcohol molecules.