Ordering Transitions in Thin, Freely-Suspended Phospholipid Films

(Received 9 January 1995, accepted 22 February 1995)

Abstract
The lyotropic phase behavior of freely suspended multilayers of the phospholipid dimyristoylphosphatidylcholine (DMPC) and water, ranging in thickness from a single bilayer (  Å) to 12 bilayers, has been investigated via optical reflectivity and reflection polarization microscopy. Specifically, equilibration of the film with a vapor phase of adjustable moisture content facilitated the continuous control of film composition, and this made it possible to maintain single bilayers for periods of days. Several optical features indicate effects of reduced dimensionality on ordering transitions from the phase into the partially ordered, lower temperature and phases. These include the reversible distortion of layer steps and the formation of dendritic growth patterns at the onset of the transition. In addition, we find the ordering transitions of thin multilayers to be characterized by an increase in the optical reflectivity. On the basis of a numerical evaluation of a suitable slab model of multibilayers, we attribute this increase primarily to a lateral layer contraction, known to accompany the ordering in bulk dispersions of these materials. Comparison of the phase behavior of thin lipid films with the known phase diagram of thick films of the same system reveals a close correspondence.