Biomedical subjects

S W de Leeuw

Publications and source records attributed to S W de Leeuw.

5 recordsLinked to original sources

PubMed2006

Computing the starting state for Gibbs-Duhem integration.

Gibbs-Duhem integration implies the numerical integration of a Clapeyron equation. To start the numerical integration, an initial coexistence point and a corresponding initial slope of the Clapeyron equation are needed. In order to apply Gibbs-Duhem integration to all kinds of systems at diverse physical conditions, one has to investigate and assess the available methods that can be used to compute these initial values. This publication focuses on vapor-liquid equilibria in binary mixtures comprising chain molecules. The initial coexistence point is either computed with the NVbeta Gibbs ensemble or with the Npbeta+test molecule method with overlapping distributions, which is introduced in this publication. Although computationally demanding, the Npbeta+test molecule method with overlapping distributions is applicable at conditions where the NVbeta Gibbs ensemble fails. We investigated three methods that can be employed to compute the initial slope of the Clapeyron equation. The Widom method and the overlapping-distributions difference method provide correct values for the initial slope. The difference method does only provide the correct answer in special cases. The possibility to judge the reliability of the results makes the overlapping-distributions difference method the safest route to the initial slope. Gibbs-Duhem integration requires the frequent computation of the slope of the Clapeyron equation. This slope depends on ensemble averages of the composition. A new bias method for efficient sampling of the composition in a semigrand-canonical simulation of chain molecules is presented. This bias method considerably enhances the composition sampling in systems comprising chain molecules of different sizes.

Journal Article↗

PubMed2006

An advanced Gibbs-Duhem integration method: theory and applications.

The conventional Gibbs-Duhem integration method is very convenient for the prediction of phase equilibria of both pure components and mixtures. However, it turns out to be inefficient. The method requires a number of lengthy simulations to predict the state conditions at which phase coexistence occurs. This number is not known from the outset of the numerical integration process. Furthermore, the molecular configurations generated during the simulations are merely used to predict the coexistence condition and not the liquid- and vapor-phase densities and mole fractions at coexistence. In this publication, an advanced Gibbs-Duhem integration method is presented that overcomes above-mentioned disadvantage and inefficiency. The advanced method is a combination of Gibbs-Duhem integration and multiple-histogram reweighting. Application of multiple-histogram reweighting enables the substitution of the unknown number of simulations by a fixed and predetermined number. The advanced method has a retroactive nature; a current simulation improves the predictions of previously computed coexistence points as well. The advanced Gibbs-Duhem integration method has been applied for the prediction of vapor-liquid equilibria of a number of binary mixtures. The method turned out to be very convenient, much faster than the conventional method, and provided smooth simulation results. As the employed force fields perfectly predict pure-component vapor-liquid equilibria, the binary simulations were very well suitable for testing the performance of different sets of combining rules. Employing Lorentz-Hudson-McCoubrey combining rules for interactions between unlike molecules, as opposed to Lorentz-Berthelot combining rules for all interactions, considerably improved the agreement between experimental and simulated data.

Journal Article↗

PubMed2005

Dynamics and lithium binding energies of polyelectrolytes based on functionalized poly(para-phenylene terephthalamide).

Polyelectrolyte materials are an interesting class of electrolytes for use in fuel cell and battery applications. Poly(para-phenylene terephthalamide) (PPTA, Kevlar) is a liquid crystalline polymer that, when sulfonated, is a polyelectrolyte that exhibits moderate ion conductivity at elevated temperatures. In this work, quasi-elastic neutron scattering (QENS) experiments were performed to gain insight into the effect of the presence of lithium counterions on the chain dynamics in the material. It was found that the addition of lithium ions decreases the dynamics of the chains. Additionally, the binding of lithium ions to the sulfonic acids groups was investigated by density functional theory (DFT) calculations. It was found that the local surroundings of the sulfonic acid group have very little effect on the lithium-ion binding energy. Binding energies for a variety of different systems were all calculated to be around 150 kcal/mol. The DFT calculations also show the existence of a structure in which a single lithium ion interacts with two sulfonic acid moieties on different chains. The formation of such "electrostatic cross-links" is believed to be the source of the increased tendency to aggregate and the reduced dynamics in the presence of lithium ions.

Journal Article↗

PubMed2001

Effect of diffusion on lithium intercalation in titanium dioxide.

A new model of Li intercalation into rutile and anatase structured titania has been developed from first principles calculations. The model includes both thermodynamic and kinetic effects and explains the observed differences in intercalation behavior and their temperature dependence. The important role of strong local deformations of the lattice and elastic screening of interlithium interactions is demonstrated. In addition, a new phase of LiTiO2 is reported.

Journal Article↗

PubMed1999

Self-motion in glass-forming polymers: a molecular dynamics study.

We present results of molecular dynamics simulations of an undercooled polymer melt, performed to study the validity of mode-coupling theory (MCT) for realistic polymer melts in general. The mean square displacements of the chain segments are computed to study the diffusion constant of the Rouse-like motion. It is shown that this diffusion constant follows a power law behavior as a function of the temperature, as predicted by the MCT. In addition, we studied the incoherent part of the intermediate scattering function and show that these functions obey the second scaling law of the MCT. We also calculated the relaxation times of the alpha-relaxation and found that they follow the same power law (gamma=2.9) as the diffusion constant. Using gamma, and the relationships given by MCT, we obtain values for a (0.27) and b (0.46) and use these exponents to describe the beta-relaxation regime. We find that the long time part of the beta-relaxation can be described accurately by the Von Schweidler relaxation over a wide range of wave numbers. In the short time regime of the beta-relaxation, no critical decay is observed.

Journal Article↗