A Critical Comparison of Adsorption Models for Soluble Surfactants

Abstract

The adsorption isotherm of a soluble anionic surfactant was measured and discussed within the framework of established thermodynamical models. The adsorbed amount derived by these models was compared with the results obtained by second harmonic generation, SHG. SHG can be used as a highly surface specific probe which allows the determination of orientation and number density. Since no thermodynamic quantities are involved in the interpretation of SHG measurements, the data can be used to assess the validity and underlying assumptions of thermodynamic adsorption models. The SHG experiments reveal a good agreement with the Frumkin model. No indication of a subsurface layer can be found.

A study of kinetic molecular exchange processes in the medium frequency range by surface SHG on an oscillating bubble

“…SHG measurements at the air-water interface with a surfactant that has the same SHG-active group as the surfactant used in this work have been reported in the literature [23,24]. The second order polarizability of the chromophore is completely dominated by its β zzz component, with z being along the long chromophore axis.…”

An oscillating bubble device for direct measurement of molecular exchange processes at the air–liquid interface in the medium frequency range

“…SHG measurements at the flat air-liquid interface of a surfactant with the same SHG-active group as F381 have been reported in the literature. 9,13 The hyperpolarizability of the chromophore is completely dominated by its ␤ zzz component, with z being along the chromophore long axis. Therefore, the other ␤ components can be neglected, and the ␤ tensor can be treated as a scalar quantity.…”

Phase Transition in an Adsorption Layer of a Soluble Surfactant at the Air–Water Interface

“…The synthesis is described in Ref. (31) together with a confirmation of the structure via elemental analysis, 1 H and 13 C NMR spectra. The azobenzene unit with CF 3 provides a sufficiently high hyperpolarizability in order to fully utilize the potential offered by second harmonic generation (SHG).…”

“…The surface-chemical purification of the aqueous solution was performed as described in (31), applying the automatic highperformance purification apparatus (34) using a 3 × 10 −3 M stock solution.…”

“…In (31) we first investigated this model compound. The aim of this study was to compare the surface excess as retrieved from Gibbs fundamental law with the corresponding value obtained by purely optical means.…”