The colloidal nature of phospholipid monolayers

Monolayers of different phospholipids and of a fluorinated fatty acid are studied by surface pressure and potential measurements and by fluorescence microscopic observation of domain movement in inhomogeneous electric fields. The data consistently show that long range electrostatic forces result from molecular dipole moments of the glycerin ester and not from the head group region of the monolayer. In coexistence with fluid phase solid lipid domains exhibit excess dipole density between 50 mD/nm2 and 200 mD/nm 2 with different signs for phospholipids and fluorinated fatty acids. The latter fact explains the opposite direction of forces on lipid domains in electric fields. Energies per domain corresponding to these forces were calculated to about 104 kT. The molecular origin of the surface dipole moments is discussed.