Dilatational rheology of a gel point network formed by nonionic soluble surfactants at the oil–water interface

Abstract

This study presents new aspects of the dilatational rheology of soluble nonionic surfactants at the water–oil interface giving a two-dimensional rheological gel-point behavior. We illustrate that these surprising results can be described by coupling the Lucassen and van den Tempel model to a power function describing the relaxation times distribution, involving the presence at the interface of two-dimensional aggregates with a wide distribution of sizes. The principal interfacial relaxation mechanism is then attributed to adsorption–desorption of surfactant clusters of various sizes. The main hypothesis for the use of the van den Tempel model is that clusters do not interact with each other while surfactants do interact within each cluster. Using the hyperscaling relation and assuming that the cluster relaxation times are related to the cluster masses by a power law, we propose a new expression of the fractal dimension d_f for interfacial aggregating systems in function of the relaxation exponent n of the form d_f = 2/(1 + n). This original study contributes to the understanding of the two-dimensional rheology of soluble monolayers with attractive surfactants, thus opening new doors for the characterization of their behavior at the interface.