THE MAIN TRANSITION OF DIPALMITOYLPHOSPHATIDYLCHOLINE MONOLAYERS - A LIQUID EXPANDED TO SOLID CONDENSED HIGH-ORDER TRANSFORMATION

We have investigated the orientation processes of a fluorescent probe, 12-(9-anthroyloxy)stearic acid (12-9-AS), embedded in a dipalmytoylph osphatidylcholine (DPPC) monolayer spread at the air/water interface. These orientation processes are represented by an orientation paramete r defined as the ratio of two dimerization constants obtained from flu orescence experiments using two perpendicular linearly polarized incid ent light beams. The orientation parameter shows three particular doma ins of orientation during the compression of the DPPC film, these doma ins being separated by two drastic changes in the orientation paramete r, one located at the LC/SC transition point (0.48 nm(2).molecule(-1)) and the other at the end of the LE/LC transition region (0.58 nm(2).m olecule(-1)). Attribution of the phases implicated in the bidimensiona l liquid/ solid transformation was thus revisited. Therefore, from the present results together with results gathered in the literature, we show that the main monolayer phase transition of DPPC implies a transf ormation from a molecular state having a characteristic area A(LE) (ph ase LE) to a molecular state of characteristic area A(SC) (phase SC) w ith an intermediate molecular state of characteristic area A(LC) (phas e LC). During the main monolayer transformation (the LE/SC phase trans ition) the highly condensed domain with a molecular area A(SC) (0.48 n m(2).molecule(-1)) are thus surrounded by a region of interfacial lipi d of molecular area A(LC) (0.58 nm(2).molecule(-1)) distributed in a f luid phase of characteristic area A(LE) (0.74 nm(2).molecule(-1)).