Calorimetric study of dilute aqueous solutions of dialkylamines: hydration of alkyl groups

Abstract

Enthalpies of solution of a series of amines R₂NH, where R = Et, Pr, i-Pr, Bu, i-Bu and s-Bu, have been measured at 5, 15 and 25° in a concentration range (m < 0.08) which corresponds to infinite dilution. Vapour pressures of the amines have been determined over the temperature range 0–60° and latent heats of evaporation calculated. Enthalpies, free energies, entropies and heat capacities associated with the transfer of amines from the vapour to an infinitely dilute aqueous solution have been obtained and are compared with similar functions for alkanes and alcohols. There is no evidence for constant enthalpy increments for each CH₂ group added to an alkyl chain. Entropies of hydration are discussed in terms of possible changes in the internal entropy of the solute molecule and in the nature of the solvent-solvent interactions.