Probing adsorption reactions at the liquid | liquid interface by area-step experiments

The results of novel area step experiments at the interface between two immiscible electrolyte solutions (ITIES) in the presence of a supporting electrolyte only and with distearoylphosphatidylcholine (DSPC) in the bulk organic phase are presented. The interface was formed by extrusion of an aqueous drop into 1,2-dichloroethane from the tip of a micropipette. An apparatus is described which allows the dimensions of the interface to be stepped with a rise time of milliseconds, and to be controlled within 5% of a mean area for many hours. In the absence of DSPC, the relaxation of the interfacial electric current after steps in the interfacial area was shown to occur in two distinct stages, typified by an initial rapid current decay on the 50 ms time scale, followed by a much slower decay over approximately 5 s. The first part was due to the relaxation of the charge stored in the interfacial capacitance with its integrated charge going through zero at the potential of zero charge. The theory is given. In addition there was a current due to ion transfer across the interface, which was distinguishable from the charging current, and relaxation of which gave the second part of the transient. The interfacial adsorption of DSPC from the bulk organic phase and the formation of elastic lipid layers were observed visually over a period of hours and via the creation of new current transients following area modulation. The presence of DSPC increased the rate of interfacial ion transfer at all potentials studied and the effect was shown to depend on surface concentration. The presence of DSPC altered the potential of zero charge negatively as shown by a shift in the potential at which the polarity of the initial transient inverted. (C) 2000 Elsevier Science S.A. All rights reserved.