Surfactant Adsorptiononto Interfaces: Measuring theSurface Excess in Time.

We propose a direct method to measure the equilibriumand dynamicsurface properties of surfactant solutions with very low criticalmicellar concentrations (CMC) using a pendant drop tensiometer. Westudied solutions of the nonionic surfactant hexaethylene glycol monododecylether (C12E6) and of the ionic surfactant hexadecyltrimethyl ammonium bromide (CTAB) with concentrated sodium bromide(NaBr). The variation of the surface tension as a function of surfaceconcentration is obtained easily without the need for complex modelsand compares well with the result obtained using the Gibbs adsorptionequation. The time-dependent surface concentration of each surfactantwas also measured, and the adsorption process was found to be diffusion-controlled.The diffusion coefficients of the two surfactants can be extractedfrom the data and were found in very good agreement with literaturevalues, further validating the method. [ABSTRACT FROM AUTHOR]