Negative surface viscosities in fluid-fluid interfaces with adsorbed insoluble surfactants

Abstract

Recent measurements of the surface viscoelasticity of liquid surfaces with adsorbed surfactant using surface light scattering have yielded negative dilational viscosities (ε’ < 0). In this paper, we present a physical explanation for this phenomenon by constructing a molecularly based theory for the interfacial viscoelasticity of an insoluble surfactant monolayer. Using the interfacial viscoelasticity, we then calculate the resultant surface light scattering from the interface. Our microscopic theory predicts the presence of two additional surface elastic constants neglected in current treatments, namely a bending modulus, κ and a coupling constant λ. We show that for thick monolayers, the coupling constant λ can lead to apparent negative values of ε’. This is the first time that this result has been explained for insoluble monolayers using a physically realistic model.