Adsorption Kinetics of Alkyl Polyethylene Oxide Surfactants at the Water-Oil Interface

The adsorption kinetics of alkyl polyethylene oxide (CiEj) nonionic surfactants onto an initially clean air-water interface has been well studied over the past two decades. In principle, the adsorption of surfactant at an air-liquid interface should be very similar to that at a liquid-liquid interface. However, we have found that the diffusion, adsorption, and desorption coefficients for the air/water/surfactant system do not provide the correct scaling to describe a surfactant-mediated tipstreaming phenomenon for producing micron-scale droplets in a microfluidic device, even though the phenomenon is believed to be controlled in particular by the desorption rate of surfactant. As a result, we have undertaken a systematic study to explore the differences between the surfactant dynamics at air-water and oil-water interfaces. In this work, we use the pendant drop method to obtain the dynamic surface tension for a homologous series of three nonionic C_iE₈ (i = 10, 11, and 12) surfactants adsorbed onto an initially clean water-silicone oil interface. Since density differences are small in oil-water systems, the experiments are performed at low Bond number, and we use a recently published fitting routine to extract the instantaneous interfacial tension. We compare our experimental results with the published sorption constants for the air/water system obtained using mixed diffusion-kinetic models, and find that the two systems have different rate constants. The impact of these results on complex interfacial flows will be discussed.