(179f) Experimental and Theoretical Investigation of the Depletion Force Generated by CTAB Micelles

The technique
of colloidal probe atomic force microscopy was used to measure the equilibrium
depletion force between a relatively large (30 μm diameter) spherical
silica particle and a flat silica plate in solutions of cetyltrimethylammonium
bromide (CTAB) surfactant. Previous work has indicated that the observed
depletion force from CTAB micelles is smaller than that predicted by
hard-sphere models. The
objective of this work was to establish if CTAB micelles could be modeled using
the well established equations for charged, hard-sphere depletants, or if
corrections to these approaches are needed to account for the unique physical
properties of micelles. Experiments were performed in solutions of CTAB below
and sufficiently above the critical micelle concentration that not only
equilibrium depletion forces but also long-range structural forces could be
discerned. The force profiles were then compared to models that have been found
to work well in systems using charged nanospheres as the depletants.