Langmuir 1988,4, 1179-1183
1179
Study of Viscoelasticity of Soluble Monolayers Using Analysis of Propagation of Excited Capillary Waves C. Stenvot and D. Langevin* Laboratoire de Spectroscopie Hertzienne de 1'E.N.S.24, rue Lhomond 75231, Paris, Cedex 05, France Received March 31, 1988. I n Final Form: May 19, 1988 We have studied the surface dilational viscoelasticity of dodecyltrimethylammonium bromide monolayers using a very accurate method: analysis of propagation of electrically excited capillary waves. We have found a considerable variation of the surface elasticity and viscosity with frequency. A simple model was proposed earlier by Lucassen to describe the relaxation process with a desorption mechanism. We have found serious discrepancies between our data and the predictions of the model when using a classical monolayer equation of state. We discuss the possible origin of these discrepancies, which might be partly due to the approximate character of the equation of state.
I. Introduction Surfactant molecules adsorb at liquid surfaces and form monolayers that possess elasticity and viscosity. As a consequence, the dynamic properties of the surface are altered. The frequency of the capillary waves is decreased, not only because the surface tension is lowered but also because these waves become coupled to longitudinal waves (density waves in the surfactant layer) whose frequency and damping depend mainly on dilational viscoelasticity. The study of capillary waves propagation therefore allows one to obtain information about surface viscoe1asticity.l As in bulk fluids, there is a second kind of viscoelasticity associated with shear and currently measured with surface shear rheometem2 The viscoelastic shear parameters are, however, usually very small, and the dilational viscoelasticity dominates surface rhe~logy.~ For practical purposes it is therefore important to investigate this type of viscoelasticity, which controls, for instance, foam and emulsion stability, thin liquid films, flow stability, et^.^ When the surfactant molecules are soluble in the liquid, the dilational viscoelasticity is strongly frequency dependent. Lucassen pointed out that this is due to the adsorption-desorption process associated with the monolayer expansion or compression during the propagation of the capillary wave.5 When the frequency v of the wave is much higher than the inverse characteristic time 7-1 for the above relaxation process, the layer behaves as if it were insoluble, and its elasticity t is at a maximum. When VT
