Locating the micellar shear plane and its relationship with the Debye screening length

The shear plane and shear potential of suspended nanoscale particles such as micelles and colloids are important parameters; however, reports detailing their determination are scarce and are restricted to relatively large colloidal and polymeric particles. In addition, for charged particles such as colloids and anionic or cationic micelles, the relationship between the Debye screening length and the location of the shear plane has not yet been established. In this paper, we have used the rotating disk electrode to determine micellar hydrodynamic radii (R-h(0)), as a function of electrolyte (KCl) concentration, for CTAC (cetyltrimethylammonium chloride) micelles at 298.0 +/- 0.1 K. The results reveal an initial decrease in R-h(0) due to collapse of the shear plane toward the micellar hard-sphere surface followed by an increase in R-h(0) resulting from electrolyte-induced linear spherical expansion of the micellar particles. Calculation of the micellar hard-sphere radii from monomer Volume considerations allows evaluation of the sheer plane thickness and its associated zeta-potential. The zeta-potential was found to be 95 +/- 10 mV and was invariant to changes in electrolyte concentration. In addition, estimations of the Debye screening length at the micellar charged surface allow direct comparison with the shear plane thickness. The relationship is shown to be a simple linear function with the shear plane collapsing toward the micellar surface as the double layer collapses with added electrolyte.