ADSORPTION BEHAVIOR OF OXYETHYLATED ANIONIC SURFACTANTS .1. ADSORPTION EQUILIBRIUM

Anionic surfactants containing one to three oxyethylene groups have be en synthesized and their adsorption behavior at the water/air interfac e studied by measurement of their surface tension isotherms. On the ba sis of a long-time approximation model of the diffusion-controlled ads orption model, the equilibrium surface tension of low dilute solutions is determined. The experimental results support the theory and show t hat in a certain time range the approximation yields realistic equilib rium surface tension values. These values are not affected by surface active impurities. Oxyethylene groups in the anionic surfactant molecu le increase surface activity. The effect of oxyethylene groups is comp arable to that of methylene groups in the alkyl chain. The analysis of the adsorption isotherms suggests that at low surface pressure the ox yethylene groups are localized within the water/air interface. With in creasing surface pressure an orientation change normal to the interfac e may occur. (C) 1997 Elsevier Science B.V.