Theory and Experiments for the Surfactant Adsorption from Micellar Solutions Onto an Initially Clean Air/Water Interface: Evidence of the Direct Micelle Adsorption Route
Fenfen Huang, Chemical Engineering, City College of New York, 140 St. and Convent ave, T305 Steinman Hall, New York, NY 10031, Alexander Couzis, City College of the City University of New York, Dept. of Chemical Engineering, 140th Street at Convent Avenue, New York, NY 10031, Ponisseril Somasundaran, Columbia University, Room 911, Mudd Bldg., 500W 120th Street, New York, NY 10027, and Charles Maldarelli, Chemical Engineering Department and Benjamin Levich Institute, City College and the Graduate Center of the City University of New York, Steinman Hall, 140th St @ Convent Ave, New York, NY 10031.
This presentation will focus on the development of a transport model and validating experiments for the adsorption of surfactant from aqueous micellar solutions onto an initially clean air/water interface, and the reduction in interfacial tension as a result of the surfactant adsorption. The adsorption of surfactant from sub-micellar solution onto a clean interface involves the straightforward kinetic adsorption of monomer from the sublayer underneath the surface onto the surface, followed by the diffusion of monomer from the bulk to the surface. Micellar aggregates in the bulk phase can augment the adsorption process by two routes. First, micelles can act as a reservoir, which releases monomers by kinetically breaking up when monomer/micelle equilibrium is disturb due to the adsorption of monomer onto the surface. The released monomers can then diffuse and kinetically adsorb onto the surface. Second, aggregates can directly adsorb onto the surface, break up and release monomers into the monolayer on the surface. The acceleration in adsorption and tension reduction, which results from adsorption from micellar solutions, is necessary for the formation of highly textured foams, and most industrial processes form such foams from solutions well above the critical micelle concentration. Most current micellar transport models, using a step wise kinetic association model, exclude direct micelle adsorption, and account for micellar break-up by formulating kinetic-diffusive equations which become tractable only when they are simplified so as to be valid only for small disturbances from equilibrium. To formulate equations valid far from equilibrium, which do not rely on the exact micelle/monomer reaction kinetic constants, we adopt asymptotic equations to describe the regime in which the time scale for disassembly of aggregates is much faster than the timescale for monomer diffusion. In this kinetic regime, the monomer concentration is kept at critical micelle concentration (CAC) since monomer and micelle are in equilibrium. At high enough total bulk concentrations of surfactant, micelles are present throughout the bulk. If aggregates do not directly adsorb onto the surface, the adsorption rate is only controlled by the kinetic adsorption of monomer (which defines the kinetic limit), and this process is independent of the total surfactant concentration since the monomer concentration in the sublayer is equal to the CAC by virtue of the micelle/monomer equilibrium Experimental results, however indicates that the rate of dynamic tension reduction continues to increase with total concentration of surfactant and goes beyond what the kinetic limit can predict, suggesting the other route— the direct adsorption of micelles. We develop a new model for two different surfactants C14E6 and C14E8, both of which fall into the category that the time scale for disassembly of aggregates is much faster than the timescale for monomer diffusion. The proposed model incorporates both monomer adsorption, and the direct adsorption of the aggregates onto the surface, their subsequent break-up and the release of monomers into the monolayer. In the model, aggregates can also desorb back into the sublayer. Kinetic parameters of micellar adsorption, desorption and break-up are found for C14E6 and C14E8 respectively. Validating experiments are also executed using the pendant bubble apparatus to show that this model correctly predicts the increase in the rate of dynamic tension reduction as the total bulk concentration of surfactant increases, and compares well with dynamic tension measurements for a range of high bulk concentrations.