Adsorption of zwitterionic gemini surfactants at the air–water and solid–water interfaces

In the present paper we report the adsorption behavior of a series of zwitterionic gemini surfactants, Cx–PO4−–(CH2)2–N+(CH3)2–Cy where x+y=22 and x≠y, at the air–water and solid–water interfaces. The critical micelle concentration (CMC), was determined by du Nouy ring tensiometry and by steady state fluorescence. The surface excess concentration of zwitterionic gemini surfactants was calculated from the surface tension versus log concentration curves by applying the Gibbs’ adsorption isotherm. The values of surface area per molecule calculated using Gibbs’ equation were very low, 20–30 Å2, considering the relatively large size of the gemini surfactants. This probably reflects some kind of surfactant aggregation at the surface. Adsorption at hydrophilic and hydrophobic solid surfaces (silica and silica treated with dichlorodimethylsilane, respectively) was investigated using reflectometry. The adsorbed amount at the hydrophilic surface was much higher than that at the hydrophobic surface. This can be interpreted as formation of either continuous bilayer structures or micelle-like aggregates on the hydrophilic substrate. Assuming monolayer packing, the area per molecule obtained at the hydrophobic surface at a surfactant concentration around the CMC was 42 and 44 Å2 for the more symmetrical geminis and 159 and 198 Å2 for the more unsymmetrical surfactants. Higher surfactant concentration resulted in formation of aggregates at the surface.