Dielectric and structural properties of aqueous nonpolar solute mixtures

The dielectric properties and molecular structure of water mixtures with different nonpolar solutes (methane and noble gases) are studied using molecular dynamics. The water-water, water-solute, and solute-solute interactions are calculated using the combination of a polarizable potential [J. Li, Z. Zhou, and R. J. Sadus, J. Chem. Phys. 127, 154509 (2007)] for water plus the Lennard-Jones potential. The effect of solute size and concentration on the solubility of the system, hydrogen bonding, dielectric constant, and dipole moment are investigated over a temperature range of 278-750 K and solute percentage mole fractions up to 30%. Solute particles affect the structure of water, resulting in the compression of oxygen-oxygen and oxygen-hydrogen radial distribution functions. The influence of the solute extends both to relatively low concentrations and high temperatures. The coordination numbers of aqueous solutions of the nonpolar solutes appear to be proportional to the size of the solute particles. Our study shows the destructive influence of the nonpolar solute on both the tetrahedral water structure and hydrogen bond formation at solute concentrations greater than 30%. The presence of nonpolar particles typically decreases both the dielectric constant and dipole moment. The decrease of dielectric constant and water dipole moment is directly proportional to the solute concentration and temperature

Research of interaction of a natural and forced convection in a vortex chamber of the chemical reactor

In this paper, a chemical reactor for producing refractory metals was considered. A physical and mathematical model of fluid motion and heat transfer in a vortex chamber of the chemical reactor under forced and free convection has been described and simulated. The numerical simulation was carried out in “velocity–pressure” variables by using an alternating direction implicit scheme. The velocity field and the temperature distribution in the reactor were obtained. Parametric studies on effects of the Reynolds, Prandtl and Rossbi criteria on the flow characteristics were also performed. The graphs presented show that natural convection has a significant impact on the hydrodynamics of the flow and intensifies the heat transfer. Reliability of the calculations was verified by comparing the results obtained by another method

Intermolecular potentials and the accurate prediction of the thermodynamic properties of water

The ability of intermolecular potentials to correctly predict the thermodynamic properties of liquid water at a density of 0.998 g/cm3 for a wide range of temperatures (298-650 K) and pressures (0.1-700 MPa) is investigated. Molecular dynamics simulations are reported for the pressure, thermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, and Joule-Thomson coefficient of liquid water using the non-polarizable SPC/E and TIP4P/2005 potentials. The results are compared with both experiment data and results obtained from the ab initio-based Matsuoka-Clementi-Yoshimine non-additive (MCYna) [J. Li, Z. Zhou, and R. J. Sadus, J. Chem. Phys. 127, 154509 (2007)] potential, which includes polarization contributions. The data clearly indicate that both the SPC/E and TIP4P/2005 potentials are only in qualitative agreement with experiment, whereas the polarizable MCYna potential predicts some properties within experimental uncertainty. This highlights the importance of polarizability for the accurate prediction of the thermodynamic properties of water, particularly at temperatures beyond 298 K

Thermodynamic properties and diffusion of water + methane binary mixtures

Thermodynamic and diffusion properties of water + methane mixtures in a single liquid phase are studied using NVT molecular dynamics. An extensive comparison is reported for the thermal pressure coefficient, compressibilities, expansion coefficients, heat capacities, Joule-Thomson coefficient, zero frequency speed of sound, and diffusion coefficient at methane concentrations up to 15% in the temperature range of 298-650 K. The simulations reveal a complex concentration dependence of the thermodynamic properties of water + methane mixtures. The compressibilities, heat capacities, and diffusion coefficients decrease with increasing methane concentration, whereas values of the thermal expansion coefficients and speed of sound increase. Increasing methane concentration considerably retards the self-diffusion of both water and methane in the mixture. These effects are caused by changes in hydrogen bond network, solvation shell structure, and dynamics of water molecules induced by the solvation of methane at constant volume conditions

Thermophysical properties of supercritical water and bond flexibility

Molecular dynamics results are reported for the thermodynamic properties of supercritical water using examples of both rigid (TIP4P/2005) and flexible (TIP4P/2005f) transferable interaction potentials. Data are reported for pressure, isochoric and isobaric heat capacities, the thermal expansion coefficient, isothermal and adiabatic compressibilities, Joule-Thomson coefficient, speed of sound, self-diffusion coefficient, viscosities, and thermal conductivity. Many of these properties have unusual behavior in the supercritical phase such as maximum and minimum values. The effectiveness of bond flexibility on predicting these properties is determined by comparing the results to experimental data. The influence of the intermolecular potential on these properties is both variable and state point dependent. In the vicinity of the critical density, the rigid and flexible potentials yield very different values for the compressibilities, heat capacities, and thermal expansion coefficient, whereas the self-diffusion coefficient, viscosities, and thermal conductivities are much less potential dependent. Although the introduction of bond flexibility is a computationally expedient way to improve the accuracy of an intermolecular potential, it can be counterproductive in some cases and it is not an adequate replacement for incorporating the effects of polarization

Atomistic water models: Aqueous thermodynamic properties from ambient to supercritical conditions

Progress in obtaining an accurate atomistic model for water is reviewed and evaluated with particular attention to thermodynamic properties such as the thermal pressure coefficient, thermal expansion coefficient, isothermal and adiabatic compressibilities, isobaric and isochoric heat capacities, Joule–Thomson coefficient, speed of sound and vapor–liquid equlibria. The different multi-site models are categorised in terms of bond rigidity/flexibility and polarization. The models are assessed for their ability to reproduce experimental values at conditions ranging from ambient to supercritical conditions. In general, three or four sites are sufficient to obtain reasonable results with the addition of more sites not consistently yielding improvements. The addition of bond flexibility, while improving agreement with experiment for such properties as phase coexistence, dielectric constants, viscosity and diffusion, does not appear to significantly improve the prediction of thermodynamic properties in general. For the supercritical heat capacities and thermal expansion coefficient the flexible TIP4P/2005f model yields less accurate values than its rigid TIP4P/2005 counterpart. In contrast, accounting for polarizability consistently results in improved agreement with experiment. For properties such as the isochoric heat capacity and thermal expansion coefficient, the polarizable MCYna model yields values that are in very close agreement with experimental data in the temperature range of 300–600 K. A quantitative ranking scheme is proposed and applied to ambient conditions. At or near ambient conditions, the overall ranking of models investigated is (iAMOEBA, MCYna) > (BKd3, TIP4P/FQ, GCPM, BK3) > (TIP4P/2005, SPC/Fw, SPC/E) > (SPC, TIP4P/2005f, NvdE, TIP5P) > (TIP4P, TIP3P) > (MCY, MCYL)