The effect of polar groups on structural characteristics of phospholipid monolayers spread at the air–water interface

Structural characteristics (structure, morphology, and relative film thickness) of dipalmitoyl phosphatidylcholine (DPPC), dipalmitoyl phosphatidylglycerol (DPPG) and dipalmitoyl phosphatidylserine (DPPS) monolayers were determined at the air–water interface at 20 °C and at pH 6 by means of surface pressure (π)–area (A) isotherms coupled with Brewster angle microscopy (BAM). At lower surface pressures, phospholipid monolayers adopted an expanded-homogeneous structure at the air–water interface. As the surface pressure increases, in the liquid-condensed phase (LC), phospholipid monolayers showed film anisotropy and domains with heterogeneous structures. The homogeneous structures observed at higher surface pressures proved the existence of parallel oriented aliphatic chains when the close-packed film molecules were in the solid state. The relative monolayer thickness increased with the surface pressure and was at a maximum at the collapse point. The phospholipid head-group has an important role on the structural characteristics of the monolayer at the air–water interface.