Adsorption of gemini surfactants at the air-water surface

We have studied the behavior of a gemini surfactant, the double chained dodecyl ammonium bromide or 12-2-12, at the surface of aqueous solutions and at room temperature. We have measured the kinetics of adsorption and the equilibrium surface tension. The surfactant has been obtained through a new synthesis method, which has the advantages of being simpler than previous ones and resulting in a product with good purity after the first crystallization. In the absence of salt, 12-2-12 behaves like other charged surfactants. The adsorption is initially diffusion limited, but as adsorption proceeds, and the surface concentration increases, the surface potential reaches large values (of around 200 mV) slowing down considerably the adsorption process. The observations are in line with existing models. The sensitivity of the gemini surfactants to salt is greater than that of single-chain surfactants, owing to their peculiar molecular structure. For single-chain nonionic surfactants or ionic surfactants, adsorption can be described by diffusion limited models. The behavior of 12-2-12 in salt solutions is very different. The very early stages of adsorption are diffusion limited, but very quickly, the behavior changes and strong deviations are observed. Such a behavior could be due to a desorption barrier, the origin of which remains to be elucidated.