The abnormal physicochemical characteristics of ethanol solvation in water are commonly attributed to the phenomenon of hydrophobic hydration. To investigate the structural organization of hydrophobic hydration in water−ethanol mixtures, we use molecular dynamics simulations based on detailed atomic models. Induced polarization is incorporated into the potential function on the basis of the classical Drude oscillator model. Water−ethanol mixtures are simulated at 11 ethanol molar fractions, from 0.05 to 0.9. Although the water and ethanol models are parametrized separately to reproduce the vaporization enthalpy, static dielectric constant, and self-diffusion constant of neat liquids at ambient conditions, they also reproduce the energetic and dynamical properties of the mixtures accurately. Furthermore, the calculated dielectric constant for the various water−alcohol mixtures is in excellent agreement with experimental data. The simulations provide a detailed structural characterization of the mixtures. A...

Molecular Dynamics Study of Hydration in Ethanol−Water Mixtures Using a Polarizable Force Field†

We have studied mixtures of alcohol and water in an extensive series of 465 molecular-dynamics simulations with an aggregate length of 713 ns, in order to study excess properties of mixing, in particular the relation between mobility and viscosity. Methanol/water, ethanol/water, and 1-propanol/water mixtures were simulated using an alcohol content of 0–100 mass % in steps of 10%, using the OPLS (optimized potential for liquid simulations) force field for the alcohol molecules and the TIP4P (transferable intermolecular potential with four particles) water model. Computed densities and energies show very good agreement with experimental data for bulk simulations and the mixtures are satisfactory as well. The shear viscosity was computed using nonequilibrium molecular-dynamics simulations. Other properties studied include diffusion constants and rotational correlation times. We find the mobility to correlate well with the viscosity data, i.e., at intermediate alcohol concentrations the viscosity is maximal and the mobility is minimal. Furthermore, we have combined the viscosity and diffusion calculations in order to compute an effective hydrodynamic radius of the particles in the mixtures, using the Stokes–Einstein relation. This analysis indicates that there is no collective diffusion of molecular clusters in these mixtures. For all properties we find that the excess values are underestimated in the simulations, which, given that the pure liquids are described rather well, raises the question whether the potential function is too simplistic to describe mixtures quantitatively. The set of simulations presented here can hence be regarded as a force-field benchmark.

Dynamic properties of water/alcohol mixtures studied by computer simulation

Density, self-diffusion coefficient, and shear viscosity of pure liquid water are predicted for temperatures between 280 and 373 K by molecular dynamics simulation and the Green–Kubo method. Four different rigid nonpolarizable water models are assessed: SPC, SPC/E, TIP4P, and TIP4P/2005. The pressure dependence of the self-diffusion coefficient and the shear viscosity for pure liquid water is also calculated and the anomalous behavior of these properties is qualitatively well predicted. Furthermore, transport properties as well as excess volume and excess enthalpy of aqueous binary mixtures containing methanol or ethanol, based on the SPC/E and TIP4P/2005 water models, are calculated. Under the tested conditions, the TIP4P/2005 model gives the best quantitative and qualitative agreement with experiments for the regarded transport properties. The deviations from experimental data are of 5% to 15% for pure liquid water and 5% to 20% for the water + alcohol mixtures. Moreover, the center of mass power spectrum of water as well as the investigated mixtures are analyzed and the hydrogen-bonding structure is discussed for different states.

https://thet.uni-paderborn.de/fileadmin/thet/Publikationen/Veroeffentlichungen/Trans_aqueous_alcohol.pdf

Prediction of self-diffusion coefficient and shear viscosity of water and its binary mixtures with methanol and ethanol by molecular simulation.

Terahertz (THz) radiation interacts strongly with the intermolecular hydrogen-bond network in aqueous liquids. The dielectric properties of liquid water and aqueous solutions in the THz spectral region are closely linked to the microscopic dynamics of the liquid solution, and hence THz spectroscopy offers an important insight into fundamental intermolecular interactions in polar liquids. At the same time, the strong and characteristic interaction between THz radiation and liquids offers a methodology for the classification of liquids inside containers, and hence the THz region is suitable for remote detection of some of the properties of bottled liquids. Here we present a review of THz spectroscopy and modeling of water-ethanol mixtures, and establish a link between the dielectric function of water-ethanol mixtures and some of their thermodynamic properties. We then review how the knowledge of the dielectric function of aqueous mixtures can be used for inspection of liquids inside bottles. Finally we draw up some of the limits to the applicability of THz reflection spectroscopy in the identification of dangerous liquids.

Terahertz reflection spectroscopy of Debye relaxation in polar liquids [Invited]

Viscosity B coefficients for tert-butyl alcohol (TBA) in water, methanol and mixtures of the two have been obtained at 25 and 35 °C. Hence solute contributions to the activation parameters for viscous flow of the solutions have been calculated, as well as the Gibbs energy, enthalpy and entropy of transfer from the ground-state solvent to the hypothetical viscous ‘transition-state solvent’. The enthalpies of mixing of the two transition-state solvents and their relative partial molar enthalpies L′1 and L′2 are also given.In water, B is large and positive and dB/dT large and negative; in methanol B is small and dB/dT near zero. The classical explanation of these observations, which is solely in terms of water–water (1,1) interactions in an ‘iceberg’ around the alkyl group, is inconsistent with the Compensation Principle because B is proportional to a Gibbs energy function, and this requires that solute–solvent (3, 1) interactions must determine B.It is proposed that in water the alkyl group occupies a ‘cage’ in which the strength of the bonding with respect to the pure solvent is of minor importance. The ‘caging’ of the alkyl group enhances its van der Waals interactions with its nearest neighbours. The expected dependence of such interactions on distance leads to energy–reaction coordinate diagrams in the form required to explain the inversion in sign of the solvation energies from ground-state solvent to transition-state solvent in highly aqueous media.These results are consistent with, though independent of, recent structural studies by neutron diffraction with isotopic substitution.

Relative viscosities and quasi-thermodynamics of solutions of tert-butyl alcohol in the methanol–water system: a different view of the alkyl–water interaction

J. A. Boyne

Papers

Enthalpies of mixture of ethanol and water at 25.degree.C