Governed by a hierarchy of short-range and long-range physical forces, electrically neutral phospholipid molecules aggregate in water to form bilayer membranes which in turn form a multilayer lattice of bilayers alternating with aqueous layers. We use x-ray diffraction to determine the bilayer thickness d_l , average molecular cross-sectional area A and bilayer separation d_w. As water is removed from the lattice not only do the bilayers come closer together,but they deform to decrease area A and to thicken the bilayer.

We measure the work of water removal and divide this work into that of overcoming long-range bilayer repulsion and that of causing bilayer deformation. Electrically neutral membranes of egg lecithin, synthetic lecithins (above and below the melting temperature of their hydrocarbon chains), and egg phosphatidylethanolamine all exhibit a very strong repulsion at less than 20 to 30 Angstrom separation. For d_w < 10 Angstroms the repulsive pressure exceeds 100 atmospheres. This force varies roughly exponentially with a decay distance of 2 to 3 Angstroms and presents a formidable barrier to close contact between membranes.

The lattices are stabilized at 20 to 30 Angstroms separation by van der Waals attraction between bilayers whose magnitude can also be estimated. This attraction is of the correct magnitude expected from the general theory of van der Waals forces.

Preliminary comparison of the work of membrane deformation with monolayer data suggest that the lateral pressure to compact molecules on the same bilayer is not related simply to the analogous surface pressure of lipids in monolayers. The lateral pressure is necessarily zero at the equilibrium packing area of the phospholipids and increases to the order of 25 dyne/cm for 25% change in area. The low work of lateral compression (relative to thermal energy) might permit fairly large lateral fluctuations in packing if the work of lateral expansion is comparably small. Our ability to measure the work of rearranging lipids mak.es it possible to test and compare theories of bilayer mechanical properties and their stability, phase transitions between disordered and ordered states of the hydrocarbon chains, and phase separation of lipids in mixed systems.

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Molecular forces between and within lipid membranes 1