Adsorption Kinetics and Structural Arrangements of Cetylpyridinium Bromide at the Silica−Aqueous Interface

Abstract

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 surface excess and adsorption kinetics are reported. AFM imaging above the critical surface aggregation concentration (CSAC) elucidates spherical surface structures in the absence of electrolyte and elongated cylindrical structures with added electrolyte. At concentrations around the CSAC, adsorption proceeds 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 range for which slow adsorption is apparent has been termed the slow adsorption region (SAR) of the adsorption isotherm. AFM imaging of surfactant adsorption in the SAR suggests that the slow adsorption kinetics are due to the gradual formation of surface structures in this r...