Complex adsorption behavior of rodlike polyelectrolyte-surfactant aggregates.

A quartz crystal microbalance (QCM) and an optical reflectometer have been used to quantify the long-term adsorption behavior of polyelectrolyte-surfactant aggregates of alkyltrimethylammonium and poly(4-vinylbenzoate) or pCnTVB at the silica-water interface. In solution, these polyelectrolyte-surfactant aggregates exist as weakly anionic semiflexible rodlike structures of several nanometers in radius and hundreds of nanometers in length. The optical reflectivity (OR) data confirmed our earlier proposed model of a two-stage adsorption process (Biggs, S.; Kline, S. R.; Walker, L. M. Langmuir, 2004, 20 (4), 1085-1094) where free CTA+ ions initially adsorb and charge reverse the silica surface, thus allowing the weakly anionic aggregates to adsorb. Combining data from the two techniques allows a distinction to be made between contributions to the measured signal from the bulk and the interface. The isotherm determined by OR showed a clear plateau at higher concentrations, whereas the isotherm obtained by QCM continues to increase across all concentrations tested. This indicates a significant influence of the bulk fluid on the measured signals from the QCM as the concentration is increased. Slow changes in the apparent adsorbed mass observed with the QCM were not reproduced in the OR data, suggesting that these effects were also caused by the bulk and were not a densification of the adsorbed layer. The combination of techniques clarifies the adsorption kinetics and mechanism of adsorption in polyelectrolyte-surfactant aggregate systems.