Development of a pressure-controlled pendant-drop surface balance

Abstract

A fully automated, pressure-controlled pendant-drop surface balance has been developed. It allows non-invasive surfactant adsorption kinetics studies at liquid-fluid interfaces at constant interfacial pressure π, permitting us to control the height of the energy barrier to adsorption, and to select the different regimes and – in the case of proteins – steps of the adsorption process (adsorption subsurface-surface – molecular rearrangement). In our device a surfactant solution droplet is formed at the tip of a capillary by means of a micro-injector. Drop profiles, extracted from digital drop micrographs, are fitted to the equation of capillarity, yielding π, the drop volume V and the interfacial area A. π is varied by changing V (and hence A). Control is based on a modulated fuzzy-logic PID algorithm able to maintain constant π (or A) over a wide range of experimental conditions. Once generated, the drop is kept at constant A, allowing the surfactant to adsorb freely until the desired π is attained. The adsorption kinetics at constant π are then studied by monitoring the variation of A in time, i.e., determining the relative area dilatation necessary at each instant to compensate the pressure increase due to incorporation of new surfactant molecules into the film or – in the case of proteins – conformational rearrangement of the adsorbed molecules. A study of the adsorption kinetics of β-casein and myoglobin at the air-water interface is presented.

Acknowledgements This work has been financially supported by MAT2001-2843-CO2-01.