Tuning interfacial and bulk self -assembly of ionic surfactants via counterions and similarly-charged polyelectrolytes

Great strides have been made in the past few decades to gain a fundamental understanding of self-assembly of simple surfactant systems. However, industrial formulations are often quite complex and include, not only surfactants, but also electrolytes, polymers and colloidal particles with high surface areas. Formulating these complex systems requires a fundamental understanding of how coupling effects between surfactants, polymers, counterions and surfaces control the self-assembly.;In this thesis I investigate the self-assembly of cationic, cetyltrimethylammonium, CTA+, surfactants both in the bulk and at the silica/water interface in the presence of various counterions (bromide, chloride and tosylate), in 0 and 10 mM background electrolyte and in the presence of a cationic polyelectrolyte, polylysine. I have specifically addressed three main questions: (1) How do similarly-charged polyelectrolytes affect the bulk self-assembly of ionic surfactants? (2) How do counterions affect interfacial self-assembly of ionic surfactants? (3) How do similarly-charged polyelectrolytes affect interfacial self-assembly of ionic surfactants?;I report, for the first time, that polyelectrolytes can change the critical micelle concentration, or cmc, of similarly-charged ionic surfactants by contributing their counterions for bulk self-assembly. The effect of counterions in interfacial self-assembly is similar to that found in the bulk: increasing either the binding affinity of the counterion or the concentration of counterions stabilizes interfacial self-assembly, decreases the curvature of the adsorbed aggregates and increases the adsorbed amount. By combining quantitative atomic force microscopy information about interfacial surfactant structures with the adsorbed amount from optical reflectometry, I found that interfacial aggregates are larger and more flattened than those found in the bulk.;Finally I found that the presence of polylysine affects the interfacial self-assembly of CTA+ surfactants on silica. By adding 10 mM electrolyte or changing the counterion to tosylate, the thermodynamic driving force for adsorption during co-adsorption is shifted to polylysine and the interfacial self-assembly of CTA+ surfactant monomers at concentrations below the cmc is prevented. Above the cmc, where surfactant micelles are better able to compete for adsorption, interfacial self-assembly does occur.