In this work, we explore the effects of mixing species of cationic and anionic surfactant on their adsorbed self-assembled structures. Such mixtures often have structures that differ greatly from those seen in single component solutions, forming vesicles and cylindrical micelles at various concentrations. When mixed, the surfactants also aggregate more easily and have higher surface activities, making their mixtures interesting topics for detergent development. We looked at the structure of micelles comprised of various mixtures of sodium dodecyl sulfate (SDS), dodecylamine HCl (DAH), and sodium palmititate (NaPa) on a graphite surface in water via atomic force microscopy. In images of pure SDS and DAH, it was found that the micelles present at the surface were hemicylindrical in structure, in agreement with previous works on the subject. Increasing surfactant concentration invariably decreased the width of these micellar lines down to a minimum value past the critical micelle concentration. This minimum micellar width was found to be roughly 4.6 nm for SDS and 8.6 nm for DAH within the studied concentrations of 5 to 15 mM solutions. Increasing the concentration of monovalent counterions in solution did not appear to affect this minimum width. When the surfactants were present in mixed solution, however, they displayed new behavior and structure at the graphite surface. Namely, they began exhibiting a fishbone pattern with a great deal of disorder and a large number of grain boundaries. Changing counterion concentration in these surfactant mixtures appears to directly affect the degree of surface disorder, with higher concentrations producing seemingly more compact structures. While the pure solutions of SDS and DAH were studied at concentrations ranging from 5 to 15 mM, their mixture could only be studied in molar ratios between 20:1 and 40:1 due to the presence of milky film. Further work will be done on the effects of both counterion and surfactant concentration on the mechanical properties of these films, such as elasticity and breakthrough force.

Effects of cationic and anionic surfactant concentrations on adsorbed self-assembled micellar structure at graphite surfaces

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.