Adsorption kinetics and structural arrangements of cetylpyridinium bromideat the silica-aqueous interface

The adsorption of cetylpyridinium. bromide to the silica -aqueous interface has been studied using optical reflectometry and atomic force microscopy ( AFM). The effects of pH, electrolyte, and surface preparation on the surfac e excess and adsorption kinetics are reported. AFM imaging above the critic al surface aggregation concentration (CSAC) elucidates spherical surface st ructures in the absence of electrolyte and elongated cylindrical structures with added electrolyte. At concentrations around the CSAC, adsorption proc eeds slowly in the absence of salt and takes hours to reach an equilibrium value. At all other concentrations and even at the CSAC when electrolyte is present, the adsorption is complete within minutes. The concentration rang e for which slow adsorption is apparent has been termed the slow adsorption region (SAR) of the adsorption isotherm. AFM imaging of surfactant adsorpt ion in the SAR suggests that the slow adsorption kinetics are due to the gr adual formation of surface structures in this region. The effects of pH and added electrolyte on surface excess and adsorption kinetics have also been studied. At moderate to high surfactant concentration with added electroly te, pH increases have little effect on surface excess. In the absence of el ectrolyte, the surface excess increases with pH as expected, but it is sugg ested that these increases are primarily due to increased solution ionic st rength and not due to increased charge on the substrate. At low surfactant concentrations, added cations compete effectively with the cationic surfact ant for adsorption sites, resulting in no detectable adsorption until a pH of similar to8 is reached.