Molecular dynamics insights into the dynamical behavior of structurally modified water in aqueous deep eutectic solvents (ADES)

Recent studies have demonstrated that the presence of water in deep eutectic solvents (DESs) significantly affects their dynamics, structure, and physical properties. Although the structural changes due to the addition of water are well understood, the microscopic dynamics of these changes have been rarely studied. Here, we performed molecular dynamics simulation of 30% (v/v) (∼0.57 molar fraction) water mixture of DES containing CH3CONH2 and NaSCN/KSCN at various salt fractions to understand the microscopic structure and dynamics of water. The simulated results reveal a heterogeneous environment for water molecules in aqueous DES (ADES), which is influenced by the nature of the cation. The diffusion coefficients of water in ADESs are significantly lower than that in neat water and concentrated aqueous NaSCN/KSCN solution. When Na+ ions are replaced by K+ ions in the ADES system, the diffusion coefficient increases, which is consistent with the measured nuclear magnetic resonance data. Self-dynamic structure factor for water and other simulated dynamic quantities, such as reorientation, hydrogen-bond, and residence time correlation functions, show markedly slower dynamics inside ADES than in the neat water and aqueous salt solution. Moreover, these dynamics become faster when Na+ ions in ADES are replaced by K+ ions. The results suggest that the structural environment of water in Na+-rich ADES is rigid due to the presence of cation-bound water and geometrically constrained water. The medium becomes less rigid as the KSCN fraction increases due to the relatively weaker interaction of K+ ions with water than Na+ ions, which accelerates the dynamical processes.

Temperature-Dependent Dielectric Relaxation Measurements of (Betaine + Urea + Water) Deep Eutectic Solvent in Hz-GHz Frequency Window: Microscopic Insights into Constituent Contributions and Relaxation Mechanisms

A combined experimental and simulation study of dielectric relaxation (DR) of a deep eutectic solvent (DES) composed of betaine, urea, and water with the composition [Betaine:Urea:Water = 11.7:12:1 (weight ratio) and 9:18:5 (molar ratio)] was performed to explore and understand the interaction and dynamics of this system. Temperature-dependent (303 ≤ T/K ≤ 343) measurements were performed over 9 decades of frequency, combining three different measurement setups. Measured DR, comprising four distinct steps with relaxation times spreading over a few picoseconds to several nanoseconds, was found to agree well with simulations. The simulated total DR spectra, upon dissection into three self (intraspecies) and three cross (interspecies) interaction contributions, revealed that the betaine-betaine self-term dominated (∼65%) the relaxation, while the urea-urea and water-water interactions contributed only ∼7% and ∼1%, respectively. The cross-terms (betaine-urea, betaine-water, and urea-water) together accounted for <30% of the total DR. The slowest DR component with a time constant of ∼1-10 ns derived dominant contribution from betaine-betaine interactions, where betaine-water and urea-water interactions also contributed. The subnanosecond (0.1-0.6 ns) time scale originated from all interactions except betaine-water interaction. An extensive interaction of water with betaine and urea severely reduced the average number of water-water H-bonds (∼0.7) and heavily decreased the static dielectric constant of water in this DES (εs ∼ 2). Furthermore, simulated first rank collective single particle reorientational relaxations (C1(t)) and the structural H-bond fluctuation dynamics (CHB (t)) exhibited multiexponential kinetics with time scales that corresponded well with those found both in the simulated and measured DR.

Identical Diffusion Distributions and Co-Cluster Formation Dictate Azeotrope Formation: Microscopic Evidences and Experimental Signatures

What selects azeotropic pairs and governs the azeotropic conditions (composition and temperature) is an open and intriguing question. A combined simulation and experimental work presented here investigates this by considering ethanol-water mixtures. We find identical distributions of center-of-mass diffusion coefficients for ethanol and water molecules under the azeotropic condition (95.5 wt % ethanol +4.5 wt % water, Tazeo = 351.1K). Moreover, the particle displacements show strong interspecies correlations at Tazeo. Interestingly, simulated reorientation time distributions become identical at Tazeo but at a composition different from that at which the translational diffusion distributions overlapped. Cluster analyses indicate that solutions at Tazeo with xwater ≤ 15 wt % are more microheterogeneous than those with higher water content, although no anomaly in the composition-dependent solution structural properties was detected. Ethanol-water and ethanol-ethanol interaction energies show pronounced nonideal composition dependence, but the size of the relative fluctuations in them remained small (∼0.5kBT). Rare water-water H-bonding, predominant water-ethanol H-bonding, and a sizable population of "free" water molecules characterize the azeotropic solutions. The red edge excitation spectroscopic (REES) measurements with a dissolved anionic fluorescent dye, coumarin343 (C343), support the predicted solution microheterogeneity by showing a nonmonotonic composition dependence of the excitation energy-induced changes in the fluorescence emission spectral frequencies and bandwidths, the largest changes being under the azeotropic condition. Subsequent dynamic anisotropy measurements reveal a nonmonotonic composition dependence of C343 rotation times with a peak under the azeotropic condition. In summary, equalization of the component translational diffusion coefficients and solution microheterogeneity with regular composition dependence of the solution structure appear to characterize the ethanol-water azeotrope.

Dynamical Anomaly of Aqueous Amphiphilic Solutions: Connection to Solution H-Bond Fluctuation Dynamics?

We have investigated the possible connection between "dynamical anomaly" observed in time-resolved fluorescence measurements of reactive and nonreactive solute-centered relaxation dynamics in aqueous binary mixtures of different amphiphiles and the solution intra- and interspecies H-bond fluctuation dynamics. Earlier studies have connected the anomalous thermodynamic properties of binary mixtures at very low amphiphile concentrations to the structural distortion of water. This is termed as "structural anomaly." Interestingly, the abrupt changes in the composition-dependent average rates of solute relaxation dynamics occur at amphiphile mole fractions approximately twice as large as those where structural anomalies appear. We have investigated this anomalous solution dynamical aspect by considering (water + tertiary butanol) as a model system and performed molecular dynamics simulations at several tertiary butanol (TBA) concentrations covering the extremely dilute to the moderately concentrated regimes. The "dynamical anomaly" has been followed via monitoring the composition dependence of the intra- and interspecies H-bond fluctuations and reorientational relaxations of TBA and water molecules. Solution structural aspects have been followed via examining the tetrahedral order parameter, radial and spatial distribution functions, numbers of H bonds per water and TBA molecules, and the respective populations participating in H-bond formation. Our simulations reveal abrupt changes in the H-bond fluctuations and reorientational dynamics and tetrahedral order parameter at amphiphile concentrations differing approximately by a factor of 2 and corroborates well with the steady-state and the time-resolved spectroscopic measurements. This work therefore explains, following a uniform and cogent manner, both the experimentally observed structural and dynamical anomalies in microscopic terms.

In Silico Elucidation of Molecular Picture of Water-Choline Chloride Mixture

Choline chloride (ChCl) is a component of several deep eutectic solvents (DESs) having numerous applications. Recent studies have reported manifold promising use of aqueous choline chloride solution as an alternative to DES, where water plays the role of the hydrogen-bond donor. The characteristic physical properties of the DESs and aqueous DES originate from the "inter-" and intraspecies hydrogen-bond network formed by the constituents. However, a detailed molecular-level picture of choline chloride and water mixture is largely lacking in the literature. This motivates us to carry out extensive all-atom molecular dynamics simulations of the ChCl-water mixture of varying compositions. Our analyses clearly show an overall increase in the interspecies association with an increase in ChCl concentration. At higher concentrations, the trimethylammonium groups of choline are stabilized by a nonpolar interaction, whereas the hydroxyl groups preferentially interact with water. Chloride ions are found to be involved in two types of interactions: one where chloride ions intercalate two or more choline cations, and the other one where they are surrounded by five to six water molecules forming solvated chloride ions. However, the relative fractions of these two types of associations depend on the concentration of ChCl in the mixture. Another important structural aspect is the disruption of the hydrogen-bonded water network due to the presence of both choline cations and chloride ions. However, chloride ions participate to partially restore the tetrahedral arrangement of partners around water molecules.

Dynamic heterogeneity and viscosity decoupling: origin and analytical prediction

The molecular-level structure and dynamics decide the functionality of solvent media. Therefore, a significant amount of effort is being dedicated continually over time in understanding their structural and dynamical features. One intriguing aspect of solvent structure and dynamics is heterogeneity. In these systems, the dynamics follow , where p is the measure of viscosity decoupling. We analytically predicted that in such cases, the Stokes-Einstein relationship is modified to due to microdomain formation, and the second term on the right-hand side leads to viscosity decoupling. We validated our prediction by estimating the p values of a few solvents, and they matched well with the literature. Overall, we believe that our approach gives a simple yet unique physical picture to help us understand the heterogeneity of solvent media.