Equilibrium and dynamic interfacial tension measurements at microscopic interfaces using a micropipet technique. 1. A new method for determination of interfacial tension

A new micropipet technique has been developed to measure the equilibrium and dynamic interfacial tensions of microscopic liquid-gas and liquid-liquid interfaces. In this technique, a liquid-gas or liquid-liquid interface with a radius of curvature ranging from 1 to 100 mum can be created inside a tapered micropipet. On the basis of the Laplace equation (a work balance between tension and applied pressure for the curved interface), the equilibrium interfacial tension between the two phases (clean or surfactant adsorbed) can be determined by measuring the radius of curvature of the interface for a series of pressure changes. With an additional surfactant-delivering micropipet, we show how this technique also offers an effective way to study adsorption/desorption dynamics upon exposure and washout for various surfactants and provides the whole history of surfactant exchange for microscopic interfaces. Here, we verify that the results of this technique are consistent with the interfacial tension values previously obtained by other methods typically conducted on macroscopic interfaces. The technique has been used to study the adsorption of PEG-40-Stearate as a monolayer at the liquid-gas interface. From a plot of the measured surface tension as a function of PEG-40-Stearate concentration, the critical micelle concentration and area per molecule have been determined to be 40 +/- 2 muM and 1.19 nm(2), respectively. In a companion paper, we report additional new data on a series of phospholipid monolayers.