Insoluble surfactant monolayers at the air/water interface undergo a phase transition from a high-temperature homogeneous state to a low-temperature demixed state, where dilute and dense phases coexist. Alternatively, the transition from a dilute phase to a dense one may be induced by compressing the monolayer at constant temperature. We consider the case where the insoluble surfactant monolayer interacts with a semi-dilute polymer solution solubilized in the water subphase. The phase diagrams of the mixed surfactant/polymer system are investigated within the framework of mean field theory. The polymer enhances the fluctuations of the monolayer and induces an upward shift of the critical temperature. The critical concentration is increased if the monomers are more attracted (or at least less repelled) by the surfactant molecules than by the bare water/air interface. In the case where the monomers are repelled by the bare interface but attracted by the surfactant molecules (or vice versa), the phase diagram may have a triple point. The location of the polymer special transition line appears to have a big effect on the phase diagram of the surfactant monolayer.
The transport of bacteria through soils is controlled in part by their adhesion to mineral surfaces. We studied the adhesion of Escherichia coli K-12 to two representative soil minerals (quartz and lepidocrocite), as the growth phase of the population, the metabolic state of the cells, and the pH of the solution were independently varied. Acid-base titrations and electrophoretic mobility measurements were used to investigate the effects of cell and mineral surface speciation and electric charge on the adhesion process. Significant adhesion to lepidocrocite was observed, decreasing at higher pH values presumably in response to the decreasing electrostatic attraction between the cells and the mineral surface. Adhesion of inactive cells (poisoned with streptomycin) was more extensive than for non-poisoned cells, for both mineral substrates. Further research is warranted to determine if other bacterial species display similar relationships between adhesion, cell metabolic state, mineral sorbent, and solution pH.
The conformation at equilibrium of a single polyelectrolyte molecule adsorbed electrostatically at an ideal planar charged surface with one of its ends constrained at a distance $z$ from the surface is investigated using different theoretical methods, and in particular a scaling theory. The force $F$$(z)$ applied at the fixed end of the chain is calculated. Exact results are obtained for a Gaussian chain when all screening effects are negligible, and at high salt concentration, using a self-consistent field theory. When the salt concentration is large enough, the force profile $F(z)$ reaches a plateau where the force is independent of chain length. When the electric field is screened by the counterions, a pseudoplateau is obtained, where the force increases logarithmically with $z.$ The chain leaves the surface before it is fully extended, although the distance of detachment from the surface is proportional to the number of monomers $N.$ The electrostatic interactions between the charged monomers of the polyelectrolyte chain can be taken into account using scaling arguments.
The phase diagram of insoluble surfactant monolayers at the air/water interface is affected by the addition of polymer in the water subphase. The case of a condensation transition is investigated within the framework of a mean-field theory. The interaction of the polymer with the interface leads to an upward shift of the critical temperature and of the critical concentration (if the monomers are more attracted by the surfactant molecules than by the bare interface). In some situations, the phase diagram can display a triple point.
Adsorption isotherms of quaternized polyvinylpyridine on the surface of Escherichia coli bacterial cells were performed using a spectrophotometer. Results show that about 1 mg of polymer can adsorb per m2 of bacterial surface. Electrophoretic mobility measurements of the cells for various quantities of adsorbed polymer indicate that the charge of the cells can be inverted by the cationic polyelectrolyte. Combining the adsorption isotherm and the electrophoretic mobility measurements, we argue that the polymer chains are adsorbed in a very flat configuration on the surface and that they form a strongly entangled network with a mesh size which decreases to a molecular size when the quantity of adsorbed polymer increases to the plateau value of the adsorption isotherm.
We study both experimentally, with an atomic force microscope (AFM), and theoretically, using scaling arguments, the stretching of a single polyelectrolyte chain adsorbed at a planar charged surface. The main result is that the force needed to pull a monomer of the chain at a distance z from the surface reaches a plateau at distances larger than the Debye screening length of the solution.
Quaternized polyvinylpyridine (PVPQ) was used as a cationic polymer to destabilize an Escherichia coli bacterial suspension. The optical density and the fraction of free cells, obtained by light scattering measurements, were recorded as a function of the introduced polymer amount, as a way to monitor the stability of the suspension. The flocculation was almost complete for polymer dosages ranging from about 28 to 47 mg of carbon of PVPQ per dry g of bacteria. These dosages correspond to still negatively charged cells, as shown by ζ potential measurements. At low polymer coverages, a less efficient flocculation is observed. At higher dosages, the suspension restabilizes. We interpret these results using our previous study on the adsorption of the polymer chains. We argue that the flocculation at low dosages is rendered possible by the strong inhomogeneities of charge on the bacterial surfaces because of the self-similar configuration of the adsorbed polymer layer and that the restabilization at large dosages is due to the small mesh size of the polymer network on the surface as well as to the Coulombic repulsion between the cells. The properties of the bacterial aggregates were investigated by light scattering. Destabilized suspensions produce aggregates with sizes decreasing as the quantity of adsorbed polymer increases. At the optimum of flocculation, the polydispersity of the aggregates is low, suggesting a diffusion-limited aggregation mechanism (DLA). The presence of the characteristic self-similar structure of DLA aggregates, with a fractal dimension on the order of 1.9, is suggested by some of the light diffusion experiments. On the other hand, at low coverages, that is, when only some regions on the surfaces are covered with polymers, the flocculation seems to obey a reaction-limited aggregation, with a large polydispersity in the size of the aggregates.
