Interaction forces between alpha-alumina fibers with coadsorbed polyelectrolyte and surfactant

Surface interactions between two a-alumina surfaces, at the point of zero c harge, with coadsorbed poly(styrene sulfonate) (PSS) and cetyltrimethylammo nium. bromide (CTAB) have been measured using the atomic force microscope. The interaction forces were found to be dependent on the order of addition of the polyelectrolyte and surfactant. When CTAB was sequentially added to a preadsorbed PSS layer, the surfaces were observed to jump into contact du e to an attractive bridging force that was not present in the absence of CT AB. This indicates that the addition of CTAB alters the PSS adsorbed confor mation such that there is significant tailing of the PSS chains away from t he interface facilitating a bridging force. In contrast, when PSS and CTAB were coadded, no bridging attraction was measured, implying a more compact adsorbed layer and, most likely, a greater surface excess of the PSS-CTAB c omplex. The surfaces did exhibit an adhesive force when retracted due to PS S chain entanglement that occurred when the adsorbed layers were in contact . At an electrolyte concentration of 10(-1) M KBr, sequential addition of C TAB to a preadsorbed PSS layer led to a marked reduction in the adhesion be tween the two surfaces, as compared to that measured under lower electrolyt e conditions, and there was no attractive jump into contact. This implies t hat the PSS remained in a relatively flat surface conformation due to the s mall degree of PSS-CTAB complexation. In the coaddition case, the adsorbed species at high salt behaved like an uncomplexed PSS chain under the same s olution conditions. These data show that a salt concentration of 10(-1) M K Br is sufficient to restrict PSS-CTAB association significantly.