Aqueous binary mixtures often exhibit dramatic departure from the predicted hydrodynamic behavior when transport properties are plotted against composition. We show by inherent structure (IS) analysis that this sharp composition dependent breakdown of the Stokes–Einstein relation can be attributed to the non-monotonic variation in the average inherent structure energy of these mixtures. Further IS analysis reveals the existence of a unique ground state, stabilized by both the formation of an optimum number of H-bonds and a favorable hydrophobic interaction at this composition. The surprisingly sharp turnaround behavior observed in the effective hydrodynamic radius also owes its origin to the same combination of these two factors. Interestingly, the temperature dependence of isothermal compressibility shows a minimum at the particular composition. Extensive studies on water–dimethyl sulfoxide and water–ethanol mixtures using two different force-fields of water reveal many features that are nearly universal. A justification of this quasi-universal behavior is provided in terms of a mode-coupling theory (MCT) of viscosity, which can serve as the starting point of a remarkable correlation observed with the nearest neighbor structure, as captured by the first peaks of the radial distribution function, and the slowdown in the intermediate scattering function at intermediate wavenumbers. Therefore, the formation of the local structure captured through IS analysis can be correlated with the MCT.
We report the existence of disparate static and dynamic correlation lengths that could describe the influence of confinement on nanoconfined water (NCW). Various aspects of viscous properties, such as anisotropy, viscoelasticity among others, of NCW are studied by computer simulations by varying the separation 'd' between the two confining hydrophobic plates. The mean square stress exhibits a slow spatial decay (measured from the surface) beyond ~1.8 nm. The static correlation length obtained from fitting the exponential decay of the mean-square stress with d is 0.75 nm, while the decay time of stress-stress time correlation function gives a dynamic correlation length of only 0.35 nm. The shortness of the latter seems to arise from the low sensitivity of orientational relaxation to confinement. In the frequency dependent viscosity, we observe a new peak at about 50 cm-1 that is not present in the bulk. This new peak is prominent even at 3nm separations. The peak is absent in the bulk, although it is close to the intermolecular -O-O-O- bending mode well known in liquid water. We further explore relation between diffusion and viscosity in NCW by varying 'd', and find disagreement with an existing theory.
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