Showing papers by "Dennis C. Prieve published in 1998"

Kinetics and Mechanism of Cationic Surfactant Adsorption and Coadsorption with Cationic Polyelectrolytes at the Silica−Water Interface

Abstract: 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 x cmc and 10 x 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.

Depletion Attraction Caused by Unadsorbed Polyelectrolytes

Abstract: 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 01−10 mM of KBr, when bot

Superfast Electrophoresis of Conducting Dispersed Particles

Abstract: Conducting particles can display electrophoretic velocities hundreds of times larger than those expected for nonconducting particles. For ion-exchange particles in which coions are excluded from the interior, superfast electrophoresis occurs when the externally applied electric field exceeds that required for producing the overlimit current through the particle. Then a secondary diffuse cloud of counterions is induced outside the primary diffuse cloud (the latter is associated with the electric double layer). This extra induced charge, which increases with the electric field strength, causes the much larger electrophoretic velocities observed. Using multiple-exposed videoimaging and a new inclined flowcell to separate the effects of sedimentation and electrophoresis, we measure the electrophoretic velocity of electronically conducting particles (Al/Mg alloy, graphite, or activated carbon; 250–500 μm diameter) which are then compared to earlier measurements with ionically conducting particles. For ionic strengths less than 1 mM, the electrophoretic mobility (velocity/electric field) of electronically conducting particles increases significantly with the electric field and the particle size, but is almost independent of the ionic strength. These trends are inconsistent with Smoluchowski's equation for the mobility of a dielectric particle, but instead are consistent with the theory (and earlier measurements on ion-exchange particles) for superfast electrophoresis. Although the electronically conducting particles move much faster than expected for dielectric particles, the velocity is not quite as high as that for ionically conducting particles. Smaller superfast electrophoresis for electronic conductors could be caused by the overpotentials which drive the redox reactions necessary to exchange electrons for ions at the particle surfaces; also both positive and negative secondary charge clouds are induced on opposite sides of an electronic conductor particle, which partially neutralizes the “superfast” effect.