Adsorption of complexes formed by cationic starch and anionic surfactants on quartz studied by QCM-D

The adsorption of complexes of cationic starch, (CS) and a series of homologous sodium alkanoates on silica was studied with the quartz crystal microbalance with dissipation (QCM-D) instrument. The systems were chosen so as to represent CS/surfactant ratios below and above the critical association concentrations of the surfactants but below their critical micelle concentrations. It was found that
–surfactants did not adsorb on cationic polymers that were very tightly bound to the surface;
–surfactants did adsorb on polyelectrolytes forming layers with loops and tails extending into the solution, provided the concentration of surfactant was at least around the critical association concentration (cac) of the surfactant/polymer system;
–adsorption of surfactant was promoted by increasing the surfactant chain length and by adding simple electrolyte that weakened the electrostatic polymer/surface interaction and
–multilayers were formed when the surfactant concentration in solution was well above the cac; their formation was promoted by increasing hydrophobic interactions, e.g. by increasing the surfactant chain length.