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

A new micropipet technique has been developed to measure the equilibrium an d 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 ta pered micropipet. On the basis of the Laplace equation (a work balance betw een 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 mic ropipet, we show how this technique also offers an effective way to study a dsorption/desorption dynamics upon exposure and washout for various surfact ants and provides the whole history of surfactant exchange for microscopic interfaces. Here, we verify that the results of this technique are consiste nt 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-ga s interface. From a plot of the measured surface tension as a function of P EG-40-Stearate concentration, the critical micelle concentration and area p er molecule have been determined to be 40 +/- 2 muM and 1.19 nm(2), respect ively. In a companion paper, we report additional new data on a series of p hospholipid monolayers.