Oscillatory Structural Forces Due to Nonionic Surfactant Micelles: Data by Colloidal-Probe AFM vs Theory

Micellar solutions of nonionic surfactants Brij 35 and Tween 20 are confined between two surfaces in a colloidalprobe atornic-force microscope (Cil-AFM). The experimentally detected oscillatory forces due to the layer-by-layer expulsion of the inicelles agree very well with the theoretical predictions for hard-sphere fluids. While the experiment gives parts of' the stable branches of the force curve, the theoretical model allows reconstruction of the full oscillatory curve. Therewith, the strength and range of (lie ordering Could be determined. The resulting aggregation number from the fits of the force curves for Brij 35 is close to 70 and exhibits a slight tendency to increase with the surfactant concentration. The last layer of micelles cannot be pressed out. The measured force-vs-distance curve has nonequilibriurn portions, which represent jumps frorn one to another branch of the respective equilibriurn oscillatory curve. In the case of Brij 35, at concentrations < 150 nM spherical rnicelles are present Lind the oscillation period is close to the micelle diarneter, slightly decreasing with the rise oCconcentration. For elongated micelles (at concentration 200 mM), no harmonic oscillations are observed anymore; instead, tile period increases with the decrease of film thickness. In the case of Tween 20, the force oscillations are almost Suppressed, which implies that the micelles of this SUrfactant are labile and are demolished by the hydrodynamic shear stresses due to the colloidal-probe motion. The comparison of the results for the two surfactants demonstrates that in some cases the inicelles can be destroyed by the CP-AFM, but in other cases they can be stable and behave as rigid particles. This behavior correlates with the characteristic times of the slow micellar relaxation process for these surfactants.