Viscoelastic relaxation of insoluble monomolecular films

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Abstract
Glycerol mono-oleate monolayers at the air-water interface have been investigated by quasielastic light scattering from thermally excited capillary waves over a wide range of wave numbers. Using a relatively novel data analysis procedure four surface viscoelastic properties were deduced ab initio from the light scattering data : surface elastic moduli and viscosities governing shear normal to the monolayer (≡ tension) and dilation in the film plane. The tension and dilational modulus were compared with classical, equilibrium values in the first rigorous comparison of its kind. Various effects suggested that the two moduli were affected by rather different relaxation processes : discrepancies between the light scattering and equilibrium values of the two elastic moduli occurred in different states of the monolayer, and the two surface viscosities (both zero for the clean subphase) behaved very differently on monolayer compression. These effects were observed to be frequency dependent. In the fully compressed monolayer state the transverse shear modulus was characterised by an exponential relaxation, of time scale ∼ 9 μs. This relaxation time fell exponentially on monolayer expansion, reaching 100 ns for molecular areas ∼ 60 Å 2. Slower processes than these were rigorously excluded. The dilational modulus was generally less well determined than that affecting transverse shear. However in the expanded monolayer state, the data sufficed to demonstrate much slower relaxation, τ ∼ 290 μs. Possible molecular mechanisms are briefly discussed

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