Thermodynamic studies on mixed molecular langmuir films: Part 2. Mutual mixing of DPPC and bovine lung surfactant extract with long-chain fatty acids

Mixing of long-chain saturated fatty acids (FAs), from dodecanoic (lauric) to octadecanoic (stearic) acid with a disaturated phospholipid, dipalmitoylphosphatidylcholine (DPPC), and the lipid-extracted materials from bovine lung surfactant (BLES), a clinical pulmonary surfactant (PS) formulation, have been studied at the air–water interface using Langmuir surface balance. BLES is a natural mixture of both saturated (solid) and unsaturated (fluid) lipids having DPPC as its major constituent; besides it contains two hydrophobic proteins SP-B and -C that help in better surface activity. Surface pressure–area behavior of DPPC and BLES, mixed with different proportions of FA, were studied. While DPPC–FA mixtures showed positive deviations from ideality, BLES–FA mixtures showed negative deviations irrespective of the chain length of FAs. Minimum repulsion was observed for palmitic acid in DPPC–FA mixtures while maximum association occurred with palmitic acid for BLES–FA mixtures, especially at higher mole fractions of FAs, as revealed from the excess free energy or free energy of mixing. The miscibilities are discussed in terms of the chain lengths of the FAs, the size of lipid and the composition of BLES.