Thermal Expansion and Structure of 1,3-Dimethylurea, Tetramethylurea, and Tetrabutylammonium Bromide Aqueous Solutions Derived from Density Measurements

Abstract

On the basis of density measurements, the determination of apparent and partial molar volumes of the solutes in the title solutions have been carried out previously at and around room temperatures both in H2O and D2O solutions as a function of composition. They showed minima at intermediate compositions, and a number of other properties of these solutions also indicated radical structural changes and differences versus composition. On this basis, we have chosen these solutions to analyze also the solvent partial molar volume and its thermal expansibility, which were not involved in the earlier studies, to these structural changes and differences. To obtain reliable thermal expansibility results, the density measurements were extended down to low temperatures. High precision density measurements of the normal water solutions were carried out in wide temperature [(274.15 to 308.15) K] and molality, m [(0.05 to 3 < 6) mol·kg−1], ranges at closely spaced temperature intervals. From the measured densities, both the solute and the solvent partial molar volumes and their thermal expansibilities were calculated. The results are analyzed in terms of the cage model of the hydrophobic effect, and they are correlated to the solvent excess enthalpy and entropy and to their H2O + D2O isotope effects. Although all the title solutions show dominant “interstitial” solution character, their structure is determined by a variety of structural equilibriums, depending mainly on the polar/apolar balance of the “mixed” solute molecules, on their H-bond donor/acceptor ability, and on the solute/solvent ratio.