title: Surfactant adsorption onto interfaces: measuring the surface excess in time.
abstract: We propose a direct method to measure the equilibrium and dynamic surface properties of surfactant solutions with very low critical micellar concentrations (CMC) using a pendant drop tensiometer. We studied solutions of the nonionic surfactant hexaethylene glycol monododecyl ether (C(12)E(6)) and of the ionic surfactant hexadecyl trimethyl ammonium bromide (CTAB) with concentrated sodium bromide (NaBr). The variation of the surface tension as a function of surface concentration is obtained easily without the need for complex models and compares well with the result obtained using the Gibbs adsorption equation. The time-dependent surface concentration of each surfactant was also measured, and the adsorption process was found to be diffusion-controlled. The diffusion coefficients of the two surfactants can be extracted from the data and were found in very good agreement with literature values, further validating the method.
authors.affiliation: Laboratoire de Physique de Solides, UMR 8502 Université Paris Sud, 91405 Orsay cedex, France.
authors.forename: He
authors.initials: H
authors.lastname: Yunfei
authors.forename: Shang
authors.initials: S
authors.lastname: Yazhuo
authors.forename: Liu
authors.initials: L
authors.lastname: Honglai
authors.forename: Langevin
authors.initials: L
authors.lastname: Dominique
authors.forename: Salonen
authors.initials: S
authors.lastname: Anniina