Preferential solvation in (amide + alcohol) binary mixtures. Part 1. The N-methylpyrrolidinone + ethanol mixture at T = 313.15 K: thermodynamic results and molecular dynamics calculations

Abstract

The preferential solvation of N-methylpyrrolidinone by ethanol was investigated by two complementary methods. First, the thermodynamic measurements of excess Gibbs energies and excess volumes of mixing are reported for {N-methylpyrrolidinone + ethanol} 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-dimethyl-acetamide and 2-pyrrolidinone. It was found that the presence of amide hydrogen (in the N–H group) nearly does not influence the local mole fractions. Similarly, even large hydrocarbon part in pyrrolidinones only slightly changes the local mole fractions. Second, the molecular dynamics calculations for the {N-methylpyrrolidinone + ethanol} binary mixture were performed, using various sets of parameters. The results obtained were compared with the thermodynamic data for this system, and with deductions derived from the experimental data using the Kirkwood–Buff theory of solutions. From calculated radial distribution functions, the solvation shell radius was estimated, and values of the local mole fractions were evaluated. A simple procedure for calculating the Kirkwood–Buff integrals from molecular dynamics results was proposed and examined. Moreover, the formation of hydrogen-bonded complexes was investigated; the lifetime of the N-methylpyrrolidinone–ethanol complexes created was estimated. We suppose that ethanol forms a dynamic cage around the amide molecule, and the mean lifetime of this cage was estimated. The general picture obtained from these calculations is consistent with the thermodynamic results and complements the “thermodynamic” point of view on the solvation of amides by ethanol molecules.