Breakthrough forces are a common means of mechanically characterizing supported lipid bilayers and monolayers using AFM. In this work, we use atomic force microscopy (AFM) to study the mechanical properties of sodium dodecylsulfate (SDS) and dodecylamine hydrochloride (DAH) films at a graphite surface. These surfactants self-assemble into hemicylindrical micelles at the graphite-water interface. The nanoscale structures they form, similar to lipid layers, have a characteristic breakthrough force required for the AFM tip to breach the film and reach the surface beneath. The breakthrough forces for these films were measured using silicon nitride cantilevers, and found to be 1.1 +/- 0.1 nN for a 10 mM DAH film and 3.0 +/- 0.3 nN for a 10 mM SDS film. Salts and hydrated ions are common contaminants in surfactant systems, and have been shown to increase the strength of supported lipid films. For 10 mM SDS and DAH films, it was found that the addition of 1.5 mM of NaCl, Na2SO4, or MgCl2 produced a 50-70% increase in measured breakthrough force. A model was developed using previous work on lipid monolayers and CMC data gathered via spectrofluorometry measurements to predict the change in normalized breakthrough forces with added salt concentrations. Using this model, it was found that the activation volume required to initiate breakthrough was roughly 0.4 nm3 for SDS and 0.3 nm3 for DAH, roughly the volume of a single molecule. Normalized breakthrough force data for SDS with added MgCl2 showed an unexpected dip at low added salt concentrations. The original model was adapted to account for changing activation volumes, and a curve of activation volume versus magnesium concentration was obtained, showing a minimum volume of 0.16 nm3. The addition of 0.2 mM SDS to a 10 mM DAH solution was found to double the measured breakthrough force of the film. Images taken of the surface showed a phase change from cylindrical hemimicelles to a planar film that may have produced the observed differences.

Mechanical characterization of self-assembled surfactant micelles at graphite surfaces via atomic force microscopy

253rd National Meeting (2017)

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