Biomedical subjects

Bjørn Hafskjold

Publications and source records attributed to Bjørn Hafskjold.

5 recordsLinked to original sources

PubMed2004

Molecular simulations of surface forces and film rupture in oil/water/surfactant systems.

We use dissipative particle dynamics (DPD) and molecular models to simulate interacting oil/water/surfactant interfaces. The system comprises sections of two emulsion droplets separated by a film. The film is in equilibrium with a continuous phase, in analogy with the surface force apparatus. This is achieved by combining DPD with a Monte Carlo scheme to simulate a muVT ensemble. The setup enables the computation of surface forces as a function of the distance between the two interfaces, as well as the detection of film rupture. We studied monolayers of nonionic model surfactants at different densities and compared oil-water-oil and water-oil-water emulsion films. Between surfactant monolayers facing each other tails-on (water-oil-water films), we observed repulsive forces due to the steric interaction between overlapping hydrophobic tails. The repulsion increases with surfactant density. Conversely, no such repulsion is observed between surfactant monolayers facing each other heads-on. Instead, the film ruptures, the monolayers merge, and a channel forms between the two droplet phases. Film rupture can also be induced in the water-oil-water films by forcing the interfaces together. The separation at rupture increases for oil-water-oil films and decreases for water-oil-water films when the surfactant density increases. The results are in qualitative agreement with existing theories of emulsion stability in creams, in particular with the channel nucleation theory based on the natural curvature of surfactants.

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PubMed2004

Chain length dependencies of the bending modulus of surfactant monolayers.

The effect of the surfactant chain length n on the bending modulus kappa of surfactant monolayers is simulated with a mesoscopic oil-water-surfactant model. We confirm a power law, kappa is proportional to np, as predicted by mean-field theory and found experimentally, and find p approximately 1.5 at a constant surface density and p approximately 1.0 at a constant interfacial tension. This agrees quite well with both mean-field theory (p=2-3, assuming constant surface density) and experiments (at constant surface tension). Our results suggest that the previously reported agreement between theory and experiment may be fortuitous and caused by the difference in surfactant types.

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PubMed2004

Simulating the effect of surfactant structure on bending moduli of monolayers.

We have used dissipative particle dynamics to simulate amphiphilic monolayers on the interface between oil and water. An ultralow interfacial tension is imposed by means of Monte Carlo to resemble the amphiphilic films that separate oil and water regions in microemulsions. We calculate the bending modulus by analyzing the undulation spectrum. By varying the surfactant chain length and topology we investigate the effect of surfactant structure and composition of the monolayer on the bending moduli. We find that increasing the thickness has a larger effect than increasing the density of the layer. This follows from the observations that at a given interfacial tension, the bending modulus increases with chain length and is larger for linear than branched surfactants. The increase with chain length is approximately linear, which is slower than the theoretical predictions at a fixed area. We also investigated a binary mixture of short and long surfactants compared to pure layers of the same average chain length. We find a roughly linear decrease in bending modulus with mole fraction of short surfactants. Furthermore, the mixed film has a lower bending modulus than the corresponding pure film for all mole fractions. Linking the bending moduli to the structure of the surfactants is an important step in predicting the stability of microemulsions.

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PubMed2002

Microscopic pressure tensor for hard-sphere fluids.

The microscopic pressure tensor, which is not uniquely defined, is analyzed for a uniform hard-sphere fluid in Cartesian and spherical coordinates. Two popular definitions, one due to Irving and Kirkwood (IK) [J. H. Irving and J. G. Kirkwood, J. Chem. Phys. 18, 817 (1950)] and the other due to Harasima (H) [A. Harasima, Adv. Chem. Phys. 1, 203 (1958)], were used in this work. The IK definition is found to give the same ensemble average of the local pressure in Cartesian and spherical coordinates for a homogeneous hard-sphere system. The pressure obtained from the H definition gives, on the other hand, different results in the two coordinate systems. In Cartesian coordinates, the H pressure is identical to the IK pressure, but in spherical coordinates, the pressure depends on R (the distance from the origin). Therefore the H definition does not give a proper pressure tensor.

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PubMed2001

A PFG-NMR Study of Restricted Diffusion in Heterogeneous Polymer Particles.

The diffusion resistance to monomers during heterogeneous polymerization of polyolefin particles may have a significant effect on the observed activity. This diffusivity is, in general, unknown. To gain more information on this diffusion resistance in such systems, PFG-NMR has been used to measure the diffusion of organic solvents in various systems of porous polymer particles. In such systems the complex morphology and geometry demands careful analysis of the PFG-NMR attenuation curve. In this study, effects from restricted diffusion, domains having different diffusivity, and internal magnetic field gradients are expected. Thus, the obtained diffusivities have to be considered carefully, and a way to analyze the data taking these effects into account is presented. Copyright 2001 Academic Press.

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