Influence of surface-active trace impurities on the surface properties of aqueous solutions of oligoethylene glycol monooctyl ethers

In this paper the influence of surface-active trace impurities in aqueous o ligoethylene glycol monooctyl ether solutions on their adsorption behavior at the air/water interface is considered. The equilibrium surface tension ( sigma(e)) vs logarithm of concentration (In c) isotherms of surfactants whe n used "as received" usually reveal a broad concentration range characteriz ed by a constant slope, da,ld In c. This linear dependence of the equilibri um surface tension on the logarithm of the surfactant concentration is obse rved for various surfactants at various fluid interfaces, which contradicts the fundamental Gibbs equation given in Eq. [1]. The same behavior was als o observed for aqueous solutions of chemically well-defined oligoethylene g lycol monooctyl ethers. In this case the paradoxical behavior did vanish af ter purification of aqueous solutions until the state of "surface-chemical purity" was reached using an apparatus for programmed high-performance puri fication of surfactant solutions. The gas chromatographic analyses of the o riginal samples used showed that they contained impurities of different che mical structure and surface activity. Analysis of material aspirated from t he surface of these alcohol ethoxylate solutions indicates that these trace impurities are enriched or impoverished at the solution's surface. A clear correlation between the surface activity parameter and the enrichment/impo verishment was found. Thus, these findings prove that the paradoxical adsor ption behavior (do,ld In c constant) of the oligoethylene glycol monooctyl ethers is caused by surface-active trace impurities. These results support the hypotheses suggested by K. Lunkenheimer and Ch. Wedler [Tenside, Surfac tants, Detergents 30, 342 (1993)] that generally it is the effect of surfac e-active trace impurities that gives rise to paradoxical adsorption behavio r with surfactant systems at fluid interfaces. (C) 1998 Academic Press.