Thermodynamic and structural studies of DMPC and DSPC with DOTAP mixed monolayers at the air–water interface

Abstract

According to the method of Langmuir-Blodgett the thermodynamic behavior and the miscibility of the two phospholipids 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) in the mixed monolayer were evaluated in terms of the excess mean molecular area, excess Gibbs free energy and compressibility coefficient. DMPC and DOTAP were miscible and a repulsive interaction appeared between DMPC and DOTAP molecules at most of selected ratios and surface pressures. Meanwhile, the interaction between DSPC and DOTAP were attractive and the most stable monolayers were formed at the 3: 2 and 1: 4 stoichiometry. The morphology observations by fluorescence microscopy reveals that the size of the domains composed of DSPC were larger than DMPC.