Pierre Schaaf

Molecular basis for the explanation of the exponential growth of polyelectrolyte multilayers

The structure of poly(l-lysine) (PLL)/hyaluronan (HA) polyelectrolyte multilayers formed by electrostatic self-assembly is studied by using confocal laser scanning microscopy, quartz crystal microbalance, and optical waveguide lightmode spectroscopy. These films exhibit an exponential growth regime where the thickness increases exponentially with the number of deposited layers, leading to micrometer thick films. Previously such a growth regime was suggested to result from an “in” and “out” diffusion of the PLL chains through the film during buildup, but direct evidence was lacking. The use of dye-conjugated polyelectrolytes now allows a direct three-dimensional visualization of the film construction by introducing fluorescent polyelectrolytes at different steps during the film buildup. We find that, as postulated, PLL diffuses throughout the film down into the substrate after each new PLL injection and out of the film after each PLL rinsing and further after each HA injection. As PLL reaches the outer layer of the film it interacts with the incoming HA, forming the new HA/PLL layer. The thickness of this new layer is thus proportional to the amount of PLL that diffuses out of the film during the buildup step, which explains the exponential growth regime. HA layers are also visualized but no diffusion is observed, leading to a stratified film structure. We believe that such a diffusion-based buildup mechanism explains most of the exponential-like growth processes of polyelectrolyte multilayers reported in the literature.

Irreversible adsorption of colloidal particles on solid substrates

Abstract In this article, we summarize the basic results relative to the field of irreversible deposition processes of colloidal particles on solid surfaces. An irreversible deposition process is defined as a process in which, once adsorbed, a particle can neither diffuse along, nor desorb from the surface. However, some extensions leading to reversible adsorption models are also outlined. First the basic tools used in these studies are introduced, notably the concept of available surface function. General results relative to these processes are then presented. We discuss, in particular, the connection between the reduced variance of the number density fluctuations of adsorbed particles and the available surface function. We then review the main models which were introduced in the literature to account for these processes. They can be divided in two classes: (i) the models which are based entirely on statistical and geometrical grounds. The most widely studied of them is the Random Sequential Adsorption (RSA) model which is discussed in detail. For the processes in which gravity plays an important role one uses the Ballistic Deposition (BD) model. We also present models which are aimed at accounting for the behavior lying between the ballistic deposition and the RSA. (ii) The second type of models corresponds to those which take explicitly the diffusion of the particles in the vicinity of the adsorption plane into account. The results relative to these models, called diffusional models, are discussed in detail. Finally, the last part of the article is devoted to experimental results. We present and discuss in a critical way experimental evidence which seems to indicate the validity of the RSA and BD models.

Effect of the Bulk Diffusion on the Jamming Limit Configurations for Irreversible Adsorption

We study numerically the diffusion random sequential adsorption (DRSA) and random sequential adsorption (RSA) of hard disks on a one-dimensional lattice segment. The saturation coverages of the two processes, although similar, are clearly different within the errors of the simulation. The difference is explained by the nonuniform adsorption of disks within the available gaps when diffusion is permitted. We conclude that a recently proposed class of generalized parking problems cannot rigorously describe the DRSA process.

Do physiological concentrations of IgG induce a direct aggregation of red blood cells: comparison with fibrinogen

Under physiological conditions, red blood cells (RBCs) form aggregates that allow blood flow in all the circulatory system. RBC aggregation is the result of local flow shear stress, erythrocyte properties and macromolecular interactions between adjacent cells. Plasma proteins like fibrinogen or IgG are considered to promote RBC aggregation by a mechanism that remains to be explained. In the present study, we have examined the precise role of IgG on RBC fast-phase aggregation, in comparison with that of fibrinogen. Under our experimental conditions, we observed no fast-phase aggregating effect for IgG, at either physiological or supraphysiological concentrations, while fibrinogen induces strong aggregation of RBC. We also suspect the other plasma proteins to play a role in the RBC aggregating process.

A PARADOX RESOLVED: APPARENTLY IDENTICAL RADIAL DISTRIBUTION FUNCTIONS, DIFFERENT DENSITY VARIANCES

The irreversible deposition of small latex particles on a surface leads to configurations whose radial distribution functions g(r) are well predicted by the random sequential adsorption (RSA) model, whereas the variance σ2 of the density of adsorbed particles is inconsistent with this model. Since g(r) is related to σ2 by a general equation, this constitutes an (apparent) paradox. This article lifts this paradox by means of a simple generalized RSA model.

Kinetics of the homogeneous exchange of α-lactalbumine adsorbed on titanium oxide surface

The homogeneous exchange process whereby α-lactalbumine molecules adsorbed on hydrophilic titanium oxide particles are replaced by α-lactalbumine molecules in solution has been investigated by means of a 125I radio-labeling technique. α-lactalbumine is a compact and highly negatively charged protein, making this study complementary to previous work devoted to the general understanding of the exchange mechanisms of adsorbed proteins on solid surfaces. The isotherm of α-lactalbumine exhibits bimodal adsorption shape, and the exchange process whereby adsorbed proteins are replaced by new incoming ones from the bulk solution has been studied at both the upper and the lower plateau of the isotherm. In the upper plateau the exchange process was found to be of first order with respect to the bulk molecules, and the release rate constant was equal to 0.914 L. mol−1 · s−1. This behavior is identical to what has been observed with other proteinic systems. In the lower plateau domain, in contrast, the protein release process is independent of the concentration of proteins in the bulk, but the release rates are higher than the pure desorption rates. This constitutes, to our knowledge, a behavior that never before has been observed and that remains to be explained.

A Scanning-Angle Reflectometry Study of Surfaces Covered with Latex Particles

Scanning-angle reflectometry around the Brewster angle has been used to measure the reflectivity of a flat silica surface sparsely seeded with latex particles. The reflectivity for such surfaces is composed of 1) the direct (Fresnel) reflection from the silica-water interface, 2) the (Mie) scattering of the adsorbed particles and 3) the interference between the two previously mentioned components. The analysis of the obtained curves of reflectivity vs. incidence angle yields three parameters: particle size, refractive index of the particle, and the adsorbed particle density.

An application of the Kubo and Nyquist relations to nucleation

We examine fluctuations in cluster evaporation and growth and express these, using the Nyquist and Kubo relations [1,2], in terms of the resistance to single-cluster motion along the coordinate of cluster size. If successful in future development, the methods introduced here will have application to the abstraction of nucleation rates from computer simulations of individual-cluster evaporation/growth events.