A simple and rapid method for the determination of the surface equations of state and adsorption isotherms for efficient surfactants

Abstract

A simple and fast method for determination of adsorption isotherms for highly surface-active surfactants is presented. Efficient surfactants often have critical micelle concentrations <0.1 mM and the determination of surface tension isotherms involves measurements at very low bulk concentrations where long equilibration times, solution depletion and trace impurities can seriously compromise accuracy. In the method described here, a bubble of known surface area, A₁, and surface excess concentration, Γ₁, is expanded rapidly to a new surface area, A₂. The surface tension at the new surface excess, Γ₂ = Γ₁A₁/A₂, is evaluated by extrapolation of the surface tension relaxation profile to t = 0. It is shown empirically that the choice of t = 0 to correct for adsorption of the surfactant to the growing bubble is ⅔ of the bubble growth time. The equation of state σ(Γ) when combined with equilibrium surface tension measurements in the vicinity of the c.m.c. is sufficient to define completely the adsorption isotherm. This method is then applied to the determination of the adsorption isotherms of the non-ionic surfactants C₁₂E₅ and C₁₂E₃ and the cationic surfactant C₁₆TAB in the presence of 0.1 M NaBr.