Investigating the adsorption of the gemini surfactant "12-2-12" onto mica using atomic force microscopy and surface force apparatus measurements

The adsorption of the cationic gemini surfactant 1,2-bis(n-dodecyldimethylammonium)ethane dibromide on mica was followed by measuring forces between mica surfaces and by atomic force microscopy (AFM) imaging. The surface charge was found to be neutralized at total surfactant concentrations between 8 x 10(-7) and 5 x 10(-6) M, depending on equilibration time, as judged by the elimination of the repulsive electrostatic double-layer force. At around this concentration, monolayer aggregates of the surfactant started to form on the surface, varying in size between 8 and 130 nm across and between 0.5 and 0.6 nm high. The coverage rapidly increased with a small increase in surfactant concentration, as seen by AFM images. In the concentration range (5 x 10(-6))-(1 x 10(-4)) M the surfactant continued to adsorb steadily as judged by the increase in the double-layer repulsion between surfaces. The hydrophobicity of the surfaces was confirmed by the magnitude of the force required to separate the surfaces, which increased from 20 mN/m in pure water to 120 mN/m at 4.6 x 10(-6) M, up to 210 mN/m at 9.0 x 10(-6) M; it then stayed virtually constant. AFM imaging showed that, in this range, a significant amount of the surfactant adsorbed on top of the monolayer, although neither technique suggested that the adsorbing material aggregated into bilayer patches. At 1.8 x 10(-4) M, a full bilayer formed on each surface, causing an increase in the compressed layer thickness from I to 4 nm, and a reduction in pull-off force to 5-10 mN/m. In this concentration range, the nucleation and growth of the complete bilayer was directly observed with AFM. It appeared to occur as isotactic growth of patches, which were initially around 70 nn in size and evenly distributed. These patches grew and joined together to form a flat bilayer over a time scale of around 2 h. Between 1.8 x 10(-4) and 7.6 x 10(-4) M, a non-DLVO (Derjaguin-Landau-Verwey-Overbeek) force was observed between 12 and 7 nm, followed by an attractive force which pulled the surfaces into bilayer-bilayer contact. The extra repulsive force, which has not been observed previously with cationic surfactant bilayers, was probably due to additional surfactant adsorbed outside the bilayer. AFM imaging confirmed that an extra layer was present above the critical micellar concentration (cmc), as indicated by a significant increase in surface roughness from 0.5 nm to 7-8 nm.