Biomedical subjects

Ger J M Koper

Publications and source records attributed to Ger J M Koper.

5 recordsLinked to original sources

PubMed2006

Nonequilibrium thermodynamics--A tool to describe heterogeneous catalysis.

In the study of multi-component mass transfer it is common to use the film model, in which all the resistance to mass transfer towards a catalytic surface is assumed to be localized in a diffusion layer in front of the surface. At the surface one furthermore assumes that the temperature and chemical potentials are continuous, while the coupling of a possible heat flux to the mass fluxes is assumed to be negligible. Both these assumptions are questionable. Using nonequilibrium thermodynamics we discuss how to integrate the coupling between heat and mass fluxes in the description of the film. Furthermore, following Gibbs, we introduce the surface as a separate thermodynamic system where the coupling between the vectorial heat flux and the scalar reaction rate is allowed and can be significant in heterogeneous catalysis. Non-equilibrium thermodynamic theory for surfaces allows one to find the proper rate equations. It allows for a consistent and complete description of mass and heat transfer through the film and subsequently from the film to the surface where the reaction takes place. Fast endo- or exothermic surface reactions in heterogeneous catalysis may give significant temperature gradients between a catalyst surface and the media, which will, when not accounted for, lead to an incorrect evaluation of the activity, stability and selectivity of a catalyst. Non-equilibrium thermodynamics is a useful tool for predicting the surface temperature as well as for analyzing the system. In this contribution we sketch how to systematically set up the complete description, in which the film and the surface "sum up" to one effective surface.

Journal Article↗

PubMed2006

Deposition of nanosized latex particles onto silica and cellulose surfaces studied by optical reflectometry.

Deposition of positively charged nanosized latex particles onto planar silica and cellulose substrates was studied in monovalent electrolyte solutions at pH 9.5. The deposition was probed in situ with optical reflectometry in a stagnation point flow cell. The surface coverage can be estimated reliably with island film theory as well as with a homogeneous film model, as confirmed with atomic force microscopy (AFM). The deposition kinetics on the bare surface was of first order with respect to the particle concentration, whereby the deposition rate was close to the value expected for a perfect collector. The efficiency coefficient, which was defined as the ratio of the experimental and theoretical deposition rate constants, was in the range from 0.3 to 0.7. Subsequently, the surface saturated and a limiting maximum coverage was attained (i.e., blocking). These trends were in qualitative agreement with predictions of the random sequential absorption (RSA) model, where electrostatic interactions between the particles were included. It was observed, however, that the substrate strongly influenced the maximum coverage, which was substantially higher for silica than for cellulose. The major conclusion of this work was that the nature of the substrate played an important role in a saturated layer of deposited colloidal particles.

Cellulose↗

PubMed2006

Ionization equilibria and conformational transitions in polyprotic molecules and polyelectrolytes.

The coupling between proton binding and conformational degrees of freedom in polyprotic molecules and polyelectrolytes is studied theoretically. Our approach combines the classical rotational isomeric state (RIS) model developed by Flory and the site binding (SB) model used to treat proton binding equilibria. The properties of the resulting SBRIS model, which treats conformational degrees of freedom and proton binding on equal footing, are studied with statistical mechanical techniques. Quantities of interest, such as titration curves, conformational probabilities, or macroscopic binding constants, are expressed as thermal averages and are evaluated by direct enumeration of states or by transfer matrix techniques. We further demonstrate that in the SBRIS model conformational degrees of freedom can be averaged out, leading to the contracted description within the SB model. In most cases, this contraction leads to higher order interactions, which may not be present at the SBRIS level (e.g., triplet interactions). Several examples are discussed to illustrate the concepts developed. The case of succinic acid exemplifies the situation in its simplest form. The model can further rationalize the very different titration behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA). In particular, the characteristic "jump" in the titration curve of PMAA is described quantitatively and is interpreted in terms of a conformational transition.

Journal Article↗

PubMed2006

Heats of transfer in the diffusion layer before the surface and the surface temperature for a catalytic hydrogen oxidation (H2 + (1/2)O2 --> H2O) reaction.

The surface temperature and surface mole fractions are calculated for a catalytic hydrogen oxidation reaction over a Pt/Al2O3 catalyst pellet. The thermodynamics of irreversible processes was used in order to ensure the correct introduction of coupled heat and mass transfer. Two pathways, one using the 4 x 4 resistivity matrix and the other using a simplified effective conductivity matrix, were proven to yield equivalent results. By using expressions for the thermal diffusion coefficients, heats of transfer, and the Maxwell-Stefan diffusion coefficients given in the literature, available experimental data could be reproduced. The Dufour effect was found to be negligible for the prediction of the surface temperature. Neglecting the Soret effect would increase the predicted value of the surface temperature significantly-more than 30 K out of an average of about 400 K. It is found that the reaction rate can be used to predict the surface temperature.

Journal Article↗

PubMedDate not supplied

Effect of oxidation rate on cross-linking of mussel adhesive proteins.

The cross-linking behavior of mussel adhesive protein Mefp-1 was studied by measuring the rate of aggregation of the protein by photon correlation spectroscopy. To be able to calculate the aggregation numbers, the hydrodynamic radius of monomer Mefp-1 (10 nm) was determined under reducing conditions. The aggregation is controlled by the redox potential of the solution, and the aggregation number varied, independent of pH, over a factor 2 within the experimentally accessible redox potential window. A kinetic model for cross-linking, based on the intricate interplay of the oxidation and auto-oxidation of the hydroquinones of Mefp-1, is proposed. The oxidation rate strongly depends on redox potential. The cross-linking rate is taken to be proportional to the rate of auto-oxidation. The model correctly predicts the experimentally observed phenomena. When the oxidation rate is slower than the auto-oxidation rate, cross-linking is efficient and controlled by the oxidation rate. When the rate of auto-oxidation rate is slower than the oxidation rate, the cross-linking is inefficient due to the quick exhaustion of the hydroquinones. The experimentally determined rate constant for cross-linking is found to be much smaller than those found for auto-oxidation of hydroquinones because of the excluded volume interactions imposed by the protein backbone. Tuning the interplay between oxidation and auto-oxidation presents the potential of controlling cross-linking density independent of the density of reactive groups.

Animals↗