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Benjamin Micklavzina1 Marjorie Longo2

1, University of California, Davis, Davis, California, United States
2, University of California, Davis, Davis, California, United States

Surfactants dissolved in water will self assemble into micellar structures when exposed to a hydrophobic surface. Our work focuses primarily on the study of these films via AFM, where they can be directly probed and their mechanical properties examined. In our earlier studies, we focused on the effect of salt additions upon the breakthrough force of sodium dodecyl sulfate (SDS) and dodecylamine HCl (DAH) films. It was found that the addition of around 1.5 mM of NaCl, MgCl2, and Na2SO4 all produced significant increases (~40-70%) in the average breakthrough force at a surfactant concentration of 10 mM. A model was developed in an effort to explain the cause of this strengthening, which predicted that the breakthrough force was tied to a free energy of micellar formation. Experiments were performed using spectrofluorometry to determine how addition of salt affected the CMC and, consequently, the free energy of formation. Our model showed excellent fits for most added salts, but struggled specifically in the case of MgCl2 added to 10 mM SDS. We theorized that the mismatch between our theory and data was the result of strong ion binding at the surface, which might have changed the activation volume at breakthrough. More recent work has focused on analyzing the force curves near these micellar surfaces. Force curves contain a wealth of information about potential surface structures, and can be used to determine mechanical moduli for thin films. We found that, although our film was only 1-2 nm thick, we observed strong repulsive forces at tip-film separations of ~15-20 nm. The decay length of these forces, as well as their magnitude, increased with added salts, implying that they were not the result of electrostatic repulsion. At low salt concentrations (<1 mM), observed trends were well explained by theory for steric repulsion. The large decay lengths of 2-3 nm for SDS and DAH at these concentrations implied that the forces were the result of collective movement of micelles, rather than protrusions of individual molecules. At high concentrations of salt (>1 mM), a change in behavior was observed for SDS: A second region with a different force decay length appeared near the surface. To explain this, we applied a model for charged polymer brushes in the osmotic regime to our surface, and found good agreement between the model and our results. DAH films did not display this decay length splitting, which we hypothesized was the result of lower surface charge and ion concentration. Using force curves for SDS, we were able to extract a value for the Young's modulus of 80+/-40 MPa, which is comparable to results from literature for lipid bilayers. The DAH film was too fragile to acquire accurate information about mechanical moduli from our trials. Current work focuses on using micropipette aspiration to compare information about nanomechanical strength to a more practically sized system.

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