ABSTRACT

They are generally formed from solutions of usual surfactants; they are called black because they do not reflect the natural light as a result of their smaller thickness compared to the visible wavelengths. Depending on the salt concentra­ tion, two types of black films can be observed. For the CBF, a balance between van der Waals attraction and the double-layer repulsion forces determines the equilibrium thickness [3,4]. The CBFs are known to be thicker (because they contain an aqueous core) than the NBFs, which involve more subtle short-range forces. Free amphiphilic black films were generally studied by class ical tech­ niques [optical reflectance, infrared (IR), absorption, electrical, and contact-angle measurements] [5-10]. We have shown that x-ray reflectivity is the most power­ ful technique because it is sensitive to electron density gradients, which are strong at the two air-film interfaces. A few years ago, we demonstrated that the NBF was thinner than usually expected and well organized [11]. It consists of two opposite walls of molecules, without any liquid water between them. The aqueous core is reduced to an ultimate hydration layer of the polar heads and the roughness is limited to the capillary waves. Such features are general even for very different surfactants [12]. The NBF is thus a very good model system for the direct study of interfacial phenomena such as interactions between surfactant bilayers. For example, we have investigated black films of amphiphilic diblock copolymers (polyelectrolytes) [13]. More recently we have finalized a method for the investi­ gation of black films, which combines the force measurement to the structure determination of the films by x-ray reflectivity, which we have employed for the study of hydration effects.