Surface rheology and phase transitions of monolayers of phospholipid/cholesterol mixtures

¹ Carl-Gustav Carus-Institut
² University of Karlsruhe (TH), Institute of Mechanical Process Engineering and Mechanics

^* To whom correspondence should be addressed. E-mail: gero.leneweit{at}carus-institut.de.

Submitted on January 19, 2007
Revised on February 20, 2007
Accepted on 31 October 2007

	Abstract

The dynamic surface elasticity and the surface dilational viscosity of three binary phospholipid/cholesterol mixtures are determined with axisymmetric drop shape analysis (ADSA) on a harmonically oscillating pendent drop. Dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylcholine (DMPC), and dioleoylphosphatidylcholine (DOPC) are used to explore rheological properties and phase transitions of mixtures of saturated and unsaturated phospholipids with cholesterol. Growth rates of surface dilational viscosity and dynamic elasticity are parallel for all film pressures studied. Characteristic breaks and plateaus can be found for these growth rates indicating phase transitions. For DPPC/cholesterol and DMPC/cholesterol mixtures phase diagrams with six regions separated by phase boundaries are found which are in good agreement to phase transitions reported in the literature for static measurements of isotherms and isobars on a Langmuir film balance and fluorescence microscopy. Some phase boundaries are only found by dynamic, not by static elasticity measurements. Imaging methods reveal phase separations produced by the formation of condensed stoichiometric complexes leading to micron sized and mostly circular domains. The effect of these complexes on monolayer rheology in liquid/liquid phases is described. Further, liquid/solid and solid phase transitions are discussed.

Key Words: axisymmetric drop shape analysis (ADSA), condensed stoichiometric complexes, dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC), surface dilational viscosity, surface elasticity