Thermodynamics of H-D isotope effects in the hydration of urea and structural features of its aqueous solutions at different temperatures: II. Compessibilities of the systems water-urea and heavy water-deuterated urea

The molar and excess molar isoentropy compressibilities of urea in water an d deuterated urea in heavy water were estimated on the basis of the ultraso und velocities in the solutions at 278.15, 283.15, 298.15, 318.15, and 338. 15 K in the range of concentrations from 0 (including infinite dilution) to 1.5 c(sm,2). The hydration numbers, compressibilities, and volumes of the H/D-urea isotopomers and their hydrate shells in H2O and D2O were estimated . In the temperature range below 300 K, destructurization of water by the a ction of the dissolved molecules is observed, whereas above 318 K the syste m heavy water-deuterated urea behaves like an ideal with respect to pressur e; the effect of urea on the solvent structure extends beyond the first hyd rate shell. Both the compressibility and the molecular volume of crystallin e urea decrease as a result of H/D-isotope exchange.