Liquid Structure of the Urea−Water System Studied by Dielectric Spectroscopy

Abstract

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.

High-precision gigahertz-to-terahertz spectroscopy of aqueous salt solutions as a probe of the femtosecond-to-picosecond dynamics of liquid water

“…In contrast to prior studies of aqueous salt solutions 41,44,47,48 , the time constants of the relaxations we observe are effectively independent of salt concentration over the entire concentration range we have explored (Fig. 3, top).…”

Protein–solvent interaction in urea–water systems studied by dielectric spectroscopy

“…2(a), which shows the high frequency dielectric spectra of BSA solutions at various urea concentrations. This dielectric loss peak is attributed to water and urea molecules based on the observation of similar behavior in urea-water solutions without proteins [12] as shown in Fig. 2(b).…”

“…Our previous study for urea-water systems without protein showed that the complex permittivity of a urea-water system can be described by two Debye processes or one Havriliak-Negami (HN) process, but it cannot be described by one CC process [12]. After more detailed data analysis, we concluded that the description by two Debye relaxation processes is more suitable than that by one HN process.…”

“…These facts indicate that conventional data analysis without a particular model is difficult for the present system. Therefore, we made the following two assumptions: (1) The introduction of the BSA protein does not alter the relaxation times that are observed for the urea-water system without BSA [12]. This implies the clusters around the BSA molecules have relatively non-uniform dynamical structures.…”

“…In particular, there is a lack of detailed discussion on the relevance of c-dispersion in protein-urea-water systems. Our previous work on urea-water solutions without protein showed clear changes in the dielectric responses with increased urea concentration, and those data allowed us to discuss the dynamic features of urea-water clusters [12]. In the present work, dielectric spectroscopy measurements were carried out on bovine serum albumin (BSA) dissolved in urea-water solutions at various urea concentrations.…”