Thermodynamics of interleaflet cavitation in lipid bilayer membranes.

Interleaflet cavitation in lipid bilayer membranes, or, shortly, intramembrane cavitation (IMC), is the formation of gas bubbles between the two leaflets of the membrane. The present paper focuses on the thermodynamics of IMC, namely, on the minimum work required to form an intramembrane cavity. The minimum work can be separated into two parts, one that depends on the volume and number of gas molecules in the bubble and another that depends on the bubble geometry. Minimization of the second part at a fixed bubble volume determines the optimized bubble shape. In homogeneous cavitation this part is proportional to the bubble surface area and therefore the bubble is spherical. In contrast, in IMC the second part is no longer a simple function of the bubble area and the optimized cavity is not spherical because of the finite elasticity of the membrane. Using a simplified assumption about the cavity shape, the geometry-dependent term is derived and minimized at a fixed cavity volume. It is found that the optimized cavity is almost spherical at large bubble volumes, while at small volumes the cavity has a lenslike shape. The optimized shape is used to analyze the minimum work of IMC.