The surface equation of state (γ vs Γ/Γref), based on the experimental data from rapid expansion of a pendant bubble, has been used on studying the adsorption kinetics for C12E6 (Pan et al.
The adsorption kinetics of a surfactant is used to apply the equilibrium surface tension data γ(C) for
the determination of the adsorption isotherm and then to apply the dynamic surface tension γ(t) on
determining the controlling mechanism, the diffusivity, and/or the sorption rate constants. A concept for
surfactants lacking low surface pressure data has been proposed (Hsu et al. Langmuir
1997, 13, 6204;
1999, 15, 1952): using only a limited range of equilibrium surface tension data to determine the adsorption
isotherm can cause a mistake on determining the adsorption mechanism and diffusion coefficient. In this
work, the above concept is extended: even though an isotherm describes the γ(C) data well, it may describe
the dynamic surface tension data incorrectly. An incorrect controlling mechanism and/or a diffusion coefficient
with a significant error may result if only the γ(C) data are applied on determining the adsorption isotherm
and model parameters. A second set of equilibrium data, equation of state γ(Γ), is useful and necessary
for studying the adsorption kinetics. A theoretical simulation and two examples (for surfactants with
either cooperative or anticooperative adsorption) are presented in this article to illustrate this concept.
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