Path dependence of adsorption behavior of mixtures containing dipalmitoyl phosphatidylcholine

The adsorption of aqueous phospholipid dispersions containing dipalmitoyl phosphatidylcholine (DPPC) is investigated at 35-37 degrees C as a function of dispersion preparation technique. Systems studied in terms of surface pressure-time (pi-t) adsorption behavior were pure DPPC, 9:1 DPPC:dipalmitoyl phosphatidylethanolamine, 7:3 DPPC:egg phosphatidylglycerol (PG), and lipids extracted from cow lung lavage. The results show that pi-t characteristics can differ significantly depending on the technique by which the DPPC-containing mixtures are initially dispersed in 0.15 M NaCl solution. Examples of path dependence include the fact that DPPC, which will not adsorb at T = 35 degrees C when placed in powdered crystals on the subphase surface, exhibits measurable pi-t changes after subphase dispersion by sonication or by mechanical vortexing. For 7:3 DPPC:PG, dispersion by sonication on ice or by mechanical vortexing gives faster adsorption than dispersion by sonication without temperature control. The effect of heating to T = 45 degrees C, which is greater than the gel to liquid crystal transition temperature of DPPC (Tc = 41 degrees C), is found to be particularly detrimental to the adsorption of 7:3 DPPC:PG. Of the phospholipid mixtures studied, extracted cow lung lipids exhibited by far the greatest adsorption capability and also showed less path dependence than 7:3 DPPC:PG. Similarly, in terms of dispersion techniques investigated, sonication on ice tended to give the most rapid adsorption for a given phospholipid mixture.