Adsorption of Mixed Cationic and Nonionic Surfactants at the Hydrophilic Silicon Surface from Aqueous Solution: The Effect of Solution Composition and Concentration

The adsorption of the mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide (C 16 TAB) and hexaethylene glycol monododecyl ether (C 12 EO 6 ) at the hydrophilic silicon−solution interface has been measured by specular neutron reflection. The effect of solution composition and concentration on the adsorbed amount, surface structure, and composition has been investigated at pH 2.4. Specular neutron reflection, in combination with H/D isotopic substitution of both the solvent and surfactant, enables detailed structural and compositional information about the adsorbed layer to be obtained. The results obtained are compared with those reported elsewhere for the adsorption of the same surfactant mixture at the air−water interface and for surfactant mixing in micelles. Consistent with other studies, the structure of the adsorbed layer is described as a “defective” bilayer or “flattened” micelles. The variation in the adsorbed amount with solution composition is a maximum at an equimolar solution composition. For solutions richer in C 16 TAB, the surface and solution compositions are identical (consistent with ideal mixing). Whereas for solutions richer in C 12 EO 6 there is a departure from ideality, and the surface is richer in the cationic surfactant.