Liquid structure of the urea-water system studied by dielectric spectroscopy

Dielectric spectroscopy measurements for aqueous urea solutions were performed at 298 K through a concentration range from 0.5 to 9.0 M with frequencies between 200 MHz and 40 GHz. Observed dielectric spectra were well represented by the superposition of two Debye type relaxation processes attributable to the bulk-water clusters and the urea-water coclusters. Our quantitative analysis of the spectra shows that the number of hydration water molecules is approximately two per urea molecule for the lower concentration region below 5.0 M, while the previous molecular dynamics studies predicted approximately six water molecules. It was also indicated by those studies, however, that there are two types of hydration water molecule in urea solution, which are strongly and weakly associated to the urea molecule, respectively. Only the strongly associated water was distinguishable in our analysis, while the weakly associated water exhibited the same dynamic feature as bulk water. This implies that urea retains the weakly associated water in the tetrahedral structure and, thus, is not a strong structure breaker of water. We also verified the model of liquid water where water consists of two states: the icelike-ordered and dense-disordered phases. Our dielectric data did not agree with the theoretical prediction based on the two-phase model. The present work supports the argument that urea molecules can easily replace near-neighbor water in the hydrogen-bonding network and do not require the presence of the disordered phase of water to dissolve into water.