VISCOELASTICITY OF SURFACTANT MONOLAYERS

Dilatational viscoelasticities of a nonionic surfactant monolayer are measured at the air/water interface using the excited-capillary-wave t echnique. The data are compared with the dif fusion-exchange model for surfactant adsorption. This model requires the knowledge of the equat ion of state for the monolayer. We have used the Frumkin equation of s tate. Although the measured elasticities show the same qualitative tre nds predicted by the model, their absolute values fall well below the calculated ones in the diffusion-dominated regime. In order to quantit atively fit the model to our data, rather high values of the diffusion coefficient must be used. However, we independently determine the dif fusion coefficient for this system, via dynamic surface tension measur ements using the relaxing pendent-drop technique, and our results agre e with the expected bulk value within the experimental uncertainty. Wh en the diffusion-model parameters are calculated numerically without a ssuming any form of equation of state to relate gamma, Gamma, and c, t he model fits the data with the bulk diffusion coefficient value withi n experimental uncertainties. This suggests that the current monolayer equations of state are not able to accurately describe the response o f a monolayer to small periodic dilatations around equilibrium. (C) 19 95 Academic Press, Inc.