Kinetic studies of spreading DMPC vesicles at the air-solution interface using film pressure measurements

DMPC vesicles were injected into a 50 mM NaF solution in water, and the kinetics of a monolayer formation at the air-solution interface was investigated by measuring changes of the film pressure, pi, as a function of time. The studies were carried out in temperature range from 5 to 35 degrees C. The solutions were stirred in order to eliminate the mass transport limitations. Under these conditions, the monolayer formation was controlled by the surface processes only. At temperatures above the critical temperature Tc, compression isotherms were measured and used to convert the pi-t curves into the gamma-t plots, with gamma being the surface concentration of DMPC. These kinetic data indicate that the monolayer formation involves fast rupture of vesicles and formation of a constant number of disk-shaped monolayer islands at the air-solution interface that grow with a constant radial rate. At higher coverages, the growth is restricted by the availability of the monolayer-free surface area. At temperatures below Tc, the pi-t curves cannot be converted into the gamma-t plots. Here the kinetics can be discussed only qualitatively. The data indicate that the kinetics of the monolayer formation involves two steps. Initially, an expanded film is formed. At higher film pressures, the expanded film is slowly transformed into a liquid condensed state.