Structuring of Nonionic Surfactant Micelles

Abstract

Micellar solutions of nonionic surfactants Brij 35 and Tween 20 are confined between two solid smooth surfaces by CP-AFM in order to know the effect of surface charge and the deformability of the colloids on their corresponding structuring compared the previously studied charged and rigid silica nanoparticles. The experimentally-detected oscillatory forces due to the layer-by-layer expulsion of the micelles are in good agreement with the theoretical predictions for hard-sphere fluids. While the experimentally measured force versus distance curve has non-equilibrium portions, which represent “jumps” from one to another branch of the respective equilibrium oscillatory curve, the theoretical model permit reconstruction of the full oscillatory curve. Thereby, the strength and range of the ordering could be determined. In the case of Brij 35 at concentrations less than 150 mM, spherical micelles are present. The oscillation wavelength is close to the micelle diameter and the decay length increases with the rise of concentration. The different dependence of the characteristic lengths on the system parameters for uncharged micelles, in comparison to the charged particles, is related to the different interaction involved. For elongated micelles (at concentration 200 mM), no harmonic oscillations are observed; instead, the oscillation peak-to-peak distance increases with the decrease of film thickness due to the reorientation of the elongated micelles. In the case of Tween 20, the force oscillations are almost suppressed, which implies that the micelles of this surfactant are labile and demolished by the hydrodynamic shear stresses resulting from the colloidal-probe motion. The comparison of the results for these two surfactants demonstrates that in some cases the soft micelles can be destroyed by the CP-AFM, while 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. In general, an optimum scanning speed is necessary to be defined for the system to rearrange after the expulsion of former layers of the micelles and thus obtain the force profiles.