Some mixtures of lipids in monolayers at the air-water interface exhibit immiscible liquid phases. Theory predicts an exponential scaling for the width of liquid domains that depends on a competition between line tension and electrostatic dipolar repulsion. It is shown that the scaling law is valid for the stripe phase in a binary mixture of lipids, dihydrocholesterol, and dimyristoylphosphatidylcholine. This stripe phase appears only near a miscibility critical point. The agreement of observed widths with theoretical scaling provides strong evidence for equilibrium in such mixtures, an essential condition for using data from monolayers to model the liquid regions of bilayers and biological membranes.

• Received 8 September 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.1602