Mechanism of Time-Dependent Adsorption for Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles: Effect of Temperature and Acyl Chain Length.

Dynamic surface tension measurements were used to track adsorption kinetics for dilauroylphosphatidylcholine (DLPC) or dimyristoylphosphatidylcholine (DMPC) from monodisperse vesicle dispersions to an air-water interface at elevated temperatures >= 30 degrees C. Effects of vesicle concentration, aqueous solubility of the lipids, and temperature T on the adsorption kinetics were determined, and the controlling transport pathway was identified. Adsorption dynamics were tracked for 0.1-10 mM DLPC at 30 and 38 degrees C and for 1-10 mM DMPC at 30, 50, and 58 degrees C. Experimental results were compared to theoretical predictions for a reaction-enhanced, molecular transport mechanism, which was previously shown to effectively predict DLPC adsorption kinetics at 22 degrees C. At higher temperatures, for DLPC concentrations >= 0.25 mM or DMPC concentrations >= 1 mM, a weak dependence of adsorption time on concentration was observed, again consistent with the reaction-enhanced molecular pathway. Molecular release rates from vesicles increased with increasing temperature or decreasing acyl chain length. At equivalent ratios T/T-m of the dispersion temperature to the lipid chain phase transition temperature T-m, measured adsorption times for DLPC were approximately 10-fold shorter than those for DMPC, suggesting that the fluidity of the acyl tails is not the only lipid property determining adsorption rates. Despite the significant difference in aqueous solubility and chain phase transition temperature between DLPC and DMPC, the results provide further evidence for an adsorption mechanism that is well described by diffusion of molecular lipid, with rates of molecular diffusion near the interface enhanced by release from nearby vesicles.