Equilibrium and Dynamic Interfacial Tension Measurements at Microscopic Interfaces Using a Micropipet Technique. 2. Dynamics of Phospholipid Monolayer Formation and Equilibrium Tensions at the Water−Air Interface

Abstract

Phospholipid monolayers at the water−air interface have been used extensively as models approximating one-half of the biological bilayer membrane and are of particular interest as stabilizers of microbubbles and emulsions. Interfacial tension is an important measure of adsorption and monolayer formation, but for macroscopic interfaces, equilibrium conditions have been difficult if not impossible to reach, especially over the wide range of temperatures necessary to study homologous series of lipids such as the phosphatidylcholines (C12−C18). By use of a new micropipet technique, however, clean, freshly prepared interfaces on the scale of microns can be rapidly and repeatably produced and exposed to monolayer-forming materials (such as phospholipids and surfactants), the equilibrium condition can be quickly achieved in minutes, and changes in the surface tension by introducing new solutions to the interface (and hence, adsorption dynamics) can be accurately measured and tracked. We have used this technique to study the formation of monolayers of various insoluble surfactant systems (pure and mixed phospholipids including cholesterol and charged lipids) at the water−air interface by measuring equilibrium and dynamic surface tensions. We show that liquid-phase lipid systems spread rapidly (50 mN/m min-1) from vesicle suspensions to form monolayers and reach the same equilibrium surface tension of 25 mN/m. The incorporation of cholesterol to create a liquid ordered phase that dramatically changes the molecular packing, elastic modulus, and tensile strength of bilayers has no effect on the rate or equilibrium surface tension of the monolayers. From the limiting surface tension of 25 mN/m for all liquid lipid monolayers (including cholesterol rich), it appears that the liquid state of the lipid acyl chains is the major influence in determining a monolayer surface tension that is similar to the bulk liquid hydrocarbon/gas surface tension. For single-component lipids in their gel phase, the equilibrium surface tension and rate of monolayer formation decreases as the temperature is lowered below Tm, consistent with a reduced spreading pressure for 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.