(742c) Shape Morphology of Dipolar Domains in Planar and Spherical Monolayers

Two-dimensional monolayers and bilayers of phospholipids exhibit a rich and interesting phase morphology. Common structural features of these systems include the emergence of condensed, liquid crystalline domains (usually around 10-20 microns) embedded in an otherwise fluid, disordered matrix of lipids. Two important consequences of this heterogeneous microstructure are (1) mechanical reinforcement of the monolayer or bilayer while maintaining its internal fluidity, and (2) spatial localization of “functional” molecules including proteins, cholesterol, and signaling agents. There is a growing body of evidence that the morphology of two-dimensional, phase-separated systems is influenced by the curvature of the underlying template. Although this geometric dependence remains poorly understood, it becomes extremely relevant to the study and manipulation of monolayers (e.g., in lung alveoli, micro- and nano-emulsions) and bilayers (in cells, organelles, and vesicles) with radii of curvature on the order of 10-100 microns. In this work, we present a continuum theory for predicting the equilibrium shape and size of dipolar domains formed during liquid-liquid phase coexistence in planar and spherical monolayers. Our main objective is to assess the impact of monolayer surface curvature on domain morphology. Following previous investigators, we base our analysis around minimizing the free energy, with contributions from line tension and electrostatic dipolar repulsions. Assuming a monodisperse system of circularly symmetric domains, we calculate self and interaction energies for planar and spherical monolayers and determine the equilibrium domain size from the energy minima. We subsequently evaluate the stability of the circularly symmetric domain shapes to an arbitrary, circumferential distortion of the perimeter via a linear stability analysis. We find that surface curvature promotes the formation of smaller, circularly symmetric domains instead of larger, elongated domains. We rationalize these results based on the repulsive interactions between splayed dipoles on a curved monolayer. A phase diagram of domain shape morphologies can be parameterized in terms of the domain area fraction and the monolayer curvature. For typical domain dimensions of 1-30 μm, our theoretical results are relevant to monolayers (and possibly also bilayers) in liquid-liquid phase coexistence with radii of curvature of 1-100 μm.