Solvation of N-methyl-2-pyrrolidone and N,N-dimethylpropanamide in cyclohexane, heptane, n-alkan-1-ols(C1–C4) and water at 298.15 K

Enthalpies of solution, densities and sound velocities of dilute solutions of N-methyl-2-pyrrolidone (NMP) and N,N-dimethylpropanamide (DMPA) in cyclohexane, heptane, n-alkan-1-ols(C1–C4) and water were measured at 298.15 K under atmospheric pressure. Excess partial molar enthalpies, excess partial molar volumes and partial molar isentropic compressibilities of these amides at infinite dilution were evaluated to study the solvation of NMP and DMPA in these solvents. Excess partial molar enthalpies of NMP and DMPA at infinite dilution in n-alkan-1-ols as well as in saturated hydrocarbons reveal that both tertiary amides are stable in n-alkan-1-ols and the extent of the stabilization ranges from 8 kJ mol−1 to 15 kJ mol−1. On the other hand, the excess partial molar enthalpies of NMP and DMPA at infinite dilution in water suggest that both NMP and DMPA form 1:2 hydrogen-bonded aggregates with water molecules. The excess partial molar volumes and partial molar isentropic compressibilities of NMP and DMPA at infinite dilution in saturated hydrocarbons and n-alkan-1-ols strongly depend on the free volumes of components as well as the enthalpic stability of the solutions. DMPA, having the hydrophobic single bondCH2single bond groups and conformational flexibility, plausibly changes its conformation to ones that are more compact, and reduces its partial molar volume at infinite dilution in water than in organic solvents.