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 t he dynamic surface tension of adsorbed surfactant measured from radial ly growing liquid drops (1). Competing effects of surfactant depletion due to interfacial stretching and surfactant replenishment from the b ulk are described. The proposed model accurately predicts the experime ntally observed transient maximum and flow-rate dependence of growing drop dynamic surface tension measurements. At ambient temperature, tra nsport of 1-decanol to the water/air interface is shown to be diffusio n 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 fro m different initial phases (i.e., decanol in the liquid, decanol in th e vapor, and decanol in both liquid and vapor phases) reveal that both liquid and vapor phases contribute significantly to the overall alcoh ol accumulation at the interface, with liquid-phase transport being sl ightly faster. The proposed modeling procedure for analyzing growing d rop dynamic tensions is a general and powerful tool for elucidating su rfactant exchange kinetics at fluid/fluid interfaces. (C) 1994 Academi c Press, Inc.