Axisymmetric drop shape analysis as penetration Langmuir balance

A new Langmuir-type pendant-drop penetration film balance has been develope d combining a Langmuir-type pendant-drop film balance with a new rapid-subp hase-exchange technique. In addition to the determination of surface pressu re-molecular area isotherms of insoluble monolayers deposited on the surfac e of a pendant drop, it allows the study of reactions with some surfactant added to the subphase. The monolayer is spread on the surface of a drop sus pended from a capillary, which is the outer one of an arrangement of two co axial capillaries connected to the different branches of a microinjector. O nce the film is brought to the desired state of compression by varying the drop volume with the microinjector, the subphase liquid in the drop can be exchanged quantitatively by means of the coaxial capillaries. This exchange is complete for a through-flow of at least three times the drop volume, an d the monolayers endure it at all tested film pressures. The determination of surface tension as a function of surface area is performed using axisymm etric drop shape analysis (ADSA) The complete set-up, i. e., the image capt uring and microinjector system is fully computer controlled by a user-frien dly and fully Windows integrated program, including the ADSA surface tensio n calculus algorithm. As a penetration film balance, pendant-drop methodolo gies offer a wide range of advantages such as a more stringent control of t he environmental conditions and therefore, more uniform temperature, pressu re and concentration along the interface, small amounts of material needed, and a 20 times greater interface/volume ratio than in conventional Langmui r toughs. (C) 1999 American Institute of Physics. [S0034-6748(99)02605-2].