Biomedical subjects

Jan van Stam

Publications and source records attributed to Jan van Stam.

3 recordsLinked to original sources

PubMed2006

Latex diffusion at high volume fractions studied by fluorescence microscopy.

The behavior of fluorescent latex probes (radii 0.05, 0.1, and 0.5 mum) in latex host particle suspensions was investigated by fluorescence microscopy with image analysis. The volume fraction of the host latex was varied between 0 and 0.50. A careful statistical analysis was performed to examine the accuracy of the fluorescence microscopy method, from which the direct observation of the Brownian motion gives the diffusion coefficient. The method was found to meet all statistical requirements. From rheological measurements, the maximum volume fraction and the intrinsic viscosity can be obtained. The Krieger-Dougherty equation can be used for the prediction of sample viscosities. The predicted viscosities were used to obtain the theoretical diffusion coefficients with the Stoke-Einstein equation. When comparing the theoretical diffusion coefficients with the experimental ones, it turned out that all models tested yielded acceptable predictions of the diffusion coefficients.

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PubMed2005

Tuning of the alpha-terthiophene radical cation coupling reaction using mixed micelles with varying charge density.

The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical.

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PubMed2003

Dynamic fluorescence microscopy as a feasible technique for estimating the diffusion coefficients of small particles in the presence of additives.

The Brownian motion of carboxylated polystyrene latex, labeled with fluorescent probe molecules, at low concentrations in aqueous solutions was investigated by dynamic fluorescence microscopy. For all three latex radii, i.e., 0.5, 0.265, and 0.1 mum, the estimated diffusion coefficients correspond well with the theoretical predictions if thermal and electrostatic contributions are included in the discussions. It was also possible to discriminate latex interactions with an added polymer. Added polyethylene glycol showed no or very weak interaction with the latex until the polymer overlap concentration was reached, at which the formation of a polymer network slowed down the latex diffusion. Polyvinylpyrrolidone, on the other hand, had a more pronounced interaction with the polystyrene latex and slowed down the diffusion even at polymer concentrations in the ppm range. The overall conclusion is that fluorescence microscopy is a feasible method for the study of the dynamic behavior of small particles in solution.

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