Dynamic properties and relaxation processes in surface layer of pulmonary surfactant solutions

In this work, dynamic surface properties of spread monolayers of DPPC and adsorbed layers of PS solutions were investigated in a broad range of surface pressure. Application of a modified Langmuir trough and a recently developed approach for the analysis of a non-linear response of the adsorption layer to large surface deformations gives a possibility to determine the dilational surface elasticity in the region of high surface pressures corresponding to the physiological state of the lung alveoli surface. Although DPPC is the main component of the complex pulmonary surfactant (PS) mixture, the dynamic surface properties of the adsorption layers of these two compounds are different. Other components in the PS adsorption layer lead to a decrease of the dynamic surface elasticity due to a looser packing of the layer, as confirmed by ellipsometry and infrared reflection absorption spectroscopy. Moreover, in the region of high surface pressures the main relaxation time of the surface stresses is much lower than that for spread DPPC monolayers. The fast relaxation in the adsorption layer can be connected with the redistribution of molecules between the surface and the subsurface layer in the course of compression. The acceleration of the relaxation processes can lead to a decrease of the number and size of collapsed parts of the layer and thereby to a decrease of the number of molecules with lost functional properties. This effect can be one of the reasons for the lower efficiency of synthetic pharmaceutical drugs, which do not have proteins in their composition, as compared with the natural PS extracted from animal lungs.