Effect of electrolyte on the depletion and structural forces in a micellar system

Abstract

The force as a function of separation is measured between two mica surfaces coated with adsorbed bilayers of cetyltrimethylammonium bromide (CTAB) and immersed in aqueous ionic micellar solutions of CTAB at different ionic strengths. At short separations, the interaction profile is a double-layer repulsion, arising solely from the dissociated counterions, free CTA+, and added salt. At larger separations, the repulsion is reduced and a secondary minimum in the interaction potential due to the depletion of the micelles from the diffuse double-layer is observed. Finally, at further separations, structural effects superimpose an oscillatory force profile. Addition of electrolyte reduces the thickness of the diffuse double-layers. In addition to the shift of the position of the secondary minimum towards smaller separations, the depletion and structural contributions are depressed progressively as the hard-sphare effective volume of micelles decreases. Finally, at high ionic strength, the structural effects are completely removed, but not the depletion one, as the micelles become highly anisotropic.