Very slow surfactant adsorption at the solid–liquid interface is due to long lived surface aggregates

Abstract

Until recently it was believed that surfactant adsorption was completed very rapidly, but now it is understood that following the initial rapid adsorption of surfactant a much slower adsorption process can continue for many hours. Adsorption isotherms for cetyltrimethylammonium bromide (CTAB) on silica have been measured using optical reflectometry (OR) in the absence and presence of 1 mM and 10 mM KBr. Adsorption from bulk concentrations ranging from 0.3 to 1 × cmc showed evidence of slow adsorption, casting some doubt on the true value of equilibrium surface excess for these concentrations. To further explore this effect a series of concentration cycling (ziggurat) experiments were performed using both OR and a quartz crystal microbalance (QCM), these serve to highlight the disparity between surface excesses determined via concentration and dilution experiments. A mechanism for slow adsorption is proposed whereby the absence of collisions for adsorbed aggregates removes the dominant mode of equilibration, which therefore proceeds slowly by monomer adsorption. The proposed mechanism should apply equally to all types of surfactant, suggesting that slow adsorption is a universal phenomenon.