The adsorption parameters of alkyldimethylphosphine oxides (from n-hexyl- to n-tetradecyl) at the air/water interface were calculated from equilibrium surface tension (σe) versus concentration (c) isotherms at 295 K. To guarantee absence of measurable surface-active impurities in the adsorption layers, stock solutions of the surfactants were purified until the state of surface-chemical purity was achieved. The evaluation of the σe vs c isotherms was performed by applying a two-state approach to the surface equation of state. All of them are described in the best way by ideal surface behavior. The critical micelle concentrations (cmc) and the calculated adsorption parameters, i.e., standard free energy of adsorption and limiting surface area demand per molecule adsorbed (A min), reveal distinct effects of alternation (even/odd phenomena). The A min values decrease with rising n-alkyl chain length. The results are discussed with respect to donor−acceptor interactions and surface conformational changes in the adsorption layer. As these surfactants' adsorption reveals ideal surface behavior, the data may serve as a reliable basis for modeling adsorption.

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The adsorption layer of soluble surfactants can be described by the well-known Gibbs equation. It states that the surface excess Γ is given by the slope of the isotherm according to Γ ) -1/(mRT)∂σ e /∂ ln a. The rigorous thermodynamic treatment requires a factor m ) 2 for 1:1 electrolyte in the case of the absence of indifferent electrolyte. This aspect is addressed in the following contribution. For this purpose, a soluble C 12 and an insoluble cationic amphiphile C 20 was designed and synthesized. Both contain the same head group which possess a sufficiently high hyperpolarizability for surface second-harmonic generation (SHG) measurements. SHG is highly surface specific, and polarization dependent measurements allow a determination of the orientational order within the adsorption layer. The intensity of the SHG signal is proportional to the square of the number density N, and with a calibration based on the SHG signal of the Langmuir monolayer of the insoluble compound the absolute number density can be determined. The corresponding (π,A) isotherm of the insoluble monolayer shows only a liquid expanded region, and experimental errors are minor since for both experiments the head group which is in an identical physical environment is responsible for the SHG signal. Furthermore, complications arising from local field corrections are eliminated. In our case, the factor m was determined to be m ) 1.28. These findings are discussed in detail.

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.

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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.

Maleic acid mon0[2-(4-&ylpiperazinyl)ethyl esters] (cB-C13) and their precursors, l-alkyl-4-(2-hydroxyethyl)piperazines, are synthesized and the pKa values of the surfactants are determined by potentiometric titration. The different ionic forms of the surfactant and their content in the solution at definite pH values are determinedwithin the range pH = 0.5-10. At pH = 6.2 the predominant component in the solution has a betaine-like structure. As this form is expected to have the highest surface activity and the concentrations of other ionic species are low in the bulk phase, this pH is selected to determine the adsorption parameters of the homologous series. A apecial method to determine equilibrium values of the surface tension is used in cases of low surfactant concentrations. To determine the adsorption parameters from a-log C curves of the aqueous surfactant solutions a Frumkin-Damaskin isotherm is used. The cmc (critical micelle concentration), the interfacial activity, and the interaction parameter show a significant evedodd alternation with the number of alkyl groups of these surfactants, in accordance with the alternation of the melting points.