Surface forces in foam films from DPPC and lung surfactant phospholipid fraction

Surface properties of foam films formed from aqueous dispersions of dipalmitoyl-phosphatidylcholine (DPPC) and from solutions of a phospholipid fraction of lung surfactant (TPL) are studied employing the foam film method. Experiments are carried out within a wide range of NaCl concentrations (C_el) and the ranges of C_el determining formation of common films (CF), common black films (CBF) and bilayer Newton black films (NBF) are found. The thickness (h) of the CF and CBF decreases with the increase of C_el until the critical electrolyte concentrations (C_{el, cr}) is reached. The determined C_{el, cr} that characterize the transition to NBF show that C_{el, cr} of the TPL films is an order of magnitude higher than that of the DPPC films. The measured h of the TPL films is higher than that of the DPPC films in the whole C_el range. Besides, only the h(C_el) curve of the DPPC films outlines a metastable C_el range where both CF and NBF are obtained. Both the h(C_el) curves and the direct measurements of the disjoining pressure isotherms of the DPPC films (Π(h) isotherms) demonstrate the role of electrostatic repulsive forces for the stability of the phospholipid films The obtained results are compared with the DLVO theory equations and the evaluated potentials of the diffuse electric layer φ₀  20 mV for the DPPC films and φ₀  100 mV for TPL films show the strong effect of the charged phospholipids in the TPL mixture on the electric properties at the film interfaces.