Colorimetric determination of the free phospholipids present in concentrated fluorocarbon emulsions indicates that the amount of phospholipids adsorbed at the fluorocarbon/water interface increases when a mixed fluorinated/hydrogenated compound, C6F13C10H21, is added as a stabilizer. The polar head surface area of the phospholipids in emulsions of C8F17Br was reduced from ca. 85.8 +/- 1.3 angstrom2 to 74.1 +/- 1.1 angstrom2, indicating tighter molecular packing of the surfactant. This effect was not observed when C10F21Br was used as a stabilizer, implying that the fluorocarbon/hydrocarbon compound is located preferentially at the fluorocarbon/water interface rather than dispersed throughout the fluorocarbon phase. The interfacial film structuring effect of the mixed fluorocarbon/hydrocarbon ''dowel'' molecule is more effective when the dispersed fluorocarbon is linear rather than cyclic. (C) 1994 Academic Press, Inc.