Anomalous Damping of the Capillary Waves at the Air-Water Interface of a Soluble Triblock Copolymer

The dilational viscoelasticity of Gibbs and Langmuir monolayers of pluronic F-68 (PEO₇₆-PPO₂₉-PEO₇₆) has been studied by capillary wave techniques over a broad frequency range. The results show that the behaviors of Gibbs and Langmuir monolayers are equivalent in the low surface pressure range. For concentrated monolayers, the dynamic elasticity ε shows a noticeable frequency dependence, and the concentration dependence of ε (at constant frequency) is not compatible with the classical Lucassen-Van den Tempel model. Moreover, in this concentration range, the dilational viscosity takes apparent negative values. The spectra of light scattered by thermal capillary waves have been analyzed in terms of two recently proposed dispersion equations. The equation of Buzza et al. (J. Chem. Phys. 1998, 109, 5008) is able to fit the experimental spectra only if one assumes that the thickness of the monolayer takes unrealistic values of the order of 0.1 μm. The equation of Hennenberg et al. (J. Colloid Interface Sci. 2000, 230, 216) is able to fit the experimental spectra, although the physical meaning of its new coupling parameter remains obscure.