Microscopic origin of breakdown of Stokes–Einstein relation in binary mixtures: Inherent structure analysis

Abstract

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.

Breakdown of the Stokes–Einstein Equation for Solutions of Water in Oil Reverse Micelles

“…The critical temperature at which this decoupling starts to occur in supercooled and glass-forming systems may at times be surprisingly high and be above the glass-transition temperature or melting temperature as observed, for example, for water, phase-change materials, several ionic liquids, and GeTe alloy, where in the latter case high atomic mobility above T g was found in large-scale molecular dynamics (MD) simulations . In theoretical treatments of glass-forming liquids, it was argued that the Stokes–Einstein breakdown is caused by a spatial heterogeneity of dynamics. , We note that short-range ordering has also been implicated as a root for deviations from the Stokes–Einstein equation, in particular five-fold icosahedral short-range order for metallic glass formers ,, as well as water aggregates around dimethylsulfoxide and ethanol in respective liquid–liquid binary systems . Nevertheless, efforts are ongoing to obtain a more detailed understanding of these glass systems that contain the presence of dynamic heterogeneity .…”

“…73,74 We note that short-range ordering has also been implicated as a root for deviations from the Stokes−Einstein equation, 75 in particular five-fold icosahedral short-range order for metallic glass formers 24,28,30 as well as water aggregates around dimethylsulfoxide and ethanol in respective liquid−liquid binary systems. 49 Nevertheless, efforts are ongoing to obtain a more detailed understanding of these glass systems that contain the presence of dynamic heterogeneity. 76 For example, coexisting regions of cage rattling-dominant motions and jump-dominated motions have been found in MD studies of supercooled GeTe.…”

“…Pronounced deviations from the Stokes–Einstein equation have also been observed in binary systems for which one would ordinarily expect the Stokes–Einstein equation to hold. Recent examples include aqueous lithium chloride solutions, , glycerol, dimethylsulfoxide, , ethanol, some eletrolytes, tetraethyleneglycol in levitated viscous aerosols, and small molecules in supercritical water . Turton and Wynne even questioned if deviations from the closely related Stokes–Einstein–Debye equation for molecular orientational diffusion are the rule rather than the exception based on their Kerr-effect spectroscopy study on guanidine hydrochloride in water and mixtures of carbon disulfide with hexadecane .…”

Breakdown of the Stokes–Einstein Relation in Supercooled Water/Methanol Binary Mixtures: Explanation Using the Translational Jump-Diffusion Approach

“…Požar et al studied the spatiotemporal heterogeneity of water/methanol mixtures at a wide range of temperatures. The amphiphilicity of methanol molecules reinforces the microheterogeneity of the structure even at room temperature as seen in aqueous solutions of other amphiphiles. ,− These microsegregated domains are longer lived at a lower temperature. The structural heterogeneity has a strong impact on the dynamic heterogeneity of the medium, which actually indicates the co-existence of mobile domains of different mobilities in the same system.…”

Nanomechanical Molecular Mass Sensing Using Suspended Microchannel Resonators

“…We plot hydrostatic pressure responsivity versus compressibility ( Figure 3 d). As we know from the gas experiments, hydrostatic compressions in this setup are an intermediate case between isothermal and adiabatic compression; therefore, we choose as compressibility value the arithmetic average of the isothermal and adiabatic compressibility factor for each solution from its nominal values [ 12 , 35 ]. By linear fitting of this data, we obtain an offset of and slope of −0.22 ± 0.04, resulting in good agreement with the values predicted in the analytical model (−0.17).…”