Biomedical subjects

M Borkovec

Publications and source records attributed to M Borkovec.

10 recordsLinked to original sources

PubMed2001

Predicting the wettability of quartz surfaces exposed to dense nonaqueous phase liquids.

A model incorporating electrostatic and van der Waals forces was used to predict the wetting behavior of several systems containing a quartz surface, water, and a dense nonaqueous phase liquid (DNAPL). The disjoining pressure between the quartz/water and a water/DNAPL interface as a function of their separation was calculated from a full solution of the nonlinear Poisson-Boltzmann equation allowing for charge regulation. For the pH regime associated with a positive pressure barrier, the quartz surface was predicted to be water wetting. The model takes into account the charging behavior of an isolated DNAPL/water interface as inferred from electrophoretic mobility data. It provides excellent predictions for four coal tar and creosote samples tested, where the quartz was oil-wetting below pH 5 and water-wetting at higher pH values. In a fifth sample, the transition occurred above pH 7, which indicates that mechanisms other than electrostatic interaction control the wetting behavior in this sample.

Environmental Pollution↗

PubMed2000

A cluster expansion method for the complete resolution of microscopic ionization equilibria from NMR titrations

The NMR chemical shift of spin 1/2 nuclei in a polyprotic molecule represents a sensitive probe of microscopic protonation equilibria. However, these equilibria are commonly parametrized in terms of microscopic equilibrium constants, whose number increases very rapidly with the number of ionizable groups. For that reason their determination was considered to be basically impossible except for the cases of the simplest molecules. On the basis of a cluster expansion of the free energy of a microstate, we propose a novel parametrization of this problem that drastically reduces the number of necessary parameters needed to specify the microscopic equilibria. Such cluster parameters can be extracted from NMR titration data in a straightforward way. Once these parameters are known, all microscopic equilibrium constants can be obtained.

Journal Article↗

PubMed1999

Exact Poisson-Boltzmann solution for the interaction of dissimilar charge-regulating surfaces.

An efficient method is proposed to calculate the electric double layer force between two flat surfaces of dissimilar composition and ionization properties. The approach is based on explicit expressions for the solution of the (nonlinear) Poisson-Boltzmann equation and allows for boundary conditions of charge regulation, i.e., chemical equilibrium of both surfaces with a bulk electrolyte at all surface separations. As an illustration, we discuss in some detail the interaction between a weakly acidic and a strongly acidic latex surface, and between an acidic (silica) surface and an amphoteric (rutile) surface.

Journal Article↗

PubMed1998

Colloidal Particles at Water-Glass Interface: Deposition Kinetics and Surface Heterogeneity.

Videomicroscopy in combination with evanescent field illumination is applied to study the sorption of colloidal particles from flow in a parallel plate channel on a glass surface. The experiments, carried out in the presence of a repulsive electrostatic barrier, reveal surprisingly complex results: The glass surface, though optically flat and well cleaned, is not homogeneous, but rather the sorption occurs at a limited number of preferred sites. Moreover, these sites are not static: new sites keep appearing at random positions on the observed surface and disappearing at a rate of kd = 1.3 x 10(-5) s-1. These findings can be understood within a simple model that takes into account slow but inevitable dissolution of the glass surface. The bulk glass contains potential adsorbers, which are continuously being exposed by the dissolution process and act as transient adsorption sites, before being washed off by the flowing buffer solution. Copyright 1998 Academic Press.

Journal Article↗

PubMed1997

Absolute Aggregation Rate Constants of Hematite Particles in Aqueous Suspensions: A Comparison of Two Different Surface Morphologies.

Measurements of absolute aggregation rate constants were performed for two samples of well-characterized spheroidal hematite particles with rather different surface morphology. At high ionic strength, the system is in the fast aggregation regime with pH independent rate constants around (1-4) x 10(-18) m3/s. At low ionic strengths, the rate constant is a function of pH and goes through a flat maximum around the point of zero charge (PZC) where fast aggregation conditions are reached. With increasing pH the rate constants increase gradually below the PZC while they decrease very rapidly above the PZC. Above the PZC the rate constants are rather well predicted by the classical DLVO theory. Below the PZC, however, it is necessary to assume a distribution in the surface potentials with a coefficient of variation around 30% to account for the observed behavior. In spite of the pronounced differences in the surface roughness of both hematite samples, their aggregation rate constants are rather similar. Copyright 1997 Academic Press.

Journal Article↗

PubMed1997

Measurement of Absolute Coagulation Rate Constants for Colloidal Particles: Comparison of Single and Multiparticle Light Scattering Techniques

The absolute coagulation rate constants of monodisperse spherical colloids in aqueous suspension were measured at the early stage of the coagulation processes by using two different techniques: single particle light scattering (SPLS) and simultaneous static and dynamic light scattering (SLS + DLS). Single particle light scattering determines the cluster-size distribution directly by counting the number of different clusters during the coagulation process and therefore can be used to test the kinetic growth model used for obtaining the coagulation rate constants. The simultaneous static and dynamic light-scattering method is an in-situ experiment, where the average cluster size is estimated by simultaneous static and dynamic light-scattering measurements at different angles on a multiangle fiber-optics-based setup. A combined evaluation of the static and dynamic light-scattering data permits the determination of the absolute rate constants without the explicit use of light scattering form factors and hydrodynamic properties for dimers. In this paper, we compare results obtained with both techniques. Good agreement was found for the absolute coagulation rate constants of two different latex particle suspensions. Furthermore, we show that the two techniques complement one another and the limitations of the first are overcome by the second and vice versa. Copyright 1997Academic Press

Journal Article↗

PubMed1997

Interpretation of Competitive Adsorption Isotherms in Terms of Affinity Distributions

In the present study we evaluate affinity distributions for competitive adsorption isotherms which involve several components. In such a multicomponent situation, the affinity distribution becomes a function of several affinity constants, and already in the case of two components, little is known about their features. In the two-component situation, we have calculated the affinity distributions from the adsorption isotherms with a numerical inversion technique. This technique is based on a constrained least-squares algorithm and uses a regularization function which biases the resulting affinity distribution toward a smooth function. The applicability of the procedure was tested with a newly derived isotherm, which is based on a fully uncorrelated affinity distribution, and with the generalized Langmuir-Freundlich (LF) isotherm, which is known to have a perfectly correlated distribution. The present study demonstrates that the extended Henderson-Hasselbalch (HH) isotherm has an underlying affinity distribution, which displays a partial correlation, while the non-ideal competitive adsorption (NICA) isotherm has an affinity distribution with a varying degree of correlation. In the competitive situation, the affinity distribution thus provides an interesting means to characterize the corresponding isotherms. As an illustration of the present techniques, experimental data of metal ion binding for a humic acid are analyzed in the same context.

Journal Article↗

PubMed1997

On the difference in ionization properties between planar interfaces and linear polyelectrolytes.

Ionizable planar interfaces and linear polyelectrolytes show markedly different proton-binding behavior. Planar interfaces protonate in a single broad step, whereas polyelectrolytes mostly undergo a two-step protonation. Such contrasting behavior is explained using a discrete-charge Ising model. This model is based on an approximation of the ionizable groups by point charges that are treated within a linearized Poisson-Boltzmann approximation. The underlying reason as to why planar interfaces exhibit mean-field-like behavior, whereas linear polyelectrolytes usually do not, is related to the range of the site-site interaction potential. For a planar interface, this interaction potential is much more long ranged if compared with that of the cylindrical geometry as appropriate to a linear polyelectrolyte. The model results are in semi-quantitative agreement with experimental data for fatty-acid monolayers, water-oxide interfaces, and various linear polyelectrolytes.

Electrolytes↗

PubMed1995

A self-report diagnostic measure of generalized anxiety disorder.

The present study tested the reliability of a self-report diagnostic measure of generalized anxiety disorder (GAD) based on DSM criteria. Among two samples of undergraduate students, 47-80% of the GAD diagnoses by questionnaire were confirmed by diagnostic interview, with the higher rate being associated with DSM-IV criteria. Categorization of a participant as Non-GAD by questionnaire was found to be 100% reliable by interview. Given these findings as well as prior empirical support for the validity of the measure, this questionnaire has utility for identifying analogue GAD and Non-GAD participants from undergraduate populations for use in preliminary investigations of the mechanisms and functions associated with chronic worry and GAD.

Anxiety Disorders↗

PubMed1992

Phenomenological theories of globular microemulsions.

Simple phenomenological theory of noninteracting microemulsion water droplets dispersed in an oil phase (or vice versa) is presented. Several characteristics quantities such as radii, polydispersities, shape fluctuation amplitudes and interfacial tensions are evaluated. In particular, the different features of a one phase microemulsion and a two phase coexistence of the microemulsion with an excess water phase are emphasized. We discuss the relations to most existent theories of microemulsion droplets, micelles and nucleating droplets and briefly review recent experimental techniques to access the relevant parameters.

Emulsions↗