A versatile modular setup is described which incorporates ellipsometry, surface plasmon spectroscopy, waveguide modes, their corresponding imaging techniques and Brewster angle microscopy in a single instrument. The important design criteria are discussed with special emphasis given to the requirements imposed by imaging under an oblique angle of incidence. Several experimental examples demonstrate the power of the instrument. Imaging nullellipsometry of a patterned monolayer on a highly reflecting support demonstrates a lateral resolution of approximately 1 μm and an accuracy in the thickness determination in the sub-nm region. The localization of the evanescent field of a surface plasmon was exploited to characterize adsorption layers in turbid and thus highly scattering solutions. An example of how an anisotropic sample can be characterized with the aid of waveguide modes is provided.
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.
Ellipsometry is a well-established, nondestructive optical method for the characterization of thin films.
An ellipsometric experiment yields in the thin film limit only a single parameter η, which is related to
changes in the state of polarization caused by reflection. The ellipsometric quantity is only subject to
certain conditions proportional to the adsorbed amount Γ. The necessary requirements leading to the
proportionably are not met for adsorption layers of soluble surfactants at the air−water interface since
the dielectric constants ε 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 η. The derived expression
cannot be inverted, and it is not justified to assume a linear relation between η and the surface excess
Γ. The aim of this contribution is to obtain an understanding what η represents for soluble surfactants
at the air−water interface. For the purpose of this study a soluble surfactant was designed which possesses
a sufficiently high hyperpolarizability to enable surface second harmonic generation (SHG) in reflection
mode to be performed. Polarization dependent SHG measurements were used to determine the orientation,
the surface excess, and the symmetry of the interface. These data were used to assess the meaning of
ellipsometric measurements. The comparison reveals that the relation between surface coverage and
ellipsometric signal is nonlinear. The ellipsometric isotherm increases at low concentration and possesses
a maximum at an intermediate coverage and then even decreases with increasing surface excess. These
features can be understood in terms of changes in the orientation of the aliphatic tails of the amphiphile
and by the prevailing ion distribution at the interface. Ellipsometry is therefore 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 convenient technique to characterize
qualitatively local and temporal variations of the molecular density at fluid interfaces.
Diblockcopolymers of polystyrene and poly(oxyethylene) were adsorbed at the toluene/water
interface. The short water soluble poly(ethylene oxide) block anchors the polymer at the interface, whereas
the long nonadsorbing Polystyrene block remains in toluene in good solvent conditions. The adsorption
layer adopts a brushlike conformation, and the system serves as a model for end-adsorbed polymers. A
film balance especially designed for liquid−liquid interfaces allows a compression of the adsorption layer
via a movable barrier and allows the control of the anchor density of the polymer. The balance was
optimized for optical reflection measurements and enables the simultaneous determination of the surface
tension. Ellipsometric measurements reveal that the density of the adsorption layer does not change
with compression. However, the thickness of the layer depends linearly on the anchor density. This
finding is in reasonable agreement with the prediction of the scaling laws.
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