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

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

Is ethaline a deep eutectic solvent?

With the present contribution we clarify the phase behaviour of choline chloride (ChCl) + ethylene glycol (EG) mixtures for ChCl mole fractions (x(ChCl)) less than 0.333 and temperatures below 323 K by providing melting points obtained by differential scanning calorimetry for samples containing <300 ppm of water. We show that ethaline, the ChCl : EG mixture of molar ratio 1 : 2 that is usually believed to be the composition of the eutectic point, actually lies in the ChCl-saturated region of the {ChCl + EG} phase diagram. The real eutectic point was found to be at the 1 : 4.85 molar ratio of ChCl : EG (x(ChCl) = 0.171) which is characterized by a melting point of 244 K. This temperature is only 16 K below the melting point of neat EG. Thus, neither from its particular composition nor from the observed melting-point depression of {ChCl + EG} mixtures can ethaline be considered a "deep eutectic solvent". Surprisingly, despite ChCl being an electrolyte dissolved in EG, the phase diagram is that of an ideal binary mixture

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

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

Variation of Density, Viscosity, and Electrical Conductivity of the Deep Eutectic Solvent Reline, Composed of Choline Chloride and Urea at a Molar Ratio of 1:2, Mixed with Dimethylsulfoxide as a Cosolvent

We investigated the variation of density (rho), dynamic viscosity (eta), and electrical conductivity (kappa) of mixtures of the deep eutectic solvent (DES) reline, composed of choline chloride and urea at a molar ratio of 1:2, with dimethylsulfoxide (DMSO) covering the entire miscibility range. Data for these properties were obtained as a function of temperature in the range 308.15 K <= T <= 363.15 K for rho and eta, whereas kappa was recorded between 308.15 and 338.15 K. While rho depended linearly on T, eta and kappa were best fitted by the empirical Vogel-Tammann-Fulcher (VFT) equation. The rho(x(1)) and eta(x(1)) data-with x(1) as the DES mole fraction-were used for calculating excess molar volumes (V-E) and excess logarithmic viscosities (In eta(E)) of the studied system. With respect to volumetric properties, reline/DMSO mixtures deviate largely from ideality, possessing negative V-E values over the entire range of T and x(1). The temperature dependence of V-E suggests that nonspecific interactions (packing effects) are mainly responsible for the system contraction. However, specific interactions (including H-bonding) cannot be excluded as In eta(E) values are positive and exhibit a temperature dependence characteristic for systems with strong interspecies interactions. Similar to conventional electrolytes and to mixtures of ionic liquids with a molecular solvent, kappa as a function of x(1) shows a maximum at x(1) approximate to 0.25, indicating a concentration at which rising viscosity and the formation of ion pairs and larger aggregates compensate the conductivity rise caused by increasing charge carrier density. Changes in the VFT parameters point at a transition from electrolyte solution-like behavior to molten salt-like behavior at x(1) approximate to 0.6

A Comprehensive Study of Density, Viscosity, and Electrical Conductivity of (Choline Chloride + Glycerol) Deep Eutectic Solvent and Its Mixtures with Dimethyl Sulfoxide

The data for density (rho) and the transport properties viscosity (eta) and electrical conductivity (kappa) of a deep eutectic solvent (DES), glyceline, composed of choline chloride (ChCl) and glycerol at a 1:2 molar ratio are presented. Density was determined in the temperature range from T = (278.15 to 363.15) K, while measurements for eta and kappa were performed at T = (278.15 to 368.15) K and T = (278.15 to 338.15) K, respectively. The results were compared to the corresponding data provided in the literature, and their possible discrepancies are discussed. Addition- ally, rho(T), eta(T), and kappa(T) were determined for mixtures of glyceline with dimethyl sulfoxide (DMSO) covering an entire miscibility range. For both neat DES and its mixtures, density data vary linearly with temperature, whereas eta(T) and kappa(T) were best fit by the empirical Vogel-Fulcher-Tammann equation. For the {glyceline+DMSO} system, the analysis of excess molar properties revealed that observed deviations from ideal density are mainly caused by packing effects. Nevertheless, the presence of strong H-bonding is supported by the sign and temperature dependence of excess viscosity. Similar to concentrated solutions of conventional electrolytes, the conductivity of {glyceline+DMSO} mixtures shows a pronounced maximum. Expectedly, its position depends significantly on temperature but is also sensitive to the type of a hydrogen bond donor used to prepare the DES

Complexation of Ni(ClO₄)₂ and Mg(ClO₄)₂ with 3‑Hydroxyflavone in Acetonitrile Medium: Conductometric, Spectroscopic, and Quantum Chemical Investigation

The complex formation of Ni­(ClO₄)₂ and Mg­(ClO₄)₂ with 3-hydroxyflavone (HL, flavonol) in acetonitrile was studied using conductometric and spectroscopic methods. It was found that interaction of nickel cation with HL leads to formation of the doubly charged [Ni­(HL)]²⁺ complex, whereas in solutions of magnesium perchlorate the complex with anion [MgClO₄(HL)]⁺ is formed. Using the extended Lee–Wheaton equation, the limiting equivalent conductivities of [Ni­(HL)]²⁺ and [MgClO₄(HL)]⁺ and thermodynamic constants of their formation were obtained at 288, 298, 308, 318, and 328 K. Calculated Stoke’s radii indicate weak solvation of the formed complexes and low temperature stability of their solvation shells. On the basis of the quantum chemical calculations and noncovalent interactions analysis, it is found that in the solvated [Ni­(HL)]²⁺ and [MgClO₄(HL)]⁺ complexes interaction of the Ni²⁺ and Mg²⁺ cations with flavonol occurs via the carbonyl group of HL. Complexation with Ni²⁺ does not change the internal structure of HL greatly: in the [Ni­(HL)]²⁺ complex, flavonol shows an intramolecular H-bond between 3-hydroxyl and carbonyl groups. When a complex with [MgClO₄]⁺ is formed, the OH group turns out of the plane of the chromone moiety that leads to rupture of an intramolecular H-bond in the ligand molecule. Moreover, in the [MgClO₄(HL)]⁺ complex, perchlorate anion possesses a strong ability to interact with HL, forming an intracomplex H-bond between hydrogen of the 3-hydroxyl group and oxygen of ClO₄^–. Its strength is more pronounced than in the intramolecular one in both [Ni­(HL)]²⁺ and uncomplexed 3-hydroxyflavone

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

The intermol. interactions and dynamics of aq. 1,​1-​dimethyurea (1,​1-​DMU) solns. were studied by examg. the concn. dependence of the solvent and solute relaxations detected by dielec. spectroscopy. Mol. dynamics simulations were carried out to facilitate interpretation of the dielec. 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 dielec. spectra of aq. solns. of 1,​1-​DMU and of its structural isomer 1,​3-​DMU. Similar to the previously studied compds. 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 mol., indicating pronounced parallel alignment of the solute dipole with two to three H2O moments. The MD simulations revealed that the involved water mols. 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 dielec. spectra, suggesting rather different hydration-​shell dynamics for 1,​1-​DMU as confirmed by the simulations. In contrast to aq. urea and 1,​3-​DMU, addn. 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 concn