In the widely accepted Stern model, an adsorption layer of an ionic surfactant at the air-water interface consists of a charged topmost amphiphilic monolayer, a so-called compact Stern layer of directly adsorbed counterions, and the Gouy-Chapman layer characterized by a diffuse ion distribution. The crux of Stern's treatment is the estimation of to what extent ions enter the compact layer and reduce the surface potential. This issue is addressed in this paper by optical means. Surface second harmonic generation, ellipsometry, and surface tension measurements have been used for an investigation of the prevailing ion distribution. Each technique probes different structural elements of the interfacial architecture, and their combination yields a deeper insight into the internal composition of the interface. The amphiphile 1-dodecyl-4-dimethylaminopyridinium bromide, C12-DMP, was used as a cationic soluble surfactant and the comparison with the experimental data obtained with the closely related nonionic betaine 2-(4′dimethylaminopyridinio)-dodecanoate provided evidence for the correctness of our interpretation of the data. A strikingly different ion distribution with increasing bulk concentration is observed and the underlying mechanism is discussed. Furthermore we are able to clarify the current discussion about the meaning of ellipsometric measurements for adsorption layers of soluble surfactants (with thickness < 2 nm). The dilemma is the impossibility of obtaining on the basis of Fresnel theory (i.e., the solution of Maxwell's equations) a one to one correspondence between measured quantities and the structural data of the monolayer. Commonly it is assumed that ellipsometry measures at least the surface excess but a recent publication questioned this [Teppner et al., Langmuir 1999, 15, 7002.]. Our simulations reveal that the effect of optical anisotropy within the layer on the ellipsometric signal is negligible as compared to the effect of a changing ion distribution. This analysis combined with the experimental results on both model systems give us the means to precisely state under which experimental prerequisites ellipsometry directly measures the surface excess as defined by Gibbs.

Surface rheology governs a great variety of interfacial phenomena such as foams or emulsions and plays a dominant role in several technological processes such as high-speed coating. Its major difference with bulk rheology resides in the high compressibility of the surface phase, which is the direct consequence of the molecular exchange between adsorbed and dissolved species. In analogy to bulk rheology, a complex surface dilational modulus, epsilon, which captures surface tension changes upon defined area changes of the surface layer, can be defined. The module epsilon is complex, and the molecular interpretation of the dissipative process that gives rise to the imaginary part of the module is subject to some controversy. In this contribution, we used the oscillating bubble technique to study the surface dilational modulus in the mid-frequency range. The dynamic state of the surface layer was monitored by a pressure sensor and by surface second-harmonic generation (SHG). The pressure sensor measures the real and imaginary part of the modulus while SHG monitors independently the surface composition under dynamic conditions. The experiment allows the assessment of the contribution of the compositional term to the surface dilational modulus epsilon. Two aqueous surfactant solutions have been characterized: a surface elastic and a surface viscoelastic solution. The elastic surface layer can be described within the framework of the extended Lucassen-van den Tempel Hansen model. The change in surface concentration is in phase with the relative area change of the surface layer, which is in strong contrast with the results obtained from the surface viscoelastic solution. Here, surface tension, area change, and surface composition are phase-shifted, providing evidence for a nonequilibrium state within the surface phase. The data are used to assess existing surface rheology models.

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