Thin Layer Wicking - A Method for the Determination of the Gradient Hydrophobicity of a Solid Surface Due to Surfactant Adsorption

Using commercial Al₂O₃ plates (60 F254, Type E, Merck) for thin layer chromatography, investigations have been conducted on the effect of adsorbed surfactants (sodium dodecyl sulfate, SDS, and cetyltrimethylammonium bromide, CTAB) on the solid surface free energy components, apolar Lifshitz-van der Waals, γ^LW_s, polar electron donor, γ^-_s and electron acceptor, γ⁺_s. The surfactants were adsorbed during wetting process. Solution of the tested surfactant (10^-4 - 10^-2 M) penetrated along 10 cm-long plate until the solution reached the opposite edge of the plate. Then the plate was dried at 150°C and used for thin-layer wicking experiments to determine the Al₂O₃ components. It was found that the components varied along the plate in the studied systems, maxima and minima in the values of the components appeared as a function of the distance. In general, the adsorbed surfactants caused a decrease in the electron donor component relative to that for the untreated surface. Although these results are somewhat preliminary, it seems that thin layer wicking method is an interesting tool for investigations of the gradient hydrophobicity of the solid surface.