Competitive surfactant adsorption of AOT and TWEEN 20 on gold using quartz crystal microbalance with dissipation

Abstract

Competitive surfactant adsorption of the anionic surfactant AOT and the nonionic surfactant Tween 20 on gold surface was investigated by using the quartz crystal microbalance with dissipation (QCM-D) at 25 ℃. The adsorption isotherm of pure AOT did not reach a plateau at the CMC, but rather adsorption continued to gradually increase at concentrations higher than the CMC before reaching a plateau. This behavior is evidence of competitive adsorption between AOT and impurities. The adsorbed layer of AOT on gold became more viscoelastic as the concentration of AOT increased. Tween 20 reached the maximum adsorption on gold before its concentration reached the CMC, suggesting that the attraction between Tween 20 and gold is very strong. The Tween 20 adsorbed layer was also rigid, when compared to the AOT adsorbed layer, as indicated by low dissipation. The addition of Tween 20 to a surface covered by AOT resulted in an increase in adsorbed mass as determined by the QCM signal, suggestive of insertion of Tween 20 into the AOT adsorbed layer. This phenomenon is expected since Tween 20 will be able to separate the repulsive head groups of AOT. However, some desorption was found for a pre-adsorbed Tween 20 layer when AOT is added. The interpretation is that some of the Tween 20 on gold was pulled by AOT to form mixed micelles in solution. Although a two-step model was applied to both AOT and Tween 20 adsorption kinetic data. AOT was found to adsorb much more slowly than Tween 20.