Anomalous swelling in phospholipid bilayers is not coupled to the formation of a ripple phase

The anomalous swelling in phospholipid bilayers was studied. Results show that the anomalous swelling exhibited by DMPC, dmDMPE, and mmDMPE bilayers appears to be related to an, unobtainable critical point that is a continuation of the fluid phase Gibbs free energy surface. The main transition is first order because the free energy of either the ripple phase or the gel phse is lower below T_M. Criticality of the anomalous swelling type is not seen in these low temperature phases, suggesting that they cannot be analytically continued to the imputed critical point. The existence or nonexistence of a ripple phase for bilayers of a particular lipid is then determined entirely by the competition between the ripple phase and the gel phase.