Characteristic Isotherms for a Mixed Molecular Layer Composed of Phospholipid and Fatty Acid

A mixed molecular layer composed of phospholipids and fatty acids (carbon number: 18) was investigated experimentally to understand the interaction between them depending on the number of double bonds in the fatty acids. The surface pressure–surface area (Π–A) isotherm for 1,2-di-oleoyl-sn-glycero-3-phosphocholine (DOPC) at an air/water interface characteristically changed with the addition of fatty acid, i.e., A at Π was not changed for oleic acid very much in comparison with other fatty acids. The degree of the decrease in A at Π for 1,2-di-stearoyl-sn-glycero-3-phosphocholine was not changed for stearic acid very much in comparison with other fatty acids, suggesting different responses to fatty acids between the two compounds. Attenuated total reflection Fourier transform infrared spectroscopy and differential scanning calorimetry were used to evaluate the interaction between DOPC and fatty acid molecules.