Biomedical subjects

Collin D Wick

Publications and source records attributed to Collin D Wick.

13 recordsLinked to original sources

PubMed2006

Simulated surface potentials at the vapor-water interface for the KCl aqueous electrolyte solution.

Classical molecular dynamics simulations with polarizable potential models were carried out to quantitatively determine the effects of KCl salt concentrations on the electrostatic surface potentials of the vapor-liquid interface of water. To the best of our knowledge, the present work is the first calculation of the aqueous electrolyte surface potentials. Results showed that increased salt concentration enhanced the electrostatic surface potentials, in agreement with the corresponding experimental measurements. Furthermore, the decomposition of the potential drop into contributions due to static charges and induced dipoles showed a very strong effect (an increase of approximately 1 V per 1M) due to the double layers formed by KCl. However, this was mostly negated by the negative contribution from induced dipoles, resulting in a relatively small overall increase ( approximately 0.05 V per 1M) with increased salt concentration.

Chemistry, Physical↗

PubMed2006

Computational observation of enhanced solvation of the hydroxyl radical with increased NaCl concentration.

Classical molecular dynamics simulations with many-body potentials were carried out to quantitatively determine the effect of NaCl salt concentration on the aqueous solvation and surface concentration of hydroxyl radicals. The potential of mean force technique was used to track the incremental free energy of the hydroxyl radical from the vapor, crossing the air-water interface into the aqueous bulk. Results showed increased NaCl salt concentration significantly enhanced hydroxyl radical solvation, which should significantly increase its accommodation on water droplets. This has been experimentally observed for ozone aqueous accommodation with increased NaI concentration, but, to our knowledge, no experimental study has probed this for hydroxyl radicals. The origin for this effect was found to be very favorable hydroxyl radical-chloride ion interactions, being stronger than those for water-chloride.

Hydroxyl Radical↗

PubMed2006

Distribution, structure, and dynamics of cesium and iodide ions at the H2O-CCl4 and H2O-vapor interfaces.

Molecular dynamics simulations utilizing many-body potentials of H2O-CCl4 and H2O-vapor interfaces were carried out at different cesium and iodide ion concentrations to compare ion distribution, interfacial orientational and structural properties, and dynamics. It was found that cesium was repelled by both interfaces, and iodide was active at both interfaces, but to a much greater degree at the H2O-vapor interface. At the interface, the iodide dipole was strongly induced, orienting perpendicular to the interface for both systems, leading to stronger hydrogen bonds with water. For the H2O-CCl4 interface, though, there was a compensation between these strong hydrogen bonds and short to moderate ranged repulsion between iodide and CCl4. Hydrogen bond distance and angular distributions showed weaker water-water hydrogen bonds at both interfaces, but generally stronger water-iodide hydrogen bonds. Both translational and rotational dynamics of water were faster at the interface, while for CCl4, its translational dynamics was slower, but rotational dynamics faster at the interface. For many of the studied systems and species, translational diffusion was found to be anisotropic at both interfacial and bulk regions.

Carbon↗

PubMed2006

Critical comparison of classical and quantum mechanical treatments of the phase equilibria of water.

The Gibbs ensemble Monte Carlo simulation technique was used to compare the phase equilibria of the rigid TIP4P water model [Jorgensen et al., J. Chem. Phys. 79, 926 (1983)] utilizing classical and quantum statistical mechanics. The quantum statistical mechanical treatment generally resulted in lower liquid densities and higher vapor densities, narrowing the phase envelope. As a result, the calculated critical temperatures and normal boiling points were lower from the quantum simulations than the classical by 22 and 17 K, respectively, but the critical densities were equal within the estimated uncertainties. When the phase diagram from the quantum statistical mechanical treatment was increased by 22 K, it agreed with the classical results quite well throughout the entire simulated temperature range. A semiclassical treatment, involving a low order expansion in Planck's constant, resulted in good agreement with the path integral results for second virial coefficients, but gave densities and vapor pressures that fluctuated between the values for the classical and quantum statistical mechanics values, with no definite agreement with either.

Journal Article↗

PubMed2005

Investigating pressure effects on structural and dynamical properties of liquid methanol with many-body interactions.

Molecular-dynamics simulations utilizing a many-body potential was used to study the pressure dependence of structural and dynamical properties for liquid methanol. The liquid density as a function of pressure agreed well with experiment, and a combination of radial and angular distribution functions were used to analyze molecular structure. From these distribution functions, it was observed that hydrogen bond strength increased with increasing pressure. This observation coincided with an increase in the molecular dipole as a function of pressure, having a significant effect on the observed increased hydrogen bond strength. Also, methanols were found to more strongly favor exactly two hydrogen bonds, with fewer methanols of zero, one, or three hydrogen bonds present at higher pressures. Furthermore, a majority of the compression with increased pressure was found to occur in regions perpendicular to the methanol hydrogen-oxygen bond vector. This was the case despite hydrogen-oxygen nonbonded distances between hydrogen bonding species being shorter, but their stiffer oxygen-hydrogen-(nonbonded) oxygen angle offsets this, resulting in their oxygen-oxygen distances being relatively unaffected. The methanol translational diffusion decreased significantly with increased pressure, while the rotational diffusion decreased at a similar magnitude around the oxygen-hydrogen and oxygen-carbon bond vectors, despite having very different overall diffusion. Finally, the hydrogen bond lifetime increased significantly with pressure, owing to the increased hydrogen bond strength, and the slower translational and rotational dynamics.

Journal Article↗

PubMed2005

Transferable potentials for phase equilibria. 7. Primary, secondary, and tertiary amines, nitroalkanes and nitrobenzene, nitriles, amides, pyridine, and pyrimidine.

The transferable potentials for phase equilibria (TraPPE) force fields are extended to amine, nitro, nitrile, and amide functionalities and to pyridine and pyrimidine. In many cases, the same parameters for a functional group are used for both united-atom and explicit-hydrogen representations of alkyl tails. Following the TraPPE philosophy, the nonbonded interaction parameters were fitted to the vapor-liquid coexistence curves for selected one-component systems. Coupled-decoupled configurational-bias Monte Carlo simulations in the Gibbs ensemble were applied to neat (methyl-, dimethyl-, trimethyl-, ethyl-, diethyl-, or triethyl-)amine, nitromethane, nitroethane, nitrobenzene, acetonitrile, propionitrile, acetamide, propanamide, butanamide, pyridine, and pyrimidine. Excellent agreement with experimental results was found, with the mean unsigned errors being less than 1% for both the critical temperature and the normal boiling temperature. Similarly, the liquid densities at low reduced temperatures are reproduced to within 1%, and the deviation for the critical densities is about 4%. Additional simulations were performed for the binary mixtures of methylamine + n-hexane, diethyl ether + acetonitrile, 1-propanol + acetonitrile, and nitroethane + ethanol. With the exception of the methylamine/n-hexane mixture for which the separation factor is substantially overestimated, agreement with experiment for the other three mixtures is very satisfactory.

Amides↗

PubMed2005

Microscopic origins for the favorable solvation of carbonate ether copolymers in CO2.

The strong desire for the wide use of carbon dioxide as an environmentally benign process solvent has spurred a large number of attempts to improve its solubility characteristics. Pioneering experimental work by Beckman and co-workers has pointed to carbonate ether copolymers as promising candidates for nonfluorous surfactants. It is demonstrated here that Gibbs ensemble Monte Carlo simulations (using configurational-bias and double-bridging strategies and the transferable potentials for phase equilibria force field) can be employed to accurately predict the phase equilibria of a carbonate ether copolymer with CO2. The simulations indicate that the greater accessibility of the carbonyl oxygen plays a major role for the CO2-philicity of this copolymer surfactant.

Journal Article↗

PubMed2005

Diffusion at the liquid-vapor interface of an aqueous ionic solution utilizing a dual simulation technique.

The recently proposed dual simulation technique [J. Phys. Chem. B 2004, 108, 6595.], with slight modification, was used to determine the diffusion coefficients for a variety of regions of a 2.2 M sodium chloride aqueous solution with a vapor-liquid interface. The diffusion of all species was shown to be isotropic far away from the interface, but at different regions in the interface, the diffusion coefficients parallel and perpendicular to the interface did not agree for water and chloride. Specifically, interfacial water diffusion parallel to the interface was significantly higher than diffusion perpendicular to the interface. Chloride ions showed even larger anisotropicity in its diffusion coefficient at the interface, with its perpendicular diffusion being similar to its bulk value, but parallel diffusion being much higher, corresponding to the region of highest chloride ion concentration. The origin for this was found to be hydrogen bonds with waters which are highly oriented perpendicular to the interface, somewhat impeding chloride ion diffusion perpendicular to the interface. While sodium ion diffusion increased at the interface, its interfacial concentration is low in that region, and its diffusion was fairly isotropic throughout all regions.

Journal Article↗

PubMed2005

Temperature dependence of hydrogen bonding: an investigation of the retention of primary and secondary alcohols in gas-liquid chromatography.

Configurational-bias Monte Carlo simulations in the Gibbs Ensemble were carried out to investigate the analyte partitioning of n-pentane, n-hexane, n-heptane, 1-propanol, and 2-propanol into a dioctyl ether retentive (stationary) phase used in gas-liquid chromatography. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to model all analytes and the solvent. The analyte partition coefficients, Gibbs free energies of transfer, and Kovats retention indexes were calculated at four different temperatures ranging from 303.15 to 348.15 K. Although hydrogen bonding is a major contributor to the retention of the alcohol analytes over the entire temperature range, its importance for the separation factor between the primary and secondary alcohol decreases substantially with increasing temperature. The enthalpies and entropies for hydrogen bond formation were also estimated from the temperature dependence of the corresponding equilibrium constants. In agreement with experimental measurements, it is observed that the hydrogen bond involving 1-propanol is enthalpically favored, but entropically disfavored compared to 2-propanol.

Journal Article↗

PubMed2004

Simulation studies on the effects of mobile-phase modification on partitioning in liquid chromatography.

Various driving forces have been suggested to explain retention and selectivity in reversed-phase liquid chromatography (RPLC). To provide molecular-level information on the retention mechanism in RPLC, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for model systems consisting of three phases: an n-hexadecane retentive phase, a mobile phase with varying water-methanol composition, and a helium vapor phase as reference state. Liquid n-hexadecane functions as a model of a hydrophobic stationary phase, and a wealth of experimental data exists for this system. Gibbs free energies for solute transfers from gas to retentive phase, from gas to mobile phase, and from mobile to retentive phase were determined for a series of short linear alkanes and primary alcohols. Although the magnitude of the incremental Gibbs free energy of transfer for a methylene segment is always larger for the gas- to retentive-phase transfer than the gas- to mobile-phase transfer, it is found that the partitioning of alkanes and alkyl tail groups is mostly affected by the changes in the aqueous mobile phase that occur when methanol modifiers are added. In contrast, the partitioning of the alcohol headgroup is sensitive to changes in both the n-hexadecane and the mobile phases. In particular, it is found that hydrogen-bonded aggregates of methanol are present in the n-hexadecane phase for higher methanol concentrations in the mobile phase. These aggregates strongly increase alcohol partitioning into the retentive phase. The simulation data clearly demonstrate that due to modification of the retentive-phase hydrocarbons by solvent components, neither the solvophobic theory of RPLC, advocated by Horvath and co-workers, nor the lipophilic theory of RPLC, advocated by Carr and co-workers, can adequately describe the separation mechanism of the hexadecane model system of a retentive phase studied here nor the more complex situation present in actual RPLC systems.

Journal Article↗

PubMed2002

Molecular simulation of concurrent gas-liquid interfacial adsorption and partitioning in gas-liquid chromatography.

The importance of adsorption at the gas-liquid interface on retention in gas-liquid chromatography has been controversial since the pioneering work of Martin in the 1960s. In particular, experimental studies using chromatographic and static techniques to quantify partitioning and adsorption of polar analytes on nonpolar liquid phases yielded conflicting results. In this work, Monte Carlo simulations were carried out for a free-standing liquid slab of squalane surrounded by a helium vapor to investigate interfacial adsorption effects for n-pentane, n-hexane, n-heptane, 1-butanol, and benzene solutes at infinite dilution. The simulations indicate preferential adsorption for the flexible alkane and alcohol solutes in a narrow region just inside the Gibbs dividing surface, but no such effect was observed for the rigid benzene solute. Nevertheless, the extent of the interfacial enrichment is small, as measured by the partition coefficient between the bulk liquid and the interfacial region (K(bulk-interface) approximately 1.5). In addition, a region that is slightly depleted for all solute molecules is found to separate the interfacial and bulk regions of the squalane slab. Thus, adsorption at the gas-liquid interface should not contribute significantly to the retentive behavior observed in gas-liquid chromatography on nonpolar capillary columns but might play a role in packed-bed columns with low bonded-phase loadings. The origin for the small enrichments and more favorable free energies for solutes at the interface is that the enthalpies of solvation decrease to a smaller relative extent than the entropies of solvation compared to the bulk liquid.

Journal Article↗

PubMed2002

Temperature effects on the retention of n-alkanes and arenes in helium-squalane gas-liquid chromatography. Experiment and molecular simulation.

Experiments and molecular simulations were carried out to study temperature effects (in the range of 323 to 383 K) on the absolute and relative retention of n-hexane, n-heptane, n-octane, benzene, toluene and the three xylene isomers in gas-liquid chromatography. Helium and squalane were used as the carrier gas and retentive phase, respectively. Both the experiments and the simulations show a markedly different temperature dependence of the retention for the n-alkanes compared to the arenes. For example, over the 60 K temperature range studied, the Kovats retention index of benzene is found to increase by about 16 or 18+/-10 retention index units determined from the experiments or simulations, respectively. For toluene and the xylenes, the experimentally measured increases are similar in magnitude and range from 14 to 17 retention index units for m-xylene to o-xylene. The molecular simulation data provide an independent method of obtaining the transfer enthalpies and entropies. The change in retention indices is shown to be the result of the larger entropic penalty and the larger heat capacity for the transfer of the alkane molecules.

Alkanes↗

PubMed2002

Influence of analyte overloading on retention in gas-liquid chromatography: a molecular simulation view.

In an attempt to elucidate the molecular basis for concentration (isotherm) effects on retention in gas-liquid chromatography, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to investigate changes in analyte partitioning caused by overloading a model chromatographic system with either an alkane or an alcohol. Squalane was used as the stationary-phase material, and the analytes included n-pentane, n-hexane, n-heptane, 1 -butanol, and 1-pentanol. Three systems were studied that differed in the mobile-phase composition: (i) a helium vapor, (ii) a n-hexane vapor, and (iii) a 1-pentanol-saturated helium vapor. While the amount of helium that partitions into the stationary phase is very small, both n-hexane and 1-pentanol partition strongly into and thereby swell the stationary phase. Although the swelling of the stationary phase leads to a reduction in the partition coefficients for the alkane solutes for both the n-hexane- and 1-pentanol-swollen stationary phases, the effects on the alcohol solutes differ markedly. Whereas saturation by n-hexane causes a decrease of the alcohol partition contants (to an extent similar to that for the alkane solutes), the saturation by 1-pentanol causes a dramatic increase of the alcohol partition coefficients; e.g., the Kovats index of 1-butanol increases by more than 150 Kovats units. The formation of hydrogen-bonded alcohol aggregates in the liquid phase is the microscopic origin for the dramatic effect of 1-pentanol saturation on the retention of alcohols.

Journal Article↗