Thermodynamic and textural characterization of DPPG phospholipid monolayers

The phase properties of DPPG (dipalmitoyl-phosphatidylglycerol, sodium salt ) monolayers are investigated at different temperatures 25 degrees C less t han or equal to T less than or equal to 34 degrees C where the surface pres sure-area (Pi-A) isotherms have large two-phase coexistence regions. The st udies are based on the thermodynamic and textural characterization of the m onolayers. The main phase transitions obtained from the Pi-A isotherms are in complete agreement with the Brewster angle microscopy results. In equili brium, compact domains are formed which are never really circular. The doma ins differ from each other in the azimuthal tilt, but they have no inner te xture. Over the entire region of the gaseous to the condensed state, the ex perimental Pi-A isotherms are well described by an equation of state recent ly derived on the basis of the generalized Volmer's equation and the quasic hemical equilibrium model of 2D aggregation. The possible dissociation effe ct 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 tempera tures. The analysis of these results shows that, unlike the micelle formati on, within the Langmuir monolayer the aggregation of the amphiphilic molecu les to condensed phase results only in an entropy decrease of the system du e to the ordering of the amphiphilic molecules.