We used scanning angle reflectometry to measure the adsorption isotherm, adsorption kinetics, and desorption kinetics for cetyltrimethylammonium bromide (CTAB) surfactants on negatively charged silica surfaces. The initial adsorption rate increased with increasing CTAB concentrations between approximately 0.2 × cmc and 10 × cmc, displaying a discontinuous increase at the critical micelle concentration. The initial desorption rate was a monotonically increasing function of the bulk concentration of the surfactant solution from which the adsorbed layer was formed, both above and below the cmc. Combining equilibrium and kinetic information, we conclude that the adsorption mechanism and the structure of the adsorbed layer both change abruptly at the cmc. Below the cmc, monomeric surfactants adsorb to an extent that is consistent with a defective bilayer structure. Above the cmc, micelles adsorb directly to the surface, to an extent that is consistent with a close-packed monolayer of micelles. The adsorption rate was apparently limited by slow rearrangements within the adsorbed layer. CTAB adsorption was significantly hindered by coadsorption with polylysine, in terms of both the rate and extent of adsorption. The effect of polylysine on CTAB adsorption was very sensitive to the ionic strength and the order in which the surfactant and the polyelectrolyte were exposed to the surface. Different pathways to the same final bulk solution composition produced much different adsorption results. This demonstrates that coadsorption of CTAB and polylysine is inherently a nonequilibrium process dominated by kinetic traps. Although it had an overall hindering effect, coadsorption with polylysine did not alter the basic difference in CTAB adsorption mechanisms above and below the cmc.

Coadsorption of polymers and surfactants is a poorly understood process that occurs in a variety of complex fluid applications. In single-component solutions, the cationic polyelectrolyte polylysine and the cationic surfactant cetyltrimethylammonium bromide (CTAB) both adsorb to negatively charged silica surfaces. Here we use scanning angle reflectometry to contrast adsorption from single-component solutions with a sequential adsorption process and a coadsorption process. When adsorbed from single-component solutions, polylysine adsorbs irreversibly, whereas CTAB adsorption is reversible. In the sequential adsorption case, CTAB neither displaces nor adsorbs to preadsorbed polylysine layers. When solutions contain both CTAB and polylysine, they coadsorb to form mixed layers. Mixed layer formation is indicated by a dramatic alteration of the kinetics and reversibility of adsorption compared to either single-component case. The amounts of CTAB and polylysine adsorbed in the mixed layers are both similar to the amounts adsorbed from the respective single-component solution.

Although amphiphilic polystyrene-poly(ethylene oxide) diblock copolymers (PS-PEO) adsorb from aqueous solutions to hydrophobic surfaces, they are unable to form an end-anchored brush. Four independent techniquessscanning angle reflectometry, hydrodynamic layer thickness measurements, streaming potential measurements, and total internal reflection microscopysindicate that PS-PEO adsorption is indistinguishable from PEO homopolymer adsorption. The surface concentrations attained by three PS-PEO diblocks are independent of molecular weight (between 67 000 and 479 000) and are indistinguishable from PEO (420 000 molecular weight) surface concentrations. Hydrodynamic layer thicknesses and electrokinetic layer thicknesses are on the order of only a few nanometers for both PEO and PS-PEO diblocks, more characteristic of a homopolymer layer than an extended brush. Furthermore, adsorbed PS-PEO imparts no detectable steric repulsion to the energy of interaction between a Brownian particle and a wall. Apparently, brush formation is prevented because the surface affinity of the large watersoluble PEO block presents a large kinetic barrier to its being completely displaced from the surface by the insoluble PS block.

Total internal reflection microscopy was used to measure the total interaction between a 6 µm glass sphere and a glass plate, separated by an aqueous solution containing 0.1-1.0 mM of KBr, when both surfaces are saturated with physisorbed polylysine. When the excess polylysine is completely removed from the solution, the sphere fluctuates around the secondary potential-energy minimum formed between double-layer repulsion and gravitational attraction. Subtracting gravity leaves a contribution from doublelayer repulsion which decays exponentially with distance; the decay length is virtually identical to the Debye length calculated for each ionic strength. However, the presence of as little as 10 ppm of unadsorbed 26 kDa polylysine (rod length of 45 nm) causes a measurable attraction, although the most probable separation distance without polymer (150 nm) is much larger than the size of the macromolecule. Increases in the attraction with unadsorbed polymer concentration and decreases in the attraction with increasing KBr concentration correlate with the calculated osmotic pressure for two different molecular weights of polylysine, indicating that the attraction arises from the depletion of the polyelectrolyte from the gap between the sphere and the plate.

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