Thermodynamic and Textural Characterization of DPPG Phospholipid Monolayers

The phase properties of DPPG (dipalmitoyl-phosphatidylglycerol, sodium salt) monolayers are investigated at different temperatures 25 °C ≤ T ≤ 34 °C where the surface pressure−area (Π−A) isotherms have large two-phase coexistence regions. The studies are based on the thermodynamic and textural characterization of the monolayers. The main phase transitions obtained from the Π−A isotherms are in complete agreement with the Brewster angle microscopy results. In equilibrium, compact domains are formed which are never really circular. The domains differ from each other in the azimuthal tilt, but they have no inner texture. Over the entire region of the gaseous to the condensed state, the experimental Π−A isotherms are well described by an equation of state recently derived on the basis of the generalized Volmer's equation and the quasi-chemical equilibrium model of 2D aggregation. The possible dissociation effect on the monolayer properties of the ionic DPPG can be largely ignored, as clarified by extending the theoretical approach under consideration of the dissociation degree of the monolayer substance. The standard thermodynamic characteristics of the 2D aggregation are calculated for different temperatures. The analysis of these results shows that, unlike the micelle formation, within the Langmuir monolayer the aggregation of the amphiphilic molecules to condensed phase results only in an entropy decrease of the system due to the ordering of the amphiphilic molecules.