SURFACE FORCES BETWEEN SILICA SURFACES IN CATIONIC SURFACTANT SOLUTIONS - ADSORPTION AND BILAYER FORMATION AT NORMAL AND HIGH PH

Forces have been measured between glass surfaces in cetyltrimethylammonium bromide (CTAB) solutions at pH 10, and the adsorption, inferred from the fitted surface potentials and the measured adhesion, has been shown to be strongly dependent on the surface charge and the competing ions. At very low CTAB concentrations adsorption can only be detected after the surfaces are left in contact, and this results in a time-dependent adhesion consistent with diffusion-controlled adsorption of CTAB in the annulus around the contact area. At slightly higher concentrations (between 3 x 10(-6) and 6 x 10(-6) M), a hydrophobic monolayer forms and purely attractive hydrophobic interactions are measured, indicating that the surfaces are close to electroneutrality. This concentration is much lower than that required to achieve neutral surfaces at pH 5.6 for glass or for mica surfaces. It is argued that after charge reversal the forces can be fitted with DLVO theory, assuming a plane of charge away from the surface. Bilayer formation occurs at the critical micelle concentration (cmc) (1 x 10(-3) M) for low pH and below the cmc at elevated pH. The density of surfactant in the outer layer of the bilayer on silica at normal and elevated pH is much lower than that on mica at a comparable bulk CTAB concentration. As a result it is possible to disrupt the bilayers and push the surfaces into a hydrophobic contact. The thickness of the bilayer and the surface charge density obtained from fitting the force law as well as the nature of the force law during compression of the bilayer lead to the conclusion that CTAB adsorbs to glass as patches of bilayers or surface aggregates. Dynamic rearrangements within the surface layer have also been observed.