On the Analysis of Ellipsometric Measurements of Adsorption Layers at Fluid Interfaces

Abstract

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.

On the Internal Structure of an Adsorption Layer of an Ionic Soluble Surfactant. The Buildup of a Stern Layer Monitored by Optical Means

“…22 A recent publication brought the frequently assumed proportionality into question. 1 The ellipsometric isotherm of a cationic soluble amphiphile was measured. The surface tension isotherm resembles all features of a classical soluble surfactant including a critical micelle concentration (cmc).…”

Ellipsometry and Infrared Reflection Absorption Spectroscopy of Adsorbed Layers of Soluble Surfactants at the Air–Water Interface

“…The radiotracer technique is one of the earliest methods which can probe surface density directly (23)(24)(25). Recently, neutron reflection (26,27), second harmonic generation (28), ellipsometry (28)(29)(30)(31), and infrared reflection absorption spectroscopy (IRRAS) (18,21) have also been used for determining the surface density of soluble monolayers. Comparisons of the strengths and weaknesses of several of these techniques in determining surface coverage, molecular orientation, and hydrocarbon chain conformational state have been given in Ref.…”

Ellipsometry in Interface Science

“…adsorption layer of nonionic surfactants at the air-water interface, there is a striking mismatch between interfacial height h and the wavelength of light λ. As a result certain peculiarities exist which are discussed in this section [8].…”