title: Adsorption of single and mixed ionic surfactants at fluid interfaces.
abstract: Two different approaches have been used in the literature to describe the effects of ionisation of surfactants on the surface pressure, pi. One approach is based on a molecular model for a charged monolayer, in which the mutual repulsion of the long-chain surfactant ions results in an additional surface pressure, piel, calculated with the Gouy-Chapman theory for the formation of a diffuse electric double layer, and with counterion binding in the Stern-Helmholtz layer adjacent to the surfactant monolayer. The other approach regards the surface as a two-dimensional solution defined as a Gibbs dividing surface, which is electroneutral by definition. In this approach, the adsorption of any ion is the sum of its excesses in the monolayer and the electrical double layer; no assumptions are made about the spatial distribution of charges. It has been shown that both models can produce a reasonable description of experimental results obtained for solutions of a single ionic surfactant (RX) with or without inorganic electrolyte (XY). In many cases, measurements of pi vs. mean ionic activity at different salt concentrations (cXY) are found to coincide on a single curve, implying that at given mean ionic activity both adsorption and pi are independent of cXY, i.e. that double-layer contributions to the surface pressure are negligible. In addition, the electroneutral 2-D solution approach has resulted in a simple explanation of several typical features of mixed ionic surfactant solutions, in particular for mixtures of anionic and cationic surfactants. In mixed solutions too, double-layer effects appear to be negligible. We present arguments for such negligibility. One reason is a significant degree of binding between adsorbed surface active ions (R) and counterions (X); another is that for 1:1 electrolytes, the contribution of the diffuse double layer to the adsorption of the combination (RX) vanishes. As a result, it is possible to interpret the same experimental data in terms of both models.
authors.affiliation: International Medical Physicochemical Centre and Institute of Technical Ecology, Donetsk, Ukraine.
authors.forename: V B
authors.initials: VB
authors.lastname: Fainerman
authors.forename: E H
authors.initials: EH
authors.lastname: Lucassen-Reynders