Competitive Coadsorption Dynamics in Surfactant−Copolymer Formulations

Abstract

The adsorption of surfactant copolymer mixtures can be complicated by interfacial competition between different components. Using a commercial formulation, Silwet L-720, based on a silicone−poly(ethylene oxide)−poly(propylene oxide) graft copolymer, this work demonstrates how one can determine the adsorption of different populations within a sample by comparing their behaviors on different surfaces. We discovered that on hydrophilic silica, the L-720 formulation appeared to adsorb strongly and quickly in a transport-limited fashion, which is the maximum possible. Surprisingly, however, adsorption was more rapid still on a model hydrophobic surface (C16 self-assembled monolayer), suggesting that only one or two populations within the formulation were actually able to adsorb onto silica. A quantitative analysis revealed the fraction able to adsorb on silica to be quite low, less than 50% of the overall formulation. Furthermore, adsorption of the same material on the hydrophobic surface produced stepwise kinetic traces, a signature of a sample in which distinctly different components adsorbed cocompetitively: Once the surface appeared saturated to a particular constituent, it stopped adsorbing and was displaced by other compounds within the formulation that continued to adsorb, up to surface saturation.