Structure and surface energy of the surfactant layer on the alveolar surface

The surface energy of the alveolar surfactant layer is determined in the scope of a modification of the structural model of Larsson et al. [(1999) J Disp Sci Technol 20:1-12], according to which this layer is built up of a lipid monolayer adsorbed at the hypophase/air interface and supported by a network of lipid bilayers immersed into the hypophase, i.e., the alveolar liquid. Formulae are derived for the dependence of the specific surface energy of the surfactant layer on the distance between the bilayers constituting the layer. It is shown that at equilibrium this energy can have values comparable with or less than 1 mJ/m2 needed for normal functioning of the alveolus during the respiration cycle. The specific surface energy of the surfactant layer with monolayer-bilayer structure can have such low values only if the layer is of optimal thickness and if the specific line energy of the monolayer-bilayer contact lines is negative and that of the bilayer-bilayer contact lines is positive. It is found that in dynamic regime the change in the specific surface energy of the alveolar surfactant layer with bilayer-monolayer structure is in qualitative agreement with that determined experimentally during lung inflation and deflation.