Thermodynamic investigation of mixed monolayers of trans-dehydrocrotonin and phospholipids

The thermodynamic behavior of mixed floating monolayers of zwitterionic phospholipids and trans-dehydrocrotonin (t-DCTN) at different molar fractions (x) of the diterpene has been investigated by measuring the surface pressure and surface potential Langmuir isotherms. Excess Gibbs energies of mixing have been calculated for different compositions of the floating monolayer at varying surface pressure values. Spreading of pure t-DCTN molecules at the air–water interface resulted in a compressed-type isotherm. For the t-DCTN/DBPC and t-DCTN/DOPC binary systems, the maximum stability was obtained at xt-DCTN = 0.375 and 0.500 at a surface pressure of 15 mN/m, with a minimum ΔGM value equal to −1.7 kJ/mol. The analyses of the obtained thermodynamic parameters such as excess areas, excess free energies and free energy of mixing confirmed the miscibility of the phospholipids and t-DCTN in the binary systems. In these mixed monolayers, the phospholipid molecules seemed to exhibit higher long-range orientation order for intermediary values of t-DCTN molar fractions. As a whole, the above observations suggest that the t-DCTN molecules exert a condensing effect on the monolayers when mixed with the phospholipids.