(38j) Constant Surface Area Surfactant Adsorption for Microtensiometers

The adsorption behavior of surfactants is important to all applications where they are present. These range from the stabilization of protein based drugs, to the solvation of dirt and oils in cleaning products, to the optimization of interfacial tension reduction in enhanced oil recovery. Capillary based microtensiometers and pendant drop tensiometers are often used to measure the time dependent adsorption of soluble surfactants to interfaces. This is achieved by holding the pressure difference between the two phases constant, and allowing the surfactant to adsorb, measuring the change in the surface tension by the change in capillary radius or drop shape. However, these changes in surface tension result in a change of the area of the microbubble or pendant drop, meaning that the area of adsorption is constantly changing (generally increasing) throughout the experiment. Of particular interest is the often extended lag time before substantial changes in surface tension are observed. These experiments are then analyzed with the Ward and Tordai diffusion-absorption equation, which assumes a constant interfacial area.

To address the experimental limitations, we designed a new capillary pressure feedback control scheme to allow absorption to occur at constant interfacial area. A new model-based control method was developed and applied to the microtensiometer so that the surface area of the bubble can be held constant during absorption by controlling the capillary pressure difference. This is shown to result in significantly faster adsorption times and reduced induction times before a decrease in surface tension is measured when compared to the constant pressure method. This method also allows for a larger difference between the maximum and minimum surface tension that can be measured, as attaining low surface tensions no longer cause the bubble to be ejected from the capillary. We can now directly compare our results to the Ward and Tordai equation to try to understand the role of micelle adsorption and breakdown during surfactant adsorption.