Urea hydration from dielectric relaxation spectroscopy: Old findings confirmed, new insights gained

© the Owner Societies 2016. We report results on urea hydration obtained by dielectric relaxation spectroscopy (DRS) in a broad range of concentrations and temperatures. In particular, the effective hydration number and dipole moment of urea have been determined. The observed changes with composition and temperature were found to be insignificant and mainly caused by the changing number density of urea. Similarly, solute reorientation scaled simply with viscosity. In contrast, we find that water reorientation undergoes substantial changes in the presence of urea, resulting in two water fractions. The first corresponds to water molecules strongly bound to urea. These solvent molecules follow the reorientational dynamics of the solute. The second fraction exhibits only a minor increase of its relaxation time (in comparison with pure water) which is not linked to solution viscosity. Its activation energy decreases significantly with urea concentration, indicating a marked decrease of the number of H-bonds among the H2O molecules belonging to this fraction. Noncovalent interactions (NCI) analysis, capable to estimate the strength of the interactions within a cluster, shows that bound water molecules are most probably double-hydrogen bonded to urea via the oxygen atom of the carbonyl group and a cis-hydrogen atom. Due to the increased H-bond strength compared to the water dimer and the rigid position in the formed complex the reorientation of these bound H2O molecules is strongly impeded

Hydration and self-aggregation of a neutral cosolute from dielectric relaxation spectroscopy and MD simulations: The case of 1,3-dimethylurea

© the Owner Societies 2017.The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielectric relaxation spectroscopy. The experiment provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a separate slow-water relaxation in addition to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration numbers were calculated, showing that each 1,3-DMU molecule effectively freezes the reorientation of 1-2 water molecules. Additionally, a significant amount of solvent molecules, decreasing from ∼39 at infinite dilution to ∼3 close to the solubility limit, is retarded by a factor of ∼1.4 to 2.3, depending on concentration. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound H2O molecules. Molecular dynamics (MD) simulations of selected solutions revealed a notable slowdown of water rotation for those solvent molecules surrounding the methyl groups of 1,3-DMU and strong binding of ∼2H2O by the hydrophilic carbonyl group, corroborating thus the experimental results. Additionally, the simulations revealed 1,3-DMU self-aggregates of substantial lifetime

Complexation of the alkaline earth metals perchlorates with 3-hydroxyflavone in acetonitrile: Precise conductometric treatment

© 2017 Elsevier B.V. The alternative models of ionic equilibria in acetonitrile solutions of alkaline earth percholorates in the presence of 3-hydroxyflavone (HL) were studied by conductometry at 288.15, 298.15, 308.15, 318.15, and 328.15 K. Using a procedure specially developed for this task it was shown that in the solutions containing Ca(ClO 4 ) 2 and Sr(ClO 4 ) 2 the complexation of the cation (M 2 + ) by the HL molecule leads to the formation of the doubly-charged [M(HL)] 2 + complex species. In contrast to them, in the presence of Ba(ClO 4 ) 2 HL interacts with the ion pair BaClO 4 + forming the singly-charged [BaClO 4 (HL)] + complex. The limiting equivalent conductivities as well as the constants of complexation were estimated. The latter w ere calculated by taking into account the ion association between cation and anion of initial salts and activity coefficients, that is the ‘true’ thermodynamic constants were found. The reliability of the proposed approach was additionally checked by analyzing artificially noised model experimental data. It was shown that the fitted parameters can be satisfactorily reproduced even at the noise level equal to 2%. The limiting equivalent conductivities of the [Ca(HL)] 2 + , [Sr(HL)] 2 + and [BaClO 4 (HL)] + complex species were interpreted in terms of the Stokes radii whose values indicate very weak solvation of the formed complexes. The variation of the constants of complexation among cations was found to be in agreement with the values of primary association constants

Hydration and self-​aggregation of a neutral cosolute from dielectric relaxation spectroscopy and MD simulations: the case of 1,​3-​dimethylurea

The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielec. relaxation spectroscopy. The expt. provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a sep. slow-water relaxation in addn. to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration nos. were calcd., showing that each 1,3-DMU mol. effectively freezes the reorientation of 1-2 water mols. Addnl., a significant amt. of solvent mols., decreasing from ~39 at infinite diln. to ~3 close to the soly. limit, is retarded by a factor of ~1.4 to 2.3, depending on concn. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound H2O mols. Mol. dynamics (MD) simulations of selected solns. revealed a notable slowdown of water rotation for those solvent mols. surrounding the Me groups of 1,3-DMU and strong binding of ~2H2O by the hydrophilic carbonyl group, corroborating thus the exptl. results. Addnl., the simulations revealed 1,3-DMU self-aggregates of substantial lifetime

消化器内科この一年

© the Owner Societies 2017.The influence of the amphiphile 1,3-dimethylurea (1,3-DMU) on the dynamic properties of water was studied using dielectric relaxation spectroscopy. The experiment provided evidence for substantial retardation of water reorientation in the hydration shell of 1,3-DMU, leading to a separate slow-water relaxation in addition to contributions from bulk-like and fast water as well as from the solute. From the amplitudes of the resolved water modes effective hydration numbers were calculated, showing that each 1,3-DMU molecule effectively freezes the reorientation of 1-2 water molecules. Additionally, a significant amount of solvent molecules, decreasing from ∼39 at infinite dilution to ∼3 close to the solubility limit, is retarded by a factor of ∼1.4 to 2.3, depending on concentration. The marked increase of the solute amplitude indicates pronounced parallel dipole alignment between 1,3-DMU and its strongly bound H2O molecules. Molecular dynamics (MD) simulations of selected solutions revealed a notable slowdown of water rotation for those solvent molecules surrounding the methyl groups of 1,3-DMU and strong binding of ∼2H2O by the hydrophilic carbonyl group, corroborating thus the experimental results. Additionally, the simulations revealed 1,3-DMU self-aggregates of substantial lifetime

On the Solvation Behavior of Graphene Oxide in Ethylene Glycol/Water Mixtures

© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The self-association and solvation pattern of graphene oxide (GO) in water, ethylene glycol (EG), and their mixtures were analyzed by means of UV/Vis spectrophotometry. A careful analysis of the absorbance dependencies vs. the GO concentration shows that self-association of the GO sheets in EG occurs at higher concentration compared to that in water. It was established that depending on the mixed solvent composition, two different types of the GO solvates are formed. The results of quantum chemical calculations allow one to suggest that in the water-rich compositions, the GO oxygen-containing groups are in direct contact with water molecules while in the glycol-rich media the EG molecules fully substitute water in the GO's first solvation layer

The Interplay of Methyl-Group Distribution and Hydration Pattern of Isomeric Amphiphilic Osmolytes

© 2018 American Chemical Society. The intermolecular interactions and dynamics of aqueous 1,1-dimethyurea (1,1-DMU) solutions were studied by examining the concentration dependence of the solvent and solute relaxations detected by dielectric spectroscopy. Molecular dynamics simulations were carried out to facilitate interpretation of the dielectric data and to get a deeper insight into the behavior of the system components at the microscopic level. In particular, the simulations allowed for explaining the main differences between the dielectric spectra of aqueous solutions of 1,1-DMU and of its structural isomer 1,3-DMU. Similar to the previously studied compounds urea and 1,3-DMU, 1,1-DMU forms rather stable hydrates. This is evidenced by an effective solute dipole moment that significantly exceeds the value of a neat 1,1-DMU molecule, indicating pronounced parallel alignment of the solute dipole with two to three H2O moments. The MD simulations revealed that the involved water molecules form strong hydrogen bonds with the carbonyl group. However, in contrast to 1,3-DMU, it was not possible to resolve a "slow-water" mode in the dielectric spectra, suggesting rather different hydration-shell dynamics for 1,1-DMU as confirmed by the simulations. In contrast to aqueous urea and 1,3-DMU, addition of 1,1-DMU to water leads to a weak decrease of the static permittivity. This is explained by the emergence of antiparallel dipole-dipole correlations among 1,1-DMU hydrates with rising concentration

Urea hydration from dielectric relaxation spectroscopy: Old findings confirmed, new insights gained

© the Owner Societies 2016. We report results on urea hydration obtained by dielectric relaxation spectroscopy (DRS) in a broad range of concentrations and temperatures. In particular, the effective hydration number and dipole moment of urea have been determined. The observed changes with composition and temperature were found to be insignificant and mainly caused by the changing number density of urea. Similarly, solute reorientation scaled simply with viscosity. In contrast, we find that water reorientation undergoes substantial changes in the presence of urea, resulting in two water fractions. The first corresponds to water molecules strongly bound to urea. These solvent molecules follow the reorientational dynamics of the solute. The second fraction exhibits only a minor increase of its relaxation time (in comparison with pure water) which is not linked to solution viscosity. Its activation energy decreases significantly with urea concentration, indicating a marked decrease of the number of H-bonds among the H2O molecules belonging to this fraction. Noncovalent interactions (NCI) analysis, capable to estimate the strength of the interactions within a cluster, shows that bound water molecules are most probably double-hydrogen bonded to urea via the oxygen atom of the carbonyl group and a cis-hydrogen atom. Due to the increased H-bond strength compared to the water dimer and the rigid position in the formed complex the reorientation of these bound H2O molecules is strongly impeded

Densities, Viscosities, and Electrical Conductivities of Pure Anhydrous Reline and Its Mixtures with Water in the Temperature Range (293.15 to 338.15) K

Density (rho), dynamic viscosity (eta), and electrical conductivity (kappa) of the deep eutectic solvent (DES) reline, composed of choline chloride (ChCl) and urea in a 1:2 molar ratio, and its mixtures with water, covering the entire miscibility range, were studied at T = (293.15 to 338.15) K. Compared to many previous studies, reline purity was significantly improved by using ultrapure urea and ChCl recrystallized from ethanol. For the investigated DES samples the mass fraction of residual water was <0.00035. This allowed checking the influence of water traces and impurities on the physicochemical properties of pure reline. It was found that the presence of small amounts of water (w(H2O) < 0.0081) only negligibly decreased reline density, not exceeding 0.14% compared to the dry sample. However, for the same amount of water decreased by similar to 36% at 298.15 K. The temperature dependence of rho was well fitted by a quadratic expression, whereas eta(T) and kappa(T) were found to follow the empirical Vogel-Fulcher-Tammann equation. For the aqueous mixtures excess properties of molar volume (V-E) and viscosity (eta(E)) showed only minor variation with composition, suggesting rather weak interactions between water and the constituents of reline. However, V-E and eta(E) depended significantly on temperature, indicating a significant contribution of H-bonding to the inherent reline structure. Similar to conventional ionic liquids, the conductivity of aqueous reline showed a broad maximum at the reline mole fraction of x(1) approximate to 0.18 associated with the border between aqueous solutions of individual reline components and reline/water mixtures. The Walden plot classifies reline as a poor ionic liquid

Dielectric relaxation of deep eutectic solvent + water mixtures: structural implications and application to microwave heating

We studied the dielectric response of deep eutectic solvents (DESs) composed of choline chloride (ChCl) and such hydrogen bond donors (HBDs) as glycerol (glyceline) and urea (reline) mixed with water atT= 298.15 K and frequencies varying from 0.05 to 89 GHz. The dielectric loss data were used to calculate normalized heating rates for these systems upon electromagnetic irradiation at operating frequencies of domestic (nu= 2.45 GHz) and industrial (nu= 900 MHz) microwave ovens. We show that due to slow dynamics and substantial Ohmic-loss contributions DES/water mixtures constitute promising solvents for microwave synthesis. Their dielectric spectra can be best fit by a superposition of relaxation processes assigned to the reorientation of dipolar DES components and water molecules. Static permittivities were found to smoothly decrease from the value of neat water (78.4) to 22.8 for glyceline and 41.2 for reline. The analysis of the obtained relaxation amplitudes suggests that the studied systems can be viewed as mixtures of individual choline, HBD and water dipoles without pronounced dipole-dipole correlations and negligible ChCl ion pairs. However, rotational motions of the dipoles are partly synchronized, leading to the slow-down of 22 water molecules for glyceline and 9.2 for reline at infinite dilution. At vanishing DES concentration ChCl-HBD interactions appear to be negligible. Relaxation times as a function of viscosity show a break point at the ChCl : HBD : H2O ratio equal to 1 : 2 : 4. This supports the suggestion of a structural transition from homogeneous electrolyte solution to a micro-heterogeneous mixture already discussed in the literature