Investigation of intermolecular interactions in solutions by means of millimeter-wave spectroscopy 4. Negative and positive hydration in aqueous solutions of urea

Measurements have been made of the absorption of EHF radiation ( 5 cm^–1) by aqueous solutions of urea at 20–70°C, over the entire interval of urea solubility. Excess absorption has been found in comparison with the additive magnitude, and this has been attributed to an increase in the fraction of water molecules in the solution with rotational degrees of freedom. A model has been proposed for the hydration of urea, based on the concept of inhomogeneity of the hydrate shell of urea: stabilization of the structure of water under the influence of carbonyl oxygen and H_cis atoms (positive hydration) and a simultaneous breakdown of the structure of water in the region of two H_trans atoms that have approached to a distance of 2.14 » (negative hydration).For Previous Communication, See [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 314–320, February, 1990.