Adsorption of nonionic amphiphiles to solid surfaces

Abstract: In this study, the adsorption of nonionic amphiphiles from aqueous solutions to solid, planar surfaces is investigated using null ellipsometry. This technique allows independent measurements of the amounts adsorbed at the surface and of the mean optical thickness of the adsorbed layer. The time resolution is sufficient for investigating the dynamics of adsorption and desorption in systems of this type. Detailed experimental studies of the processes involved in the formation and dissolution of pure and binary polyethylene glycol monoalkyl ether surface structures at the silica-water interface were carried out. A theoretical model was developed for interpreting the experimental data and for gaining an understanding of these processes at a molecular level. For the pure surfactant systems, the combination of theory and experiment led to an increased understanding of the influence that a number of surfactant and flow-cell related parameters have on the adsorption and desorption kinetics. An extension of the work to binary surfactant systems revealed the importance of considering the individual paths of adsorption and desorption for each component. Large changes in the surface composition upon rinsing were discovered in binary systems in which there was a significant difference in surfactant cmc. Studies of the addition of long-chain alcohols to aqueous solutions of polyethylene glycol monoalkyl ethers in contact with a silica surface demonstrated that small amounts of alcohol that are added can give rise to large increases in the total amount adsorbed. Just as with the binary surfactant systems, mixtures of surfactants with alcohol or with oil were shown to produce segregated desorption paths. Finally, adsorption from an aqueous solution of a fatty acid to a hydrophobic surface was found to be very sensitive to the pH of the solution. A close correlation was observed between the pH dependence of the fouling of the hydrophobic membranes and the experimentally determined adsorption at the planar hydrophobic surface.