| Abstract
| - A quantitative model for the kinetics of adsorption of ionic surfactants to an expanding liquid surfaceis presented for surfactant concentrations below and above the critical micelle concentration (cmc). Forsurfactant concentrations below the cmc, the electrostatic double layer is accounted for explicitly in theadsorption isotherm. An overflowing cylinder (OFC) was used to create nonequilibrium liquid surfacesunder steady-state conditions. Experimental measurements of the surface excess for solutions of cationicsurfactants CH3(CH2)n-1N+(CH3)3 Br- (CnTAB, n = 12, 14, 16) and the anionic fluorocarbon surfactantsodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) in the OFC are in excellent agreementwith the theoretical predictions for diffusion-controlled adsorption for all concentrations studied below thecmc. For surfactant concentrations above cmc, the diffusion of micelles and monomers are handled separatelyunder the assumption of fast micellar breakdown. This simplified model gives excellent agreement for thesystem C14TAB + 0.1 M NaBr above the cmc. Agreement between theory and experiment for C16TAB +0.1 M NaBr is less good. A plausible explanation for the discrepancy is that micellar breakdown is no longerfast on the time scale of the OFC (ca. 0.1 s).
|
