THE UNBINDING TRANSITION AND LAMELLAR PHASE LAMELLAR PHASE COEXISTENCE

The unbinding transition predicted by Lipowsky and Leibler (Lipowsky, R.; Leibler, S. Phys. Rev. Lett. 1986, 56, 2541) involving a continous change from a bound state with two membranes in van der Walls contact to an unbound state with essentially independently distributed membranes is examined. A mean field treatment of the phase equilibria is presented, and on this level it is shown that the system behaves in a way analogous to a fluid showing a first-order liquid-gas transition below a critical pressure, with a coexistance between a bound and an unbound region. Furthermore, the fluctuations occurring at the unbinding transition are analyzed. It is concluded, by use of renormalization group theory, that the fluctuations leading to a continuous transition are localized but of large amplitude. For two bilayers, restricted not to form stacks, there is not sufficient cooperativity in the system to give a first-order transition. However, for stacked membranes, as for example in lyotropic liquid crystals, such large amplitude fluctuations cannot develop independently in adjacent layers, and it is concluded that the transition in such a case is first order, that the mean field calculations are qualitatively correct, and that there is no continous unbinding transition. These conclusions are found to be consistent with the experimental observations of lamellar phase-lamellar phase coexistence and also with the observation that a continuous unbinding transition remains to be reported for a liquid crystalline system. A phase diagram for the binary system lecithin/water is calculated, and it is pointed out that a current controversy concerning the limited or unlimited swelling in this system might be resolved by the presence of a coexistence between a bound and a highly swollen phase. © 1990, American Chemical Society. All rights reserved.