FORMATION AND STRUCTURE OF SURFACE-FILMS - CAPTIVE BUBBLE SURFACTOMETRY

The adsorption model for soluble surfactants has been modified for sus pensions of pulmonary surfactant. The dynamic adsorption behavior may be governed by a two-step process: (1) the transfer of molecules betwe en the surface layer and the subsurface layer, which has a thickness o f a few molecular diameters only; (2) the exchange of molecules betwee n the subsurface and the bulk solution. The first step is an adsorptio n process and the second step is a mass transfer process. Between the subsurface and the bulk solution is an undisturbed boundary layer wher e mass transport occurs by diffusion only. The thickness of this bound ary layer may be reduced by stirring. Rapid film formation by adsorpti on bursts from lipid extract surfactants, as observed in the captive b ubble 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 ch ange in the association of the specific surfactant proteins and the su rfactant lipids during adsorption. The negative change in the free ene rgy during film formation more than compensates for the energy barrier related to the film surface pressure. In the traditional view, the ex tracellular alveolar lining layer is composed of two parts, an aqueous subphase and a surfactant film, believed to be a monolayer, at the ai r-water interface. The existence and continuity of the aqueous subphas e has recently been demonstrated by Bastacky and coworkers, and a cont inuous polymorphous film has recently been shown by Bachofen and his a ssociates, 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 osmi ophilic film which covers the entire alveolar surface, including the p ores of Kohn, is demonstrated. By transmission electron microscopy, th e surface film frequently appears multilaminated, not only in the alve olar corners or crevices, but also at the thin air-blood barrier above the capillaries. Disk-like structures or multilamellar vesicles appea r partially integrated into the planar multilayered film. In corners a nd crevices, tubular myelin appears closely associated with the surfac e film. Tubular myelin, however, is not necessary for the generation o f a multilaminated film. This is demonstrated in vitro by the fixation for electron microscopy of a film formed from lipid extract surfactan t on a captive bubble. Films formed from relatively high surfactant co ncentration (1 mg/ml of phospholipid) are of variable thickness and fr equent multilayers are seen. In contrast, at 0.3 mg/ml, only an amorph ous film can be visualized. Although near zero minimum surface tension s can be obtained for both surfactant concentrations, film compressibi lity and mechanical stability are substantially better at the higher c oncentrations. This appears to be related to the multilaminated struct ure of the film formed at the higher concentration. (C) 1998 Elsevier Science B.V. All rights reserved.