Pendant drop tensiometry enhanced by video-image digitization is shown to be a useful tool for the experimental measurement of the relaxation in interfacial tension due to the adsorption of surfactant at a fluid interface. Using this method, profiles of the relaxation in surface tension of a diffusion-controlled, nonionic polyethoxy surfactant were measured. A diffusion coefficient was computed by comparing these profiles with numerical solutions of the bulk surfactant diffusion equation and a Frumkin equilibrium adsorption isotherm. This comparison was made for the entire relaxation period. This method establishes a more reproducible diffusion coefficient than current techniques that utilize only the short-or long-time parts of the relaxation spectrum. In addition, lower bounds on the kinetic constants for the sorption process are inferred for the polyethoxy surfactant used by comparing numerical solutions of mixed diffusion and surface kinetic transfer with the diffusion-limited result.

It is well-known that there exists a minimum critical micelle concentration (cmc) in the cmc-temperature curve. It is found that the temperature of minimum cmc, T min , for both nonionic and ionic surfactants increases as the hydrophobicity of surfactants decreases. The temperature dependence of cmc is used to calculate the enthalpies and entropies of micelle formation for six different homologous series of surfactants. The enthalpyentropy compensation plot exhibits an excellent linearity. It is found that all the compensation lines for surfactants in a homologous series are parallel to one another and the intercept of these compensation lines is a linear function of the hydrophobic chain length of surfactants.

The adsorption of C12E8 onto a clean air-water interface is studied by using a video-enhanced pendant bubble tensiometry. The controlling mechanism for mass transfer changes as a function of bulk concentration; it shifts from diffusion control at dilute concentration to mixed diffusion-kinetic control at more elevated bulk concentration. The adsorption of C 12E8 is found to be anticooperative from the equilibrium surface tension data compared with the prediction of the (generalized) Frumkin model. Relaxation profiles of surface tension for C12E8 molecules absorbing onto a freshly created air-water interface for 21 different bulk concentrations are obtained. Comparison is made for the entire relaxation period of the tension data and the model predictions. Values of the diffusion coefficient and the adsorption/ desorption rate constants of C 12E8 are calculated from these dynamic surface tension profiles.

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The adsorption of C10E8 onto a clean air−water interface and the desorption out of an overcrowded interface due to a sudden shrinkage of a pendant bubble in a quiescent surfactant solution are studied. Video-enhanced pendant bubble tensiometry is employed for the measurement of the relaxation in surface tension. Relaxation profiles of surface tension for C10E8 molecules absorbing onto a freshly created air−water interface and desorbing out of a compressed air−water interface are obtained. The adsorption of C10E8 is found to be anticooperative from the equilibrium surface tension data compared with the prediction of the Frumkin model. The controlling mechanism of the adsorption process changes as a function of bulk concentration; it shifts from diffusion control at dilute concentration to mixed diffusive-kinetic control at more elevated bulk concentration. It is also confirmed that the desorption of C10E8 out of a compressed interface is a mixed diffusive-kinetic controlled process. Comparison is made for the entire relaxation period of the tension data and the model predictions. Values of the diffusivity and the adsorption/desorption rate constants of C10E8 are calculated from these dynamic surface tension profiles. The values of the kinetic rate constants obtained from the desorption experiment are the same as that obtained from the clean adsorption experiment.