Abstract
Surface light scattering was used to study thermally excited capillary waves on aqueous solutions of tetrahydrofuran. The data indicate the presence of various surface viscoelastic relaxation processes in this system. In particular, the surface dilatational elastic modulus shows clear evidence of a relaxation with time constant ∼5 μs, which is here associated with reorientation of the adsorbed tetrahydrofuran molecules. The time constant of this process increases significantly for surface films which are older than 5 h; this change is associated with other changes in the surface viscoelasticity, including the rate of increase of the surface pressure. The dilatational surface viscosity deduced from the surface light scattering data is negative, as has been found in other recent studies of soluble surfactants. These negative surface viscosities are interpreted as effective properties only, reflecting a reduction in the dilatational wave damping due to some process or processes not included in the theory used in the data analysis. The transverse shear surface viscoelastic modulus exhibits a complex frequency dependence, more than one relaxation process of different time constant being involved. Possible molecular mechanisms are discussed.
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