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 s urface second-harmonic generation (SHG) for the characterization of adsorpt ion layers of soluble surfactants are discussed with the aid of exemplary m easurements carried out with a SHG-active amphiphile. SHG provides an intri nsic surface specifity and the analysis of polarization-dependent SHG measu rements yields the symmetry of the interface and the number density and the orientation of the amphiphile. These data can be used to assess some pecul iar features of adsorption layers of soluble surfactants. The experimental work focuses on two items: the linear range observed in the sigma(c) isothe rm and the correct description of ionic amphiphiles. In the case of our mod el system major deviations were found between optical data and data obtaine d using the Gibbs model. The observed discrepancies could be bridged by the introduction of a coupling between cation and anion within the surface lay er in the derivation of the Gibbs equation. The model system was also used to assess the meaning of ellipsometric measurements for the characterizatio n of adsorption layers of soluble surfactants. In the ultra-thin-film limit an ellipsometric experiment yields only a single parameter eta. Usually et a 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 com parison between ellipsometric and SHG data showed that eta is not necessari ly proportional to the amount adsorbed. The ellipsometric isotherm even pos sesses a maximum at an intermediate bulk concentration far below the critic al micelle concentration. Hence, we have to conclude that ellipsometry is n ot a suitable alternative to surface tension measurements, neutron reflecto metry or nonlinear optical investigations for the determination of the surf ace excess of soluble surfactants although it is a convenient and valuable tool to monitor qualitatively local and temporal variations of the molecula r density at fluid interfaces.