H. D. Cochran

Simulation of supercritical water and of supercritical aqueous solutions

Molecular dynamics (MD) calculations have been performed to determine equilibrium structure and properties of systems modeling supercritical (SC) water and SC aqueous solutions at two states near the critical point using the simple point charge (SPC) potential model of Berendsen et al. for water. Both thermodynamic and dielectric properties from the simulations for pure water are accurate in comparison with experimental results even though the SPC model parameters were fitted to properties of ambient water. Details of the near‐critical clustering in SC water have been predicted which have not been measured to date. MD studies have also been undertaken of systems that model sodium and chloride ions and neutral argon in SC water at the same states. The first solvation shell in SC water is observed to be similar to that in ambient water, and long‐range solvation structures in SC water are similar to those observed for simple SC solvents. An excess of water molecules is observed clustering around ionic solute...

Na+–Cl− ion pair association in supercritical water

Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent‐separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean‐force potential from simulation and the impact of the assumptions involved.

Molecular dynamics simulations of the rheology of normal decane, hexadecane, and tetracosane

Extensive nonequilibrium molecular dynamics simulations have been carried out for liquid decane, hexadecane, and tetracosane at densities corresponding to atmospheric pressure and near ambient temperatures. The strain‐rate‐dependent viscosity has been obtained for strain rates ranging over several orders of magnitude. At high strain rate, the viscosities for all alkanes studied here have similar values and exhibit similar power‐law shear‐thinning behavior with a slope between about −0.40 and −0.33. Accompanying this shear thinning is the onset of orientational order and the alignment of the alkane molecules with the flow direction. The alignment angle tends to 45° at very low strain rate and is significantly smaller at high strain rate. This suggests that the chains substantially align in the flow direction and that the dominant motion at high strain rate is the sliding of the chains parallel to the flow. At low strain rate, the shear viscosity shows a transition to Newtonian behavior. The Newtonian visco...

Molecular simulation of the transition from liquidlike to solidlike behavior in complex fluids confined to nanoscale gaps

We report molecular dynamics simulations at ambient temperature and pressure of dodecane films of thickness between three and eight molecular layers confined between mica surfaces. We use an accurate united-atom model for dodecane and an effective interaction between the dodecane and the confining mica surfaces that is consistent with the surface energy of a mica surface. At ambient normal pressure, the strong surface–fluid interaction leads to increased dodecane density as the wall spacing is narrowed, crossing into a density region corresponding to bulk solid when the confined film becomes narrower than six molecular layers. Correspondingly, we observed a dramatic transition from a liquidlike to an ordered, solidlike structure when the confined dodecane film is reduced from seven to six molecular layers, consistent with experimental observation of many orders of magnitude increase in viscosity at the same film thickness. The solidlike structure is characterized by the layering as well as the in-plane or...

Molecular simulation study of solvation structure in supercritical aqueous solutions

We report new molecular dynamics simulations of aqueous solutions that consist of various solutes (xenon, NaCl, benzene, toluene, and benzonitrile) at infinite dilution in supercritical water at two thermodynamic states: one at the critical density and at 5% above the critical temperature, the other at 50% above the critical density and along the critical isotherm. These new simulations complement our previous simulations of infinitely dilute supercritical aqueous solutions of argon, Na+, Cl−, and methanol at the same state points. The aim of this work is to determine the solvation structure around various solutes in supercritical water with the goal of developing a molecular understanding of the solubility of nonpolar noble gases, ionic fluids and organics in supercritical water. Such an understanding can provide important insight into fundamental aspects of supercritical water oxidation of organic wastes. In addition, we report thermodynamic properties and solute—solvent structural quantities for the various solutes.

Solvation in supercritical water

Abstract Cochran H.D., Cummings P.T. and Karaborni S., 1992. Solvation in supercritical water. Fluid Phase Equilibria, 71: 1-16. The aim of this work is to determine the solvation structure in supercritical water compared with that in ambient water and in simple supercritical solvents. Molecular dynamics studies have been undertaken of systems that model ionic sodium and chloride, atomic argon, and molecular methanol in supercritical aqueous solutions using the simple point charge model of Berendsen for water. Because of the strong interactions between water and ions, ionic solutes are strongly attractive in supercritical water, forming large regions of increased local water density around each ion comparable to the solvent structures surrounding attractive solutes in simple supercritical fluids. Likewise, the deficit of water molecules surrounding a dissolved argon atom in supercritical aqueous solutions is comparable to that surrounding repulsive solutes in simple supercritical fluids. Methanol appears to be a weakly attractive or even repulsive solute in supercritical water. Only a small number of excess water molecules (if any) surround a methanol molecule in supercritical water, and this becomes a deficit at higher density. The number of hydrogen bonds per water molecule in supercritical water was found to be about one-third the number in ambient water. The number of hydrogen bonds per water molecule surrounding a central particle in supercritical water was only weakly affected by the identity of the central particle — atom, molecule or ion. These results should be helpful in developing a qualitative understanding of important processes which occur in supercritical water.

Supercritical carbon dioxide extraction of caffeine from guaraná

Abstract A single-pass system was designed to study extraction of caffeine from wet guarana seeds using supercritical carbon dioxide. Extraction was studied as a function of pressure and temperature to determine caffeine solubility at equilibrium conditions between 136.1 and 272.2 atm and temperatures of 35, 45, and 55 °C. Solubility of caffeine into supercritical CO 2 was affected directly by CO 2 density. Caffeine solubility was modeled by using thermodynamic relationships and the Peng-Robinson equation of state on a water-free basis. Effective diffusion coefficients of caffeine in ground guarana seeds were determined by conducting long-term extraction experiments.

Molecular dynamics study of the nano-rheology of n-dodecane confined between planar surfaces

Realistic molecular simulations agree with previously published surface force experiments that n-dodecane confined between mica surfaces displays shear-thinning starting at shear rate orders of magnitude less than in the bulk fluid. We probe the origin of this behavior by studying rotational and diffusional relaxations in the simulated fluid and find a freezing-out of the rotational degrees of freedom and a power-law diffusional relaxation, resulting in over seven orders of magnitude increase in the relaxation time.

Rheology of lubricant basestocks: A molecular dynamics study of C30 isomers

We have performed extensive equilibrium and nonequilibrium molecular dynamics (EMD and NEMD) simulations of three isomers of C30H62 at temperatures of 311 and 372 K employing a united atom model. Using the rotational relaxation time calculated from the EMD simulation, the Rouse model predicts a zero-shear viscosity for n-triacontane within 16% of the value determined by NEMD. Compared to experiment, NEMD and the united atom model underpredict the kinematic viscosities of n-triacontane and 9-n-octyldocosane but accurately predict the values for squalane (within 15%). In addition, the predicted values of the kinematic viscosity index for both 9-n-octyldocosane and squalane are in quantitative agreement with experiment and represent the first such predictions by molecular simulation. This same general potential model and computational approach can be used to predict this important lubricant property for potential lubricants prior to their synthesis, offering the possibility of simulation-guided lubricant des...

Molecular dynamics simulation of interfacial electrolyte behaviors in nanoscale cylindrical pores

Molecular simulations have been carried out on aqueous electrolytes in cylindrical pores a few nanometers in diameter, with uncharged wall or with dispersed, discrete charges modeling silica. The results show a classical Stern layer of adsorbed counterions near the wall, and then a diffuse layer with depleted ion concentration in the interfacial region. The depletion region coincides roughly with the more ordered water structure in the interface. Examination of the interaction energy shows that hydration energy of the ions disfavors the interior, suggesting the importance of the solvent structure near the wall. Comparison with the linearized Poisson–Boltzmann theory indicates qualitative differences in the predicted interfacial behavior.