Thermodynamic analysis and AFM study of the interaction of palmitic acid with DPPE in Langmuir monolayers

Palmitic acid (PA), which could induce the apoptosis of endothelial cells, is related to kinds of diseases. The influence of PA on 1, 2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) monolayer was studied in this paper, which is essential to understand those physiological and pathological processes. The behaviors and microstructures of monolayer of PA/DPPE binary systems on subphase of pH 6.2 and 3 were investigated by using a combination of Langmuir technique and Atomic force microscopy (AFM) in this paper. The two kinds of molecular arrangement situations were provided at different molar ratios. The thermodynamic properties of PA/DPPE mixed monolayer have been analyzed quantitatively based on the regular solution theory. The results have clearly manifested that excess molecular area (Aex), excess Gibbs energy (ΔGex) and interaction parameter (ω) of PA/DPPE mixed monolayers obtained positive deviation for XPA = 0.1, 0.3, 0.5 and 0.7, suggesting an intermolecular repulsive interaction existed, the stability of the system decreased and the binary systems were metastable. The negative deviation was found at XPA = 0.9, suggesting an attractive force existed between molecules and was beneficial to the pack of molecules and increasing the stability of the system as well. The value of activity coefficient f1 and f2 of mixed systems presented opposite changing trend at subphase of pH 6.2. However, the trend was similar at subphase of pH 3. These results provided some experiment evidences and theory references to cure atherosclerosis and acute coronary syndrome caused by excess free fatty acid.