Comparison of DLPC and DPPC in controlling the dynamic adsorption and surface tension of their aqueous dispersions

The adsorption of dilauroylphosphatidyleholine (DLPC) at the air/water interface was investigated with tensiometry, infrared reflection-absorption spectroscopy (IRRAS), and ellipsometry. The tension dynamics at 25 degreesC at constant and pulsating area depends strongly on the concentration (10-1000 ppm) and sizes of dispersed DLPC particles, which are liposomes or vesicles. Dynamic surface tensions as low as 1-5 mN/m are observed for DLPC. For dipalmitoylphosphatidylcholine (DPPC), such tensions are also observed, but only for certain special preparation procedures. Direct probing of the surface by IRRAS and ellipsometry indicates that DLPC adsorbs by a molecular adsorption mechanism, controlled largely by the rate of dissolution of the dispersed particles. The surface layer is a monolayer with no particles attached to it, unlike DPPC in which the surface monolayer forms by particles which reach the interface and remained attached to it (Wen, X.; Franses, E. I. Langmuir 2001, 17, 3194). Spectroturbidimetry and dialysis experiments, used to determine the DLPC solubility in water as 4 1 ppm. and the DPPC solubility as essentially zero, support the above mechanisms. For DLPC, a simple diffusion/adsorption model with molecular diffusion and variable effective diffusion length can account for the dynamic surface tension data.