Biomedical subjects

Clare McCabe

Publications and source records attributed to Clare McCabe.

11 recordsLinked to original sources

PubMed2006

Predicting the phase behavior of nitrogen + n-alkanes for enhanced oil recovery from the SAFT-VR approach: examining the effect of the quadrupole moment.

The phase behavior of nitrogen + n-alkanes is studied within the framework of the statistical associating fluid theory for potentials of variable range (SAFT-VR). The effect of the quadrupole moment of nitrogen on the phase behavior is considered through an extension of the SAFT-VR equation that includes an additional contribution to the Helmholtz free energy due to quadrupolar interactions. A significant improvement in the description of the phase diagram of the binary mixtures of nitrogen with different n-alkanes is obtained with the new approach when compared to predictions from the original SAFT-VR EOS (i.e., without the quadrupolar term). The experimental value for the quadrupole moment of nitrogen is used in the new equation; thus, no additional parameters are employed. Given the nonideal nature of the binary mixtures, a binary interaction parameter is needed to describe the full-phase diagram and high-pressure critical lines of these systems; however, this can be fitted to a single system and successfully used to predict the phase behavior of other binary mixtures without additional fitting. Furthermore, only a single, transferable, cross-energy parameter is required when the quadrupolar term is considered, whereas a cross-range parameter is also needed with the original SAFT-VR approach. The inclusion of the quadrupolar term in the equation of state therefore reduces the need to use effective parameters by explicitly including at the molecular level interactions due to the quadrupole moment.

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PubMed2006

Prediction of viscosity for molecular fluids at experimentally accessible shear rates using the transient time correlation function formalism.

Nonequilibrium molecular dynamics (NEMD) simulations were performed and the transient time correlation function (TTCF) method applied to calculate the shear viscosity of n-decane. Using the TTCF method we were able to calculate the viscosity at shear rate orders of magnitude lower than is possible by direct NEMD simulation alone. For the first time for a molecular fluid, we were able to simulate shear rates accessible by experimental measurements, which are typically performed at shear rates well below those accessible by NEMD simulation. The TTCF method allows us to close the gap between the lowest shear rates accessible by MD simulation and the highest shear rates possible in experimental studies. Additionally a multiple time step method for Gaussian thermostatted SLLOD equations of motion was developed following earlier work [G. A. Pan et al., J. Chem. Phys. 122, 4114 (2005)] for atomic fluids.

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PubMed2006

Phase behavior of dipolar fluids from a modified statistical associating fluid theory for potentials of variable range.

A statistical associating fluid theory for potentials of variable range to model dipolar fluids is presented. The new theory, termed the SAFT-VR+D equation (the statistical associating fluid theory for potentials of variable range plus dipole), explicitly accounts for dipolar interactions and their effect on the structure of the fluid. This is achieved through the use of the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. Isothermal-isobaric and Gibbs ensemble Monte Carlo simulations have been performed in order to test the new theoretical approach. Predictions for the thermodynamic properties and phase behavior of dipolar square-well monomer and chain fluids, in which one or more segments are dipolar, are considered and compared with new computer simulation data. The results show that the equation of state in general provides a good description of the phase behavior of dipolar monomer and chain fluids. Some deviations are seen between the simulation data and theoretical predictions for monomer fluids with large dipole moments and for molecules composed of segments with dipoles in different orientations. This is addressed through the replacement of the GMSA by the linearized exponential approximation.

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PubMed2006

Organic-inorganic telechelic molecules: solution properties from simulations.

We report molecular dynamics simulations for telechelic molecules composed of two polyhedral oligomeric silsesquioxane (POSS) cages connected by one hydrocarbon backbone dissolved in liquid normal hexane. Silsesquioxanes are novel hybrid organic-inorganic molecules that are useful as building blocks for the synthesis of nanostructured materials. By including POSS molecules within a polymeric material it is possible to modify mechanical properties such as resistance to heat and glass transition temperatures. Unfortunately, the molecular mechanisms responsible for these enhancements are at present not completely understood. In an effort to elucidate the molecular phenomena responsible for these effects, we have studied the conformation of telechelic POSS molecules in solution, as well as their self-diffusion coefficients, as a function of the length of the hydrocarbon backbone. We focus on molecules in which the radius of gyration of the alkane backbone is comparable to the size of the silsesquioxane cages. Our results indicate that the backbone has a significant influence on both the equilibrium and the transport properties of dissolved telechelic hybrid molecules. These observations are useful for developing strategies to direct the self-assembly of nanostructured materials.

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PubMed2006

Evaluation of force fields for molecular simulation of polyhedral oligomeric silsesquioxanes.

Polyhedral oligomeric silsesquioxanes (POSS) are nanometer-size molecules suitable for the production of organic-inorganic nanocomposite materials. These organic-inorganic nano-building blocks show promise for enabling the production of polymeric materials of exceptional mechanical properties as well as novel composite materials. While the experimental studies of these materials have rapidly evolved in the past decade, their theoretical investigation is still in its infancy. Toward the validation of force fields for the molecular simulation of POSS-containing systems, we present the charge-transfer reactive (CTR) force field for the molecular simulation of polyhedral oligomeric silsesquioxane (POSS) molecules and compare the ability of this, and several force fields taken from the literature, to predict the thermophysical properties of POSS-containing systems. The literature force fields compared include the universal force field (UFF) and the COMPASS and Hybrid-COMPASS force fields. Predictions from molecular dynamics simulations of the structural parameters (unit cell vectors), melting temperature, and FT-IR spectra of crystals of POSS monomers are presented. The POSS monomers investigated are octahydride, octamethyl, and octapropyl POSS. Predicted quantities are compared to experimental results where available and provide molecular-level physical insight into several aspects of the behavior of POSS molecules. While all the force fields tested perform reasonably well, our results indicate that the Hybrid-COMPASS and CTR force fields predict structural properties that are in good agreement with experimental data.

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PubMed2005

Thermodynamic and transport properties of polyhedral oligomeric sislesquioxanes in poly(dimethylsiloxane).

Polyhedral oligomeric silsesquioxane (POSS) molecules when functionalized appropriately and dispersed in polymers show promise as monodisperse organic-inorganic hybrid nanocomposite materials characterized by superior mechanical properties. We report here molecular-simulation results for POSS-POSS radial distribution functions, potentials of mean force, and self-diffusion coefficients for POSS monomers dissolved in poly(dimethylsiloxane) in the temperature range of 300-1000 K. Our results demonstrate that it is possible to modulate the effective POSS-POSS interaction by increasing the temperature or by substituting the hydrogen atoms in the POSS monomer with methyl groups. In addition, our results indicate that the motion of POSS monomers dissolved in poly(dimethylsiloxane) follows a hopping mechanism.

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PubMed2005

Predicting mixture phase equilibria and critical behavior using the SAFT-VRX approach.

The SAFT-VRX equation of state combines the SAFT-VR equation with a crossover function that smoothly transforms the classical equation into a nonanalytical form close to the critical point. By a combinination of the accuracy of the SAFT-VR approach away from the critical region with the asymptotic scaling behavior seen at the critical point of real fluids, the SAFT-VRX equation can accurately describe the global fluid phase diagram. In previous work, we demonstrated that the SAFT-VRX equation very accurately describes the pvT and phase behavior of both nonassociating and associating pure fluids, with a minimum of fitting to experimental data. Here, we present a generalized SAFT-VRX equation of state for binary mixtures that is found to accurately predict the vapor-liquid equilibrium and pvT behavior of the systems studied. In particular, we examine binary mixtures of n-alkanes and carbon dioxide + n-alkanes. The SAFT-VRX equation accurately describes not only the gas-liquid critical locus for these systems but also the vapor-liquid equilibrium phase diagrams and thermal properties in single-phase regions.

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PubMed2005

Operator splitting algorithm for isokinetic SLLOD molecular dynamics.

We apply an operator splitting method to develop a simulation algorithm that has complete analytical solutions for the Gaussian thermostated SLLOD equations of motion [D. J. Evans and G. P. Morriss, Phys. Rev. A 30, 1528 (1984)] for a system under shear. This leads to a homogeneous algorithm for performing both equilibrium and nonequilibrium isokinetic molecular dynamics simulation. The resulting algorithm is computationally efficient. In particular, larger integration time steps can be used compared to simulations with regular Gaussian thermostated SLLOD equations of motion. The utility and accuracy of the algorithm are demonstrated through application to the Weeks-Chandler-Anderson fluid. Although strict conservation of the kinetic energy suppresses thermal fluctuations in the system, this algorithm does not allow simulations at lower shear rates than those normally afforded by older nonequilibrium molecular dynamics simulations.

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PubMed2004

Equation of state and liquid-vapor equilibria of one- and two-Yukawa hard-sphere chain fluids: theory and simulation.

The accuracy of several theories for the thermodynamic properties of the Yukawa hard-sphere chain fluid are studied. In particular, we consider the polymer mean spherical approximation (PMSA), the dimer version of thermodynamic perturbation theory (TPTD), and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). Since the original version of SAFT-VR for Yukawa fluids is restricted to the case of one-Yukawa tail, we have extended SAFT-VR to treat chain fluids with two-Yukawa tails. The predictions of these theories are compared with Monte Carlo (MC) simulation data for the pressure and phase behavior of the chain fluid of different length with one- and two-Yukawa tails. We find that overall the PMSA and TPTD give more accurate predictions than SAFT-VR, and that the PMSA is slightly more accurate than TPTD.

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PubMed2003

Molecular simulations of liquid-liquid interfacial properties: water-n-alkane and water-methanol-n-alkane systems.

Direct molecular dynamics simulations of the liquid-liquid interface of water-n-alkane and water-methanol-n-alkane systems have been performed in order to study the interfacial properties of these systems. The simulations were carried out using the NERD revised force field of Nath et al. for the n-alkanes, the simple point charge extended model for water, and the optimized potential for liquid simulations model for methanol. In order to validate the model employed in this work for the n-alkanes we calculated the coexisting densities, surface tension, and thickness of the interface for pure n-pentane. For all the systems studied the interfacial tension and thickness were calculated at 298.15 K. Our results show that, by adjusting the number of molecules to reproduce the liquid densities in the direct simulation method of the liquid-liquid interface in multicomponent systems, we are able to reproduce available experimental data for interfacial tension. The interfacial thickness is underpredicted and a constant negative deviation of approximately 2.5 A from the experimental data is usually observed. We find that methanol acts like surfactant when it is added to the water-n-alkane mixtures, reducing the interfacial tension of the liquid-liquid ternary system. The interfacial tension results agree quantitatively well for the range of concentrations of methanol studied.

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PubMed2002

Comparison of nonequilibrium molecular dynamics with experimental measurements in the nonlinear shear-thinning regime.

Nonequilibrium molecular dynamics of a low-molecular-weight fluid (squalane) are compared with experimental measurements in both the linear (Newtonian) and nonlinear (non-Newtonian) regimes. The experimental and simulation data are shown to follow the same time-temperature superposition master curve. This represents the first comparison of the nonlinear rheology predicted by nonequilibrium molecular dynamics with experiment, and is thus the first experimental test of nonequilibrium molecular dynamics simulations in the nonlinear regime.

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