Dynamic tension behavior of aqueous octanol solutions under constant-area and pulsating-area conditions

Dynamic tension data for 0.3 and 0.6 mM aqueous octanol solutions at 25°C and constant-area conditions were obtained and compared with previous diffusion-controlled and mixed kinetics adsorption models. The results show that tension drops more slowly than predicted by the diffusion-controlled model. The modified Langmuir-Hinshelwood equation in a mixed kinetics model describes the data quite well. Moreover, the same solutions were examined with a pulsating bubble surfactometer at 10-80 cycles min^-1, with the bubble radius oscillating between 0.40 and 0.55 mm. The dynamic tension oscillates between a tension maximum γ_max > γ_e and a minimum γ_min < γ_e, where γ_e is the equilibrium tension. The tension amplitude (γ_max - γ_min) increases with frequency, because the adsorption process is too slow to follow exactly the area changes. The amplitude decreases with increasing concentration from 0.3 to 3 mM. Phase lags between γ(t) and A(t) and low γ_min can be predicted by convective-diffusion mass transfer models at spherical coordinates, or even by simple planar mass transfer models, without considering intrinsic surface rheology effects. Comparisons of pulsating-area tension data with the models indicate that intrinsic adsorption-desorption rates must be considered in the overall rate of adsorption. Certain discrepancies between the model and the data are attributed in part to measurement errors and in part to the use of the Frumkin equation of state for nonequilibrium surface densities. The results are relevant to foam generation and lung surfactants.