Langmuir, Vol.18, No.22, 8318-8323, 2002
Dynamic Interfacial Tension at Electrified Liquid/Liquid Interfaces
The use of a differential pressure transducer provides an accurate and rapid measurement of interfacial tension at an electrified liquid-liquid interface, by application of the Laplace equation to the continuously measured pressure of an aqueous droplet in the organic phase. This new method can measure the dynamics of response of interfacial tension both to changes of interfacial potential difference (e.g., during cyclic voltammetry)and to changes of area. It has been applied to the study of an electrified water/1,2-dichloroethane interface in the absence and presence of distearoylphosphatidylcholine (DSPC), to deduce the potential-dependent kinetics of lipid adsorption and desorption.Two modes of operation were used: variable interfacial potential difference at stationary drops and constant potential at expanding droplets. Because of the adsorption of DSPC, the interfacial tension decreased at negative interfacial potential differences (the aqueous wrt the organic phase). However, at positive potentials, the interfacial tension approached the values at a bare interface. At low scan rates, the interfacial tension superimposed on forward and reverse scans, while at scan rates above 2 mV/s increasing hysteresis was observed. Using a theory based on Langmuirian adsorption isotherm and kinetics, the dynamic interfacial tension at expanding droplets was interpreted to yield values of the potential-dependent adsorption equilibrium constant and the rate constant of the adsorption process as a function of the potential difference between the phases. The adsorption kinetics were found to be fast with a forward rate constant larger than 0.1 cm/s irrespective of the potential difference.