Using neutron diffraction and a specially constructed high pressure cell suitable for aligned multibilayer systems, we have studied, as a function of pressure, the much observed anomalous swelling regime in dimyristoyl- and dilauroyl-phosphatidylcholine bilayers, DMPC and DLPC, respectively. We have also reanalyzed data from a number of previously published experiments and have arrived at the following conclusions. (a) The power law behavior describing anomalous swelling is preserved in all PC bilayers up to a hydrostatic pressure of 240 MPa. (b) As a function of increasing pressure there is a concomitant decrease in the anomalous swelling of DMPC bilayers. (c) For PC lipids with hydrocarbon chains $>~13$ carbons the theoretical unbinding transition temperature $T^{\star }$ is coupled to the main gel-to-liquid crystalline transition temperature $T_M.$ (d) DLPC is intrinsically different from the other lipids studied in that its $T^{\star }$ is not coupled to $T_M.$ (e) For DLPC bilayers we predict a hydrostatic pressure $(>\mathrm{}290\mathrm{}\mathrm{MPa})$ where unbinding may occur.

• Received 6 August 2003

DOI:https://doi.org/10.1103/PhysRevE.69.031906