Axisymmetric drop shape analysis as penetration film balance applied at liquid–liquid interfaces

A new application of the axisymmetric drop shape analysis (ADSA) has been developed, using the pendant-drop film balance technology for penetration studies of insoluble monolayers at the liquid–liquid interface. A monolayer is spread on the surface of a drop of a buffer solution suspended from a capillary, which is the outer one of an arrangement of two coaxial capillaries connected to the two different branches of a microinjector. Next the monolayer-coated buffer drop is immersed into the oil phase. The interfacial area is varied changing the drop volume with the microinjector, and the determination of surface tension as a function of the interfacial area or time is performed using ADSA. Once the monolayer is brought to the desired compression state, the aqueous phase is exchanged quantitatively by through-flow through the coaxial capillaries, permitting to study reactions with some surfactant added to the subphase. The complete set-up, i.e. the image capturing and microinjector system is fully computer controlled by a user-friendly, Windows-integrated program, including the ADSA surface tension calculus algorithm. Compared with ‘conventional’ oil–water interface film balances, the new device presents several advantages such as a more stringent control of the environmental conditions and therefore more uniform temperature, pressure and concentration along the interface, small amounts of material needed and a 20-times greater interface/volume ratio than in conventional Langmuir troughs. © 1999 Elsevier Science B.V. All rights reserved.