Finite two-dimensional phospholipid crystals

Abstract

Phase transitions between solid and fluid phases in monolayer films of phospholipid at the air-water interface can be studied with epifluorescence microscopy. Solid domains appear as dark regions in a white background when a low concentration of fluorescent lipid probe is included in the monolayer, providing the probe partitions preferentially in the fluid phase. It has been shown previously that the shapes and lateral distributions of the solid-phase domains in the fluid background are determined by combinations of long-range electrostatic dipole-dipole repulsions and line tension between the fluid and solid phases. These forces can also affect other properties of these monolayer films. In the present discussion, it is shown that the electrostatic and line-tension effects can affect the shape of pressure-area isotherms, giving deviations from the isotherm shapes expected for simple first-order transitions in the absence of these long-range forces. At equilibrium, the interface between any two distinct thermodynamic phases is electrically polarized. For isotropic phases, this polarization can be thought of in terms of an equivalent dipole density, ,A, with an orientation perpendicular to the interface. The electrical potential difference between two phases, AV, is related to the dipole density A by the equation