PHOSPHOLIPIDS AS EMULSION STABILIZERS .1. INTERFACIAL-TENSIONS

Surface and interfacial tensions of symmetric diacyl phosphatidylcholi nes [(C-n)(2)PC], 6 less than or equal to n less than or equal to 16, at the air-water and perfluorooctyl bromide-water interfaces have been studied. The interfacial tension kinetics were found to be very sensi tive to the phospholipid chain length, decreasing by more than an orde r of magnitude for each pair of -CH2- groups (one per each chain) adde d. At n > 12, equilibration required more than one day, beyond the acc essible experimental range of the Wilhelmy plate method. The surface a nd interfacial tension data reported in the literature for long-chain phospholipids are argued to be measured under nonequilibrium condition s and, because of this, heavily overestimated. Equilibrium interfacial tensions above the critical micelle concentration varied from 9.8 dyn /cm (n = 6) to 4.0 dyn/cm (n = 12). These values are substantially low er than those observed with conventional nonfluorinated surfactants (e .g., Pluronics or sodium dodecyl sulfate). The lower Ostwald ripening rates observed in phospholipid-stabilized emulsions, in comparison wit h those stabilized by other hydrocarbon surfactants, can plausibly be explained by the lower equilibrium interfacial tensions found in the p hospholipid systems.