Equilibrium and metastable states in lecithin films.

We have considered whether lecithin surface films below the gel-liquid crystal transition temperature, Tc, are in unique physical states. In general, below Tc, equilibrium films do not exist when surface pressures, pi, exceed about 0.1 dyn/cm. Since surface pressure-surface area isotherms of lecithin films below Tc always encompass pi's much greater than 0.1 dyn/cm, the film states are metastable. We show that the film properties under these conditions depend strongly on the history of the film, particularly the method of film formation. Lecithin surface films below Tc are thus in arbitrary metastable states, so that pi-area isotherms are difficult to interpret. The physical significance of such isotherms remains to be determined. The utility of pure lecithin surface layers below Tc as models for biological systems is also challenged by our results.