SURFACTANT EXCHANGE KINETICS AT THE AIR-WATER-INTERFACE FROM THE DYNAMIC TENSION OF GROWING LIQUID-DROPS

An extended Ilkovic diffusion-kinetic model is presented to quantify the dynamic surface tension of adsorbed surfactant measured from radially growing liquid drops (1). Competing effects of surfactant depletion due to interfacial stretching and surfactant replenishment from the bulk are described. The proposed model accurately predicts the experimentally observed transient maximum and flow-rate dependence of growing drop dynamic surface tension measurements. At ambient temperature, transport of 1-decanol to the water/air interface is shown to be diffusion limited in both liquid and vapor phases for all times greater than 0.1 s. Dynamic tension data when decanol is supplied to the surface from different initial phases (i.e., decanol in the liquid, decanol in the vapor, and decanol in both liquid and vapor phases) reveal that both liquid and vapor phases contribute significantly to the overall alcohol accumulation at the interface, with liquid-phase transport being slightly faster. The proposed modeling procedure for analyzing growing drop dynamic tensions is a general and powerful tool for elucidating surfactant exchange kinetics at fluid/fluid interfaces. (C) 1994 Academic Press, Inc.