Nanomechanical Characterization of Micellar Surfactant Films via Atomic Force Microscopy at a Graphite Surface

In this work, we study the mechanical properties of sodium dodecyl sulfate (SDS) and dodecylamine hydrochloride (DAH) micellar films at a graphite surface via atomic force microscopy (AFM). Breakthrough forces for these films were measured using silicon nitride cantilevers and were 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. For 10 mM SDS films, it was found that the addition of 1.5 mM of NaCl, Na₂SO₄, or MgCl₂ produced a 50-70% increase in the measured breakthrough force. Similar results were found for 10 mM DAH films when NaCl and MgCl₂ were added. A model was developed on the basis of previous work on lipid films and CMC data gathered via spectrofluorometry measurements to predict the change in normalized breakthrough forces with added salt concentrations for SDS and DAH films. Using this model, it was found that the activation volume required to initiate the 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 films with added MgCl₂ showed an unexpected dip at low added salt concentrations. The 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 0.21 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. The pH of the bulk solution was varied for both 10 mM SDS and DAH films and was found to have little effect on the breakthrough force.