Deposition of monolayers by retraction from solution: Ellipsometric study of cetyltrimethylammonium bromide adsorption at silica-air and silica-water interfaces

摘要

Adsorption of cetyltrimethylammonium bromide (CTAB) at the silica-water interface and the adsorption deposition that occurs when the plate is retracted from solution were studied by ellipsometry. Characteristic times of adsorption at the silica-water interface are of the order of seconds near and above the critical micelle concentration (CMC) (10(-3) M), but due to diffusion control adsorption times become larger at smaller concentrations. Desorption by substitution of pure water for the solution proceeds rapidly, in less than a minute at all concentrations. Equilibrium adsorption increases from almost zero at concentrations below 5 x 10(-5) M to a plateau value with an area per molecule of about 40 Angstrom(2) above the CMC. The amount of CTAB deposited on the plate by retraction is large, starting from very small concentrations in the solution, for which adsorption at the coexisting solid-liquid and liquid-vapor interfaces is negligible. Around the CMC the area per molecule on the surface in air, about 50 Angstrom(2), does not depend on the speed of retraction. At lower concentrations the amount deposited decreases with increasing the speed. Here the process is controlled by the rate of supply of surfactant molecules to the three-phase line (solid, liquid, and vapor coexisting line) from which the deposition occurs. When the plate with deposited monolayer is immersed in pure water or dilute solution, desorption occurs rapidly, as for the liquid-phase experiment when CTAB solution is replaced by water. The results are in accord with conclusions based on dynamic and equilibrium contact angle data. The possibility of structural transformation of the silica gel layer on drying is considered, and the importance of surfactant purity is outlined. Implications of the reversibility of adsorption in mechanisms of de-wetting transitions and hydrophobic attractions supplement the discussion. References: 23