Biomedical subjects

Timothy I Morrow

Publications and source records attributed to Timothy I Morrow.

3 recordsLinked to original sources

PubMed2006

Isomolar-semigrand ensemble molecular dynamics: application to vapor-liquid equilibrium of the mixture methane/ethane.

The isomolar-semigrand ensemble molecular dynamics (iSGMD) method is applied to the simulation of the binary system methane/ethane. The vapor-liquid equilibrium properties of this system at a temperature of 192.37 K are computed using the Gibbs-Duhem integration method. The iSGMD method, which resembles conventional hybrid Monte Carlo (MC) but is applicable to phase equilibrium calculations, is designed to overcome the difficulties associated with performing standard Monte Carlo-type particle transformations in liquid systems that are very dense and/or are comprised of complex molecules with many intramolecular degrees of freedom. This work shows that particle transformations using the iSGMD method for the simple system methane/ethane are at least 25 times more successful than standard MC-type transformations. The P-x-y curve for the system methane/ethane at 192.37 K computed using iSGMD simulations agrees very well with the experimental P-x-y curve as well as results of a previous MC study.

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PubMed2005

Isomolar semigrand ensemble molecular dynamics: development and application to liquid-liquid equilibria.

An extended system molecular dynamics method for the isomolar semigrand ensemble (fixed number of particles, pressure, temperature, and fugacity fraction) is developed and applied to the calculation of liquid-liquid equilibria (LLE) for two Lennard-Jones mixtures. The method utilizes an extended system variable to dynamically control the fugacity fraction xi of the mixture by gradually transforming the identity of particles in the system. Two approaches are used to compute coexistence points. The first approach uses multiple-histogram reweighting techniques to determine the coexistence xi and compositions of each phase at temperatures near the upper critical solution temperature. The second approach, useful for cases in which there is no critical solution temperature, is based on principles of small system thermodynamics. In this case a coexistence point is found by running N-P-T-xi simulations at a common temperature and pressure and varying the fugacity fraction to map out the difference in chemical potential between the two species A and B (mu(A)-mu(B)) as a function of composition. Once this curve is known the equal-distance/equal-area criterion is used to determine the coexistence point. Both approaches give results that are comparable to those of previous Monte Carlo (MC) simulations. By formulating this approach in a molecular dynamics framework, it should be easier to compute the LLE of complex molecules whose intramolecular degrees of freedom are often difficult to properly sample with MC techniques.

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PubMed2004

Why Is CO2 so soluble in imidazolium-based ionic liquids?

Experimental and molecular modeling studies are conducted to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ionic liquids. CO2 absorption isotherms at 10, 25, and 50 degrees C are reported for six different ionic liquids formed by pairing three different anions with two cations that differ only in the nature of the "acidic" site at the 2-position on the imidazolium ring. Molecular dynamics simulations of these two cations paired with hexafluorophosphate in the pure state and mixed with CO2 are also described. Both the experimental and the simulation results indicate that the anion has the greatest impact on the solubility of CO2. Experimentally, it is found that the bis(trifluoromethylsulfonyl)imide anion has the greatest affinity for CO2, while there is little difference in CO2 solubility between ionic liquids having the tetrafluoroborate or hexafluorophosphate anion. The simulations show strong organization of CO2 about hexafluorophosphate anions, but only small differences in CO2 structure about the different cations. This is consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations. Computed and measured densities, partial molar volumes, and thermal expansion coefficients are also reported.

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