The surface viscoelasticity of aqueous solutions of surface chemically pure n-decanoic acid has been studied by light scattering from thermally excited capillary waves of frequency between 5 × 10 4 and 9 × 10 5 s -1 . The surface dilatational elastic modulus measured by light scattering, corresponding to such frequencies, agrees with the equilibrium value derived from the π-ln(c) variation for concentrations below 8 × 10 -6 M and above 6 × 10 -5 M, but systematically exceeds the equilibrium variation between these concentrations. This intermediate concentration range has been described as one of transition between two different states of adsorption (Lunkenheimer and Hirte, J. Phys. Chem. 1992, 96, 8683). The behavior of the elastic modulus can be explained by relaxation involving molecular reorientation within the adsorbed layer, with a relaxation time . 4 µs. Such reorientation would be entirely consistent with a change of adsorption state, such as has been suggested for this system. Other aspects of the surface viscoelasticity are briefly discussed.

Surface light scattering has been used to study thermally excited capillary waves on solutions of a nonionic surfactant, n-decanoic acid in 0.005 M aqueous HCl. The capillary waves show no trace of the unexpected behavior previously found for ionic surfactants, suggesting that processes thought to act to reduce the stability of the dilatational surface waves in the ionic case are absent for the present system. The implications of this observation are discussed.

The dilational elastic modulus of monomolecular films at the air-water interface can be determined by surface light scattering. For two different amphiphilic molecules, n-pentadecanoic acid and glycerol monooleate, this modulus increases discontinuously at a particular surface concentration on film compression. The magnitude of the increase, in units of kBT/A, where A is the area per molecule in the film, is the same in both cases and is compatible with the value 4 pi . This suggests that the effect arises from the transition of the quasi-two-dimensional film from a hexatic to a solid phase.

Q uasi-elastic light scattering studies of model membranes have yielded much information on their structure and dynamics. Unusually, the q dependence of the dynamic structure factor S(q, w) carries no significant structural information. Instead the information on membrane structure has derived from changes in the scattered intensity as the membrane is passed through a phase transition or other change, or from fluctuations in the dynamic part of the scattering factor in situations where the signal is non-stationary. We concentrate here on the latter aspect, and consider the information on the structure ofvarious model membrane systems which has been gathered.

Electrocapillary methods for excitation of capillary-gravitational waves at liquid surfaces have been modified for application to liquid-liquid interfaces. The necessary experimental modifications are described. Typical results are presented for a water-hexadecane interface. The observed propagation of the generated waves accords with theoretical expectation.