CHARACTERIZATION OF PHOSPHOLIPID LAYERS AT LIQUID INTERFACES .3. RELAXATION OF SPREADING PHOSPHOLIPID MONOLAYERS UNDER HARMONIC AREA CHANGES

A modified Langmuir trough with an oscillating barrier was used to inv estigate the relaxation behaviour and the dilational rheology of sprea d phospholipid monolayers of L-alpha-dipalmitoyl phosphatidylcholine ( DPPC) and L-alpha-dimyristoyl phosphatidylethanolamine (DMPE). The har monic area perturbation initiates relaxation processes in the monolaye r and provides information on the dilational elasticity and viscosity of a monolayer. In the coexistence region, the monolayer relaxation is dominated by the domain exchange. This exchange was visualised with B rewster angle microscopy at an oscillation frequency of compression/ex pansion cycles of 0.1 Hz. In single phase regions, the relaxation is c onsidered to be the result of rearrangements and orientations of the a liphatic chains. Close to the pure condensed phase, a significant incr ease of the phase angle between stress and strain arises corresponding to the increase of dilational elasticity and viscosity. Possible orig ins of this relaxation are discussed.