Adsorption behavior of oxyethylated anionic surfactants .1. Adsorption equilibrium

Anionic surfactants containing one to three oxyethylene groups have been synthesized and their adsorption behavior at the water/air interface studied by measurement of their surface tension isotherms. On the basis of a long-time approximation model of the diffusion-controlled adsorption model, the equilibrium surface tension of low dilute solutions is determined. The experimental results support the theory and show that in a certain time range the approximation yields realistic equilibrium surface tension values. These values are not affected by surface active impurities. Oxyethylene groups in the anionic surfactant molecule increase surface activity. The effect of oxyethylene groups is comparable to that of methylene groups in the alkyl chain. The analysis of the adsorption isotherms suggests that at low surface pressure the oxyethylene groups are localized within the water/air interface. With increasing surface pressure an orientation change normal to the interface may occur. (C) 1997 Elsevier Science B.V.