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

Abstract

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.

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

“…Notice that these reorientational correlations, although becoming faster upon increasing the solution temperature, are spread over a large span of time scale, ranging from subpicosecond to a few nanoseconds. It is quite interesting to note that the simulated reorientational correlations for both urea and water in this DES depict severely slowed down dynamics compared to those predicted for neat liquids , and aqueous binary mixtures . Parameters obtained from fitting each of these decays to a sum of three exponential functions of time are summarized in Table S11.…”

“…It is quite interesting to note that the simulated reorientational correlations for both urea and water in this DES depict severely slowed down dynamics compared to those predicted for neat liquids 126,148 and aqueous binary mixtures. 149 Parameters obtained from fitting each of these decays to a sum of three exponential functions of time are summarized in Table S11. A direct comparison between the simulated C 1 (t) decay time constants and measured DR time scales (Table 1) suggests that the slowest DR time scale (τ 1 ) corroborate well with the slowest time scale of C 1 (t) decay of betaine molecules, and may also contain contributions from water reorientation dynamics.…”

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

“…When ethanol is present at dilute concentrations, water engages them nearly completely through ethanol–water hydrogen bonding, allowing no or scarce ethanol–ethanol direct interactions. This is reflected in the relative position of E e–e int at 75 wt % of water (∼0.12 mole fraction of ethanol), a concentration at which anomaly has been found to be the maximum in the composition-dependent studies of spectral shifts via steady-state UV–vis absorption spectroscopic experiments , and alcohol concentration fluctuations via X-ray scattering measurements. The distribution of the water–water interaction energy, E w–w int , at 4.5 wt % water (∼0.88 mole fraction of ethanol) displays the reverse (with respect to E e–e int at 75 wt % of water) where water–water direct interaction is severely inhibited by the surrounding ethanol molecules.…”

Polarity of Organic Solvent/Water Mixtures Measured with Reichardt’s B30 and Related Solvatochromic Probes—A Critical Review

“…Obviously, the trapping of probes is also determined through the solvent cage property of the BE/water mixtures. This is in contrast to 2-methyl-2-propanol/water mixtures, where strong thermal fluctuations in partial concentrations occur [195,196]. Thus, in 2-methyl-2-propanol/water mixtures there is no true hydrophobic solvation of B30, but the partial water structures are changed depending on the The crucial region of low BE content (at N av,x = 0.0497 mol/cm 3 ) deserves special attention, since, at this composition, the agglomeration of BE molecules occurs [x(BE) ≈ 0.02; or N av,xCH = 0.017 mol/cm 3 ] [184].…”