Bulk and interfacial properties of a cationic micellar system near the critical point

Surface light scattering in reflection geometry: capabilities and limitations

In the present study, the capabilities and limitations of surface light scattering (SLS) experiments in reflection geometry are investigated. Based on the study of the transparent reference fluid toluene at 303.15 K over a wide range of wave vectors between (0.3and6.6)×10⁵m^-1, the performance of two different detection schemes analyzing light scattered from the vapor-liquid interface in a perpendicular and non-perpendicular direction is assessed. Considering various aspects such as the quality of the heterodyne correlation functions, the input information for data evaluation, and the line-broadening effects, both detection schemes show comparable overall efficiency. For wave vectors larger than 4.5×10⁵m^-1, where line-broadening effects are suppressed, the results obtained for liquid viscosity and surface tension agree with measurements in transmission geometry, validating the capability of the apparatus. For wave vectors smaller than 1.5×10⁵m^-1, the SLS signals are distinctly affected by line-broadening effects, which will result in erroneous values for surface tension and in particular viscosity, even if empirical fitting approaches commonly used in literature are applied. The modeling of the influence of line broadening on the measurements results by a simple Gaussian-weighted sum of individual damped oscillations reveals the increasing complexity of the underlying wave vector distribution toward smaller wave vectors chosen for the scattering geometry.

Light scattering by liquid surfaces, new developments

The surface light scattering technique is presented, highlighting recent technical improvements and describing studies of various types of surfaces. The technique is non-invasive, but delicate to handle and no commercial instruments are available yet. The technique gives however interesting information difficult to obtain otherwise, for instance on out-of-equilibrium surfaces, surfaces of very low tension, or systems close to solidification. Many studies were performed with monolayers of surface-active molecules at the surface of water. In this case, surface viscoelastic parameters can be determined at high frequencies (10 kHz- 1 MHz), complementing usefully the data obtained at lower frequencies with other techniques. As with these other techniques, inconsistencies such as negative surface viscosities are sometimes reported. The origin of these anomalies is not yet fully clarified. The problem deserves further work, in order to achieve a satisfactory description of the motion of surfactant or polymer-laden surfaces.