What do linear and nonlinear optical techniques have to offer for the investigation of adsorption layers of soluble surfactants?

In this contribution the potential and the limitation of ellipsometry and surface second-harmonic generation (SHG) for the characterization of adsorption layers of soluble surfactants are discussed with the aid of exemplary measurements carried out with a SHG-active amphiphile. SHG provides an intrinsic surface specifity and the analysis of polarization-dependent SHG measurements yields the symmetry of the interface and the number density and the orientation of the amphiphile. These data can be used to assess some peculiar features of adsorption layers of soluble surfactants. The experimental work focuses on two items: the linear range observed in the sigma(c) isotherm and the correct description of ionic amphiphiles. In the case of our model system major deviations were found between optical data and data obtained using the Gibbs model. The observed discrepancies could be bridged by the introduction of a coupling between cation and anion within the surface layer in the derivation of the Gibbs equation. The model system was also used to assess the meaning of ellipsometric measurements for the characterization of adsorption layers of soluble surfactants. In the ultra-thin-film limit an ellipsometric experiment yields only a single parameter eta. Usually eta is proportional to the amount adsorbed; however, for adsorption layers of soluble surfactants (layer thickness h < 2 nm, dielectric constant epsilon approximate to 2) none of the underlying assumptions required to establish the direct proportionality from first principles (Maxwell's equations) are met. It is not obvious what eta represents under these conditions. The comparison between ellipsometric and SHG data showed that eta is not necessarily proportional to the amount adsorbed. The ellipsometric isotherm even possesses a maximum at an intermediate bulk concentration far below the critical micelle concentration. Hence, we have to conclude that ellipsometry is not a suitable alternative to surface tension measurements, neutron reflectometry or nonlinear optical investigations for the determination of the surface excess of soluble surfactants although it is a convenient and valuable tool to monitor qualitatively local and temporal variations of the molecular density at fluid interfaces.