Experimental and theoretical investigation of the depletion and structural forces produced by ionic micelles

The technique of colloid probe atomic force microscopy was used to measure the force profile between a silica particle and a flat silica plate in solutions of tetradecyltrimethylammonium bromide (C14TAB) and sodium dodecyl sulfate (SDS) at varying concentrations. With increasing surfactant concentration the onset of a depletion well and structural forces was observed. The measured results were compared to the predicted results of the model of Walz and Sharma [26] for hard, charged, spherical depletants. For C14TAB the measured depletion force was substantially smaller in magnitude than that predicted by this model. In solutions of SDS micelles, a similar overestimation of the depletion force was observed but not to the same degree as for the C14TAB solutions. It was found that by reducing the magnitude of the electrostatic repulsive interactions in the system via lowering the effective surface potentials, the measured force profiles could be accurately matched. These results suggest that modeling ionic micelles as simple hard, charged spheres may not be appropriate and that the level of deviation from such a modeling approach depends on the specific properties of each type of micelle.