Characterisation 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 investigate the relaxation behaviour and the dilational rheology of spread phospholipid monolayers of L-alpha-dipalmitoyl phosphatidylcholine (DPPC) and L-alpha-dimyristoyl phosphatidylethanolamine (DMPE). The harmonic area perturbation initiates relaxation processes in the monolayer 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 Brewster angle microscopy at an oscillation frequency of compression/expansion cycles of 0.1 Hz. In single phase regions, the relaxation is considered to be the result of rearrangements and orientations of the aliphatic chains. Close to the pure condensed phase, a significant increase of the phase angle between stress and strain arises corresponding to the increase of dilational elasticity and viscosity. Possible origins of this relaxation are discussed.