Measurement of depletion interaction in semi-dilute solutions of worm-like surfactant aggregates

(Received 19 January 1994, accepted 9 March 1994)

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 solutions of elongated and semi-flexible micelles of CTAB. For the range of concentrations studied, where the ionic strength is kept constant, the worm-like micelles form a transient network similar to an entangled polymer solution (semi-dilute regime). The electrical double-layer repulsion between the adsorbed bilayers is preceded at larger separations by an attraction with two distinct regimes. The first one is the strongest and is attributed to a complete depletion of micelles. As predicted by the theory, the range of the interaction is reduced while the strength is increased as the volume fraction is increased. The second regime appears at larger separations where an oscillation-like shape is superimposed on a weak attractive background. It is suggested that the attraction results from an orientation depletion of the anisotropic micelles aligning along the surfaces. The oscillatory behavior is further evidenced with measurement of the structure factor by SANS.