Title: The Equilibrium Spreading Tension of Pulmonary Surfactant
Abstract: Monomolecular films at an air/water interface coexist at the equilibrium spreading tension with the bulk phase from which they form. For individual phospholipids, the spreading tension is single-valued, and separates conditions at which hydrated vesicles adsorb from tensions at which overcompressed monolayers collapse. With pulmonary surfactant, isotherms show that monolayers compressed on the surface of bubbles coexist with the three-dimensional collapsed phase over a narrow range of surface tensions. Dispersed surfactant vesicles adsorb through this range to reach surface tensions at the lower end of the coexistence-region. Between the upper and lower ends of this range, rates of collapse for spread and adsorbed films decrease substantially. Changes during adsorption across this narrow region of coexistence between the two- and three-dimensional structures at least partially explain how alveolar films of pulmonary surfactant become resistant to collapse.