Sound propagation in water–ethanol mixtures at low temperatures. II. Dynamical properties

Volume viscosity and ultrasonic relaxation of ethanol-water mixtures studied by molecular dynamics simulations

The volume viscosity of ethanol-water mixtures at various compositions was calculated by means of equilibrium molecular dynamics simulations, and the results were compared with acoustic experiments. The volume viscosity exhibited a strong maximum around the ethanol mole fraction of 0.27, which is in agreement with experiments. The relaxation in the 100 ps regime, which had been revealed by ultrasonic spectroscopy, was also reproduced. Analysis of the two-body density and the adiabatic pressure fluctuation demonstrated that the large volume viscosity of the mixture originates from the long-range concentration fluctuation.

Comparative analysis of ethanol dynamics in aqueous and non-aqueous solutions

In this study, we compare the results for vibrational, reorientational and hydrogen bond dynamics of ethanol in water and in hexane across the whole concentration range. Water and hexane are both commonly used as solvents, but so far, it has been unclear to what extent they modify the solute dynamics. Ethanol is chosen as the solute because it is an aliphatic molecule that is miscibile with both solvents. It is known that ethanol forms micelle-like domains in water and cyclic clusters resembling loops in hexane. This structural micro-heterogeneity is well known both in experiments and in simulations. The main question we raise here is: is there a signature of micro-heterogeneity in the dynamical quantities of ethanol? We focus on quantities such as the vibrational spectra, the reorientational correlation functions, the self-diffusion coefficients, the ethanol-ethanol hydrogen bond correlation functions and the corresponding hydrogen bond histograms. For the first time ever, we compute the van Hove functions to reveal the dynamical variations of spatial correlations in these systems. All these results complement each other and provide a unifying dynamical description of ethanol in binary mixtures.

Density, ultrasound velocity, acoustic impedance, reflection and absorption coefficient determination of liquids via multiple reflection method

The accuracy of density, reflection coefficient, and acoustic impedance determination via multiple reflection method was validated experimentally. The ternary system water-maltose-ethanol was used to execute a systematic, temperature dependent study over a wide range of densities and viscosities aiming an application as inline sensor in beverage industries. The validation results of the presented method and setup show root mean square errors of: 1.201E-3 g cm(-3) (±0.12%) density, 0.515E-3 (0.15%) reflection coefficient and 1.851E+3 kg s(-1) m(-2) (0.12%) specific acoustic impedance. The results of the diffraction corrected absorption showed an average standard deviation of only 0.12%. It was found that the absorption change shows a good correlation to concentration variations and may be useful for laboratory analysis of sufficiently pure liquids. The main part of the observed errors can be explained by the observed noise, temperature variation and the low signal resolution of 50 MHz. In particular, the poor signal-to-noise ratio of the second reflector echo was found to be a main accuracy limitation. Concerning the investigation of liquids the unstable properties of the reference material PMMA, due to hygroscopicity, were identified to be an additional, unpredictable source of uncertainty. While dimensional changes can be considered by adequate methodology, the impact of the time and temperature dependent water absorption on relevant reference properties like the buffer's sound velocity and density could not be considered and may explain part of the observed deviations.

Possible relation of water structural relaxation to water anomalies

The anomalous behavior of thermodynamic response functions is an unsolved problem in the physics of water. The mechanism that gives rise to the dramatic indefinite increase at low temperature in the heat capacity, the compressibility, and the coefficient of thermal expansion, is unknown. We explore this problem by analyzing both new and existing experimental data on the power spectrum S(Q, ω) of bulk and confined water at ambient pressure. When decreasing the temperature, we find that the liquid undergoes a structural transformation coinciding with the onset of an extended hydrogen bond network. This network onset seems to give rise to the marked viscoelastic behavior, consistent with the interesting possibility that the sound velocity and response functions of water depend upon both the frequency and wave vector.