Lung surfactant monolayers phase-separate in two dimensions, resulting in liquid-condensed (LC) domains dispersed within a less-ordered, liquid-expanded (LE) matrix. The organization of lipids, fatty acids, proteins, and cholesterol dictates the morphology of domain shapes in coexisting LC-LE monolayers, which, in turn, affects the surface-tension-lowering and respreading ability of lung surfactant. An open question, with implications for surfactant replacement formulations, is how these various chemical components combine to effect certain domain morphologies, and what these morphologies imply about the physicochemical properties of the monolayer. A related question is how physiologically realistic curvature of the pulmonary alveoli – 40-150 microns in diameter – impacts these morphologies. To address these questions, we present insights derived from continuum-mechanical theory into the physics governing domain shapes in planar and spherical monolayers. In the first part of the talk, we present an application of H. M. McConnell’s classical theory to predict droplet-to-stripe shape transitions in planar monolayers of varying chemical composition. Fluorescence imaging experiments reveal the droplet phase to be highly polydisperse, whereas the stripe phase exhibits a highly uniform width distribution that is tightly controlled by the monolayer composition. This enables, for the first time, unambiguous determination of the ratio of the squared dipole moment density to the line tension (2D Bond number) by measuring stripe widths instead of droplet radii. In the second part of the talk, we present an extension of McConnell’s theory to spherical monolayers and the effect of surface curvature. Through a linear stability analysis of the phase energetics, we show that surface curvature delays the onset of the droplet-to-stripe shape transition by attenuating the strength of dipolar repulsions between phospholipids. Our theory suggests that the curvature of the lung alveoli plays a nontrivial role in selecting LC domain morphologies, which could not have been uncovered from traditional Langmuir-trough experiments.

Domain shape morphologies in monolayer films of lung surfactant

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