Unstable topography of biphasic surfactant monolayers

– We study the conformation of a heterogeneous surfactant monolayer at a fluid-fluid interface, near a boundary between two lateral regions of differing elastic properties. The monolayer attains a conformation of shallow, steep ‘mesas’ with a height difference of up to 10 nm. If the monolayer is progressively compressed (e.g., in a Langmuir trough), the profile develops overhangs and finally becomes unstable at a surface tension of about K(δc0) , where δc0 is the difference in spontaneous curvature and K a bending stiffness. We discuss the relevance of this instability to recently observed folding behavior in lung surfactant monolayers, and to the absence of domain structures in films separating oil and water in emulsions. Insoluble (Langmuir) monolayers of amphiphilic molecules, lying at water-air or water-oil interfaces, have been extensively studied in the past decades [1]. Such monolayers are found in many applications, including, e.g., surface-tension reduction, emulsification and coating. Of particular interest are phospholipid monolayers, which are used in various studies to model the surface of cell membranes [2]. Lipid monolayers are encountered in other biological systems, such as the lung surfactant monolayer coating the alveoli in lungs [3]. An interesting issue is the departure of a monolayer, upon lateral compression, from a flat, two-dimensional conformation to a buckled, three-dimensional one. This aspect is particularly important in the case of lung surfactant monolayers, which undergo compression/expansion cycles during breathing. The buckling transition was theoretically studied in previous works [4–6]. These studies focused on the overall conformation of the entire monolayer, seeking an instability with respect to a single extended mode (or a prescribed combination of modes [6]) of undulation. They yield a buckling transition at a practically vanishing surface tension (i.e., very high compression). In many circumstances the monolayer is inhomogeneous. The heterogeneity may arise in single-component monolayers from the coexistence of expanded and condensed domains [7]; multi-component monolayers may phase-separate to form domains of different composition. (∗) E-mail: [email protected]