Determination of the Dynamic Surface Excess of a Homologous Series of Cationic Surfactants by Ellipsometry

Abstract

Ellipsometry is a powerful experimental technique for studying the surfaces of surfactant solutions both at and away from equilibrium. From a theoretical perspective, there is no simple relationship between ellipsometric parameters at the air−water interface and the surface excess, Γ, of the surfactant. Here we report neutron reflection measurements of the dynamic surface excess, Γ dyn, of the cationic surfactants C n TAB ( n = 14, 18) in an overflowing cylinder (OFC) as a function of the bulk surfactant concentration, to complement earlier measurements on C 16 TAB. These results are compared with ellipsometric measurement of the coefficient of ellipticity, ρ, of the surfactant solutions in an OFC in order to generate a calibration curve ρ( Γ ). For each surfactant, ρ was found to be a linear function of surface excess. For the shorter homologue, n = 12, surface tension measurements on equilibrium solutions have been used to determine the adsorption isotherm and the surface excess was then compared with the coefficient of ellipticity. Again, the relationship between ρ and Γ is linear. A protocol is described for determining surface excesses ellipsometrically in this class of surfactants, and the errors in the procedure are quantified.