Equilibrium and dynamic interfacial tension measurements at microscopic interfaces using a micropipet technique. 2. Dynamics of phospholipid monolayer formation and equilibrium tensions at water-air interface

Phospholipid monolayers at the water-air interface have been used extensive ly as models approximating one-half of the biological bilayer membrane and are of particular interest as stabilizers of microbubbles and emulsions. In terfacial tension is an important measure of adsorption and monolayer forma tion, but for macroscopic interfaces, equilibrium conditions have been diff icult if not impossible to reach, especially over the wide range of tempera tures necessary to study homologous series of lipids such as the phosphatid ylcholines (C12-C18). By use of a new micropipet technique, however, clean, freshly prepared interfaces on the scale of microns can be rapidly and rep eatably produced and exposed to monolayer-forming materials (such as phosph olipids and surfactants), the equilibrium condition can be quickly achieved in minutes, and changes in the surface tension by introducing new solution s to the interface (and hence, adsorption dynamics) can be accurately measu red and tracked. We have used this technique to study the formation of mono layers of various insoluble surfactant systems (pure and mixed phospholipid s including cholesterol and charged lipids) at the water-air interface by m easuring equilibrium and dynamic surface tensions. We show that liquid-phas e lipid systems spread rapidly (50 mN/m min(-1)) from vesicle suspensions t o form monolayers and reach the same equilibrium surface tension of 25 mN/m . The incorporation of cholesterol to create a liquid ordered phase that dr amatically changes the molecular packing, elastic modulus, and tensile stre ngth of bilayers has no effect on the rate or equilibrium surface tension o f the monolayers. From the limiting surface tension of 25 mN/m for all liqu id lipid monolayers (including cholesterol rich), it appears that the liqui d state of the lipid acyl chains is the major influence in determining a mo nolayer surface tension that is similar to the bulk liquid hydrocarbon/gas surface tension. For single-component lipids in their gel phase, the equili brium surface tension and rate of monolayer formation decreases as the temp erature is lowered below Tm, consistent with a reduced spreading pressure f or gel-phase lipids, Finally, unscreened interfaces in deionized water were not readily coated by DOPC/DOPG lipids, while in salt solution, monolayers were readily formed, showing that even in the presence of large amounts of lipid as vesicles in suspension, the water-air interface can remain clean when electrostatically stabilized.