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

Abstract

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.

Charge fluctuations and counterion condensation

“…Thus, this condensation picture may also be crucial to understanding the like-charged attraction [19]. Furthermore, there are some recent experimental [20] and simulation [21] indications that are consistent with the predicted condensation effect. The experiments [20] were performed with a monolayer of cationic surfactant where surface density of the surfactant and the counterion/salt density are controlled with high accuracy.…”

Ellipsometric study of nonionic polymer solutions

“…Then typically only one difference − 0 is higher than the corresponding error and the evaluation of both parameters, n 1 and δ, becomes impossible. Moreover, if the investigated layer is formed from molecules of ionic surfactant, in certain concentration regions the measured difference − 0 is no more proportional to the adsorbed amount as was demonstrated by means of novel optical method-second harmonic generation (SHG) [32].…”

Liquid Expanded Monolayers of Lipids As Model Systems to Understand the Anionic Hofmeister Series: 1. A Tale of Models

“…Monolayers of charged lipids have often been used in the past to investigate specific salt effects with considerable success. [81][82][83][84][85][86][87] With zwitterionic lipids, the direct Coulombic interactions, which strongly attract one of the two ions of the electrolyte toward the interface and strongly repel the other, are subdued.…”