Infrared study of water–benzene mixtures at high temperatures and pressures in the two- and one-phase regions

Self-diffusion of water-cyclohexane mixtures in supercritical conditions as studied by NMR and molecular dynamics simulation

The self-diffusion coefficients of water (D_w) and cyclohexane (D_ch) in their binary mixtures were determined using the proton pulsed field gradient spin-echo method from medium to low densities in subcritical and supercritical conditions. The density (ρ), temperature (T), and water mole fraction (x_w) are studied in the ranges 0.62-6.35 M (M = mol dm^-3), 250-400 °C, and 0.109-0.994, respectively. A polynomial fitting function was developed for a scaled value of Ξ = ρDT^-1/2 with ρ, T, and x_w as variables in combination with a comprehensive molecular dynamics (MD) simulation. The NMR and MD results agree within 5% for water and 6% for cyclohexane, on average. The differences between D_w and D_ch in the dependence on ρ, T, and x_w are characterized by the activation energy E_a and the activation volume ΔV_Ξ ^‡ expressed by the scaled fitting function. The decrease in the ratio D_w/D_ch and the increase in the E_a of water with increasing x_w are related to the increase in the number of hydrogen bonds (HBs). The D_w value for a solitary water molecule at a low x_w is controlled by the solvation shell, most of which is occupied by nonpolar cyclohexane molecules that provide less friction as a result of weaker interactions with water. A microscopic diffusion mechanism is discussed based on an analysis of the HB number as well as the first-peak height of the radial distribution functions that are taken as measures of the potential of the mean field controlling self-diffusion.

Molecular level properties of the free water surface and different organic liquid/water interfaces, as seen from ITIM analysis of computer simulation results

Molecular dynamics simulations of the interface of water with four different apolar phases, namely water vapour, liquid carbon tetrachloride, liquid dichloromethane (DCM) and liquid dichloroethane (DCE) are performed on the canonical ensemble at 298 K. The resulting configurations are analysed using the novel method of identification of the truly interfacial molecules (ITIM). Properties of the first three molecular layers of the liquid phases (e.g. width, spacing, roughness, extent of the in-layer hydrogen bonding network) as well as of the molecules constituting these layers (e.g., dynamics, orientation) are investigated in detail. In the analyses, particular attention is paid to the effect of the polarity of the non-aqueous phase and to the length scale of the effect of the vicinity of the interface on the various properties of the molecules. The obtained results show that increasing polarity of the non-aqueous phase leads to the narrowing of the interface, in spite of the fact that, at the same time, the truly interfacial layer of water gets somewhat broader. The influence of the nearby interface is found to extend only to the first molecular layer in many respects. This result is attributed to the larger space available for the truly interfacial than for the non-interfacial molecules (as the shapes of the two liquid surfaces are largely independent of each other, resulting in the presence of voids between the two phases), and to the fact that the hydrogen bonding interaction of the truly interfacial water molecules with other waters is hindered in the direction of the interface.

Anisotropic United Atom Model Including the Electrostatic Interactions of Benzene

An optimization including electrostatic interactions has been performed for the parameters of an anisotropic united atoms intermolecular potential for benzene for thermodynamic and transport property prediction using Gibbs ensemble, isothermal-isobaric (NPT) Monte Carlo, and molecular dynamic simulations. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data (saturation pressure, vaporization enthalpy, and liquid density) at ambient conditions and at 350 and 450 K. A comprehensive comparison of the new model is given with other intermolecular potentials taken from the literature. Overall thermodynamic, structural, reorientational, and translational dynamic properties of our optimized model are in very good agreement with experimental data. The new model also provides a good representation of the liquid structure, as revealed by three-dimensional spatial density functions and carbon-carbon radial distribution function. Shear viscosity variations with temperature and pressure are very well reproduced, revealing a significant improvement with respect to nonpolar models.

Structure of Hydrophobic Hydration of Benzene and Hexafluorobenzene from First Principles

We report on the aqueous hydration of benzene and hexafluorobenzene, as obtained by carrying out extensive (>100 ps) first principles molecular dynamics simulations. Our results show that benzene and hexafluorobenzene do not behave as ordinary hydrophobic solutes, but rather present two distinct regions, one equatorial and the other axial, that exhibit different solvation properties. While in both cases the equatorial regions behave as typical hydrophobic solutes, the solvation properties of the axial regions depend strongly on the nature of the pi-water interaction. In particular, pi-hydrogen and pi-lone pair interactions are found to dominate in benzene and hexafluorobenzene, respectively, which leads to substantially different orientations of water near the two solutes. We present atomic and electronic structure results (in terms of Maximally Localized Wannier Functions) providing a microscopic description of benzene- and hexafluorobenzene-water interfaces, as well as a comparative study of the two solutes. Our results point at the importance of an accurate description of interfacial water to characterize hydration properties of apolar molecules, as these are strongly influenced by subtle charge rearrangements and dipole moment redistributions in interfacial regions.

Dynamical and structural properties of benzene in supercritical water

We have employed an anisotropic united atom model of benzene (R. O. Contreras, Ph.D. thesis, Universitat Rovira i Virgili 2002) that reproduces the quadrupolar moment of this molecule through the inclusion of seven point charges. We show that this kind of interaction is required to reproduce the solvation of these molecules in supercritical water. We have computed self-diffusion coefficient and Maxwell-Stefan coefficients as well as the shear viscosity for the mixture water-benzene at supercritical conditions. A strong density and composition dependence of these properties is observed. In addition, our simulations are in qualitative agreement with the experimental evidence that, at medium densities (0.6 g/cm(3) and 673 K), almost half of the benzene molecules have one hydrogen bond with water molecules. We also observe that these bonds are longer lived than the corresponding hydrogen bonds between water molecules. Similarly, we obtain an important reduction of the dielectric constant of the mixture with the increment of the amount of benzene molecules at medium and high densities.

Spectroscopic study of water-NaCl-benzene mixtures at high temperatures and pressures

Near-infrared and ultraviolet spectra of water-NaCl-benzene mixtures have been measured in the 473-573 K and 100-400 bar range and 373-498 K and 50-300 bar range, respectively. Concentrations of water in the benzene-rich phase and benzene in the water-rich phase were estimated from integrated intensities of the absorption bands. It is found that addition of NaCl in the aqueous phase suppresses transfer of water into the benzene-rich phase, and the relative decrease in water solubility in benzene exhibits good correlation with an increase in density of the aqueous NaCl solution relative to that of neat water. The salting-out constant for the water-NaCl-benzene system, which is estimated from a relative decrease in benzene solubility in the aqueous phase by addition of sodium chloride, increases significantly with increasing temperature. It is suggested that the effect of sodium chloride on the water-benzene mutual solubilities can be explained by ion-induced electrostriction of the aqueous phase.

Computer Simulation Investigation of the Water−Benzene Interface in a Broad Range of Thermodynamic States from Ambient to Supercritical Conditions

The dependence of the properties of the water-benzene system on the thermodynamic conditions in a broad range of temperatures and pressures has been investigated by computer simulation methods. For this purpose, Monte Carlo simulations have been performed at 23 different thermodynamic states, ranging from ambient to supercritical conditions. The density profiles of the water and benzene molecules have been determined at each of the thermodynamic states investigated. Information on the dependence of the mutual solubility of the two components in each other as well as of the width of the interface on the temperature and pressure has been extracted from these profiles. The width of the interface has been found to increase with increasing temperature up to a certain point, where it diverges. The temperature of this divergence corresponds to the mixing of the two phases. The determination of the critical mixing temperature at various pressures allowed us to estimate the upper critical curve, separating the two-phase and one-phase liquid systems, of the phase diagram of the simulated water-benzene system. In analyzing the preferential orientation of the interfacial molecules relative to the interface, it has been found that the main orientational preference of the benzene molecules is to lie parallel with the plane of the interface, and the water molecules penetrated deepest into the benzene phase prefer to stay perpendicular to the interface, pointing by one of their O-H bonds almost straight toward the benzene phase, whereas the waters located at the aqueous side of the interface are preferentially aligned parallel with the interfacial plane. Although the strength of the observed orientational preferences decreases rapidly with increasing temperature, the preferred orientations themselves are found to be independent of the thermodynamic conditions. Remains of the orientational preferences of the molecules are found to be present up to temperatures as high as 650 K. The analysis of the relative orientation of the neighboring water-benzene pairs has revealed that the radius of the first hydration shell of the benzene molecules is independent of the thermodynamic conditions, even if the system consists of one single phase. It has been found that the nearest water neighbors of the benzene molecules are preferentially located above and below the benzene ring, whereas more distant water neighbors, belonging still to the first hydration shell, prefer to stay within the plane of the benzene molecule. In the two-phase systems the dipole vector of the nearest waters has been found to be preferentially perpendicular to the vector pointing from the center of the benzene molecule to the water O atom.

Spectroscopic study of mutual solubilities of water and benzene at high temperatures and pressures

Near-infrared and ultraviolet absorption of water-benzene mixtures has been measured at temperatures and pressures in the ranges of 323-673 K and 50-400 bar, respectively. Concentrations of water and benzene in both the water-rich phase and the benzene-rich phase of the mixtures were obtained from absorption intensities of near-infrared bands of water and benzene and ultraviolet bands of benzene. Mutual solubilities in molar fractions increase remarkably with increasing temperature at pressures in the two-liquid-phase coexistence region, and are consistent with previously reported values. It proves that the solubility of benzene in water is an order of magnitude smaller than that of water in benzene throughout the two-phase region. In addition, it is found that effect of pressure on the solubilities is opposite between water in benzene and benzene in water. These solubility properties are discussed on the basis of a cavity-based solvation model. It is suggested that the asymmetry in the mutual solubility and the opposite direction of the pressure effect are caused by difference in molecular size and difference in thermal compressibility, respectively, between water and benzene.

A vibrational spectroscopic study of structure evolution of water dissolved in supercritical carbon dioxide under isobaric heating

A combination of Raman scattering spectroscopy and infrared absorption was applied to investigate the structural evolution of water dissolved in supercritical carbon dioxide under isobaric heating (T=40-340 degrees C, P=250 bar). Quantitative analysis of experimental spectra allowed us to determine that at relatively moderate temperatures water dissolved in CO(2)-rich phase exists only under monomeric form (solitary water surrounding by CO(2) molecules), but hydrogen-bonded species, namely, dimers, begin to appear upon heating. At the same time, the ratio of dimers to monomers concentration increases with further temperature increase and at temperatures close to the temperature of total miscibility of the mixture (T=366 degrees C, P=250 bar), water dimers only are present in the CO(2)-rich phase.

Near infrared study of water-benzene mixtures at high temperatures and pressures

Near-infrared absorption of water-benzene mixtures has been measured at temperatures and pressures in the ranges of 473-673 K and 100-400 bar, respectively. Concentrations of water and benzene in the water-rich phase of the mixtures were obtained from the integrated absorption intensities of the OH stretching overtone transition of water and the CH stretching overtone transition of benzene, respectively. Using these concentrations, the densities of the water-rich phase were estimated and compared with the average densities before mixing, which were calculated from literature densities of neat water and neat benzene. It is found that anomalously large volume expansion on the mixing occurs in the region enclosed by an extended line of the three-phase equilibrium curve and the one-phase critical curve of the mixtures, and the gas-liquid equilibrium curve of water. Furthermore, magnitude of the relative volume change increases with decreasing molar fraction of benzene in the present experimental range. It is suggested that dissolving a small amount of benzene in water induces a change in the fluid density from a liquidlike condition to a gaslike condition in the vicinity of the critical region.