On the analysis of ellipsometric measurements of adsorption layers at fluid interfaces

Ellipsometry is a well-established, nondestructive optical method for the c haracterization of thin films. An ellipsometric experiment yields in the th in film limit only a single parameter eta, which is related to changes in t he state of polarization caused by reflection. The ellipsometric quantity i s only subject to certain conditions proportional to the adsorbed amount Ga mma. The necessary requirements leading to the proportionably are not met f or adsorption layers of soluble surfactants at the air-water interface sinc e the dielectric constants epsilon of all media are very similar. It is not possible to establish from first principles (Maxwells equations) a unique relation between state of the monolayer and eta. The derived expression can not be inverted, and it is not justified to assume a linear relation betwee n eta and the surface excess Gamma. The aim of this contribution is to obta in an understanding what eta represents for soluble surfactants at the air- water interface. For the purpose of this study a soluble surfactant was des igned which possesses a sufficiently high hyperpolarizability to enable sur face second harmonic generation (SHG) in reflection mode to be performed. P olarization dependent SHG measurements were used to determine the orientati on, the surface excess, and the symmetry of the interface. These data were used to assess the meaning of ellipsometric measurements. The comparison re veals that the relation between surface coverage and ellipsometric signal i s nonlinear. The ellipsometric isotherm increases at low concentration and possesses a maximum at an intermediate coverage and then even decreases wit h increasing surface excess. These features can be understood in terms of c hanges in the orientation of the aliphatic tails of the amphiphile and by t he prevailing ion distribution at the interface. Ellipsometry is therefore not a suitable alternative to surface tension measurements, neutron reflect ometry, or nonlinear optical investigations for the determination of the su rface excess of soluble surfactants although it is convenient technique to characterize qualitatively local and temporal variations of the molecular d ensity at fluid interfaces.