On the internal structure of an adsorption layer of an ionic soluble surfactant. The-buildup of a stern layer monitored by optical-means

In the widely accepted Stem model, an adsorption layer of an ionic surfacta nt at the air-water interface consists of a charged topmost amphiphilic mon olayer, a-so-called compact Stern layer of directly adsorbed counterions, a nd 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 th e compact layer and reduce the surface potential. This issue is addressed i n this paper by optical means: Surface second harmonic generation, ellipsom etry, and surface tension measurements have been used for an investigation of the prevailing ion distribution. Each technique probes different structu ral elements of the interfacial architecture, and their combination yields a deeper insight into the internal composition of the interface. The amphip hile 1-dodecyl-4-dimethylaminopyridinium bromide, C12-DMP, was used as a ca tionic soluble surfactant and the comparison with the experimental data obt ained with the closely related nonionic betaine 2-(4'dimethylaminopyridinio )-dodecanoate provided evidence for the correctness of our interpretation o f the data. A strikingly different ion distribution with increasing bulk co ncentration is observed and the underlying mechanism is discussed. Furtherm ore we are able to clarify the current discussion about the meaning of elli psometric measurements for adsorption layers of soluble surfactants (with t hickness < 2 nm). The dilemma is the impossibility of obtaining on the basi s of Fresnel theory (i.e., the solution of Maxwell's equations) a one to on e 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., La ngmuir 1999, 15, 7002.]. Our simulations reveal that the effect of optical anisotropy within the layer on the ellipsometric signal is negligible as co mpared to the effect of a changing ion distribution. This analysis combined with the experimental results on both model systems give us the means to p recisely state under which experimental prerequisites ellipsometry directly measures the surface excess as defined by Gibbs.