Multilayering of Surfactant Systems at the Air–Dilute Aqueous Solution Interface

Abstract

In the last 15 years there have been a number of observations of surfactants adsorbed at the air-water interface with structures more complicated than the expected single monolayer. These observations, mostly made by neutron or X-ray reflectivity, show structures varying from the usual monolayer to monolayer plus one or two additional bilayers to multilayer adsorption at the surface. These observations have been assembled in this article with a view to finding some common features between the very different systems and to relating them to aspects of the bulk solution phase behavior. It is argued that multilayering is primarily associated with wetting or prewetting of the air-water interface by phases in the bulk system, whose structures depend on an overall attractive force between the constituent units. Two such phases, whose formation is assumed to be partially driven by strong specific ion binding, are a concentrated lamellar phase that forms at low concentrations and a swollen lamellar phase that is not space-filling. Multilayering phenomena at the air-water interface then offer a delicate and easy means of studying the finer details of the incompletely understood attraction that leads to these two phases, as well as an interesting new means of self-assembling surface structures. In addition, multilayering is often associated with unusual wetting characteristics. Examples of systems discussed, and in some cases their bulk phase behavior, include surfactants with multivalent metal counterions, surfactants with oligomers and polymers, surfactant with hydrophobin, dichain surfactants, lung surfactant, and the unusual system of ethanolamine and stearic acid. Two situations where the air-water surface is deliberately held out of equilibrium are also assessed for features in common with the steady-state/equilibrium observations.

Controlling water evaporation through self-assembly

“…The structure/transport coupling paves the way toward homogeneous coatings through drying, which requires to avoid the coffee ring effect (21). It can also provide molecular explanations to numerous observations of multilayer films at the air-liquid interfaces (22)(23)(24)(25), with important consequences such as the reduction of water evaporation from water reservoirs (26) and the stabilization of foams through the buildup of multilayer surfactant films (27). To conclude, we have unveiled a homeostatic mechanism illustrating the moderation principle announced by Le Chatelier, which states that the system responds to a nonequilibrium perturbation by opposing it.…”

Molecular insights of adsorption and structure of surfactants and inorganic ions at fluid – liquid interfaces

“…Unlike the traditional experimental techniques, these modern tools are capable of investigating different interfaces at molecular resolution. To name a few, the radiotracer method (Dixon et al, 1949;Aniansson et al, 1950;Matuura et al, 1959), ellipsometry (Mang et al, 1980), X-ray and neutron reflectivity Thomas et al, 2015), ion scattering spectroscopy (Andersson et al, 2014) and non-linear optical techniques, i.e. second harmonic generation (SHG) spectroscopy (Corn et al, 1994) and sum frequency generation (SFG) spectroscopy (Richmond, 2002) have already been used to determine the interfacial distribution and structure of the adsorption layers of surfactants and inorganic ions.…”

“…This common view has also been adopted by the community when applying the other models such as the Langmuir isotherm to many surfactant solutions. Nonetheless, recent studies have demonstrated strong evidence for the existence of multilayers at air/liquid interfaces (Thomas et al, 2015). The monolayer adsorption assumption has recently caused an argument about the applicability of the Gibbs isotherm to the analysis of surface tension data .…”

“…oil-water or air-water, and chemical composition of the adsorption layer Lu et al, 1995;. In addition, these techniques have been capable of detecting the multilayer adsorption of some surfactants in the form of lamellar (Thomas et al, 2015) and micellar layers lying beneath the solution surface (Fig. 2.13).…”

“…The dashed lines show the MD simulation results (Boecker et al, 1992). (Thomas et al, 2015). Molecular dynamics (MD) simulations have been another useful tool for studying the microscopic structure of interfaces and adsorption layers.…”