Formation and structure of surface films: captive bubble surfactometry.
The adsorption model for soluble surfactants has been modified for suspensions of pulmonary surfactant. The dynamic adsorption behavior may be governed by a two-step process: (1) the transfer of molecules between the surface layer and the subsurface layer, which has a thickness of a few molecular diameters only; (2) the exchange of molecules between the subsurface and the bulk solution. The first step is an adsorption process and the second step is a mass transfer process. Between the subsurface and the bulk solution is an undisturbed boundary layer where mass transport occurs by diffusion only. The thickness of this boundary layer may be reduced by stirring. Rapid film formation by adsorption bursts from lipid extract surfactants, as observed in the captive bubble system, suggests that the adsorption process as defined above is accompanied by a relatively large negative change in the free energy. This reduction in the free energy is provided by a configurational change in the association of the specific surfactant proteins and the surfactant lipids during adsorption. The negative change in the free energy during film formation more than compensates for the energy barrier related to the film surface pressure. In the traditional view, the extracellular alveolar lining layer is composed of two parts, an aqueous subphase and a surfactant film, believed to be a monolayer, at the air-water interface. The existence and continuity of the aqueous subphase has recently been demonstrated by Bastacky and coworkers, and a continuous polymorphous film has recently been shown by Bachofen and his associates, using perfusion fixation of rabbit lungs with slight edema. In the present chapter, we have described a fixation technique using a non-aqueous fixation medium of perfluorocarbon and osmium tetroxide to fix the peripheral airspaces of guinea pig lungs. A continuous osmiophilic film which covers the entire alveolar surface, including the pores of Kohn, is demonstrated. By transmission electron microscopy, the surface film frequently appears multilaminated, not only in the alveolar corners or crevices, but also at the thin air-blood barrier above the capillaries. Disk-like structures or multilamellar vesicles appear partially integrated into the planar multilayered film. In corners and crevices, tubular myelin appears closely associated with the surface film. Tubular myelin, however, is not necessary for the generation of a multilaminated film. This is demonstrated in vitro by the fixation for electron microscopy of a film formed from lipid extract surfactant on a captive bubble. Films formed from relatively high surfactant concentration (1 mg/ml of phospholipid) are of variable thickness and frequent multilayers are seen. In contrast, at 0.3 mg/ml, only an amorphous film can be visualized. Although near zero minimum surface tensions can be obtained for both surfactant concentrations, film compressibility and mechanical stability are substantially better at the higher concentrations. This appears to be related to the multilaminated structure of the film formed at the higher concentration.