On the impedance of a lipid-modified Hg|electrolyte interface

The impedance of a lipid-modified mercury I electrolyte interface was investigated, concentrating on the adsorption and desorption of a dioleoyl phosphatidylcholine (DOPC) monolayer adsorbed from the gas I solution (G I S) interface as well as that formed from liposome adsorption. The impedance measurements showed two remarkable effects. First, when compared to the electrolyte in the absence of lipid, the high frequency resistance increased when DOPC was adsorbed as well as when it was desorbed. Second, the double layer capacitance exhibited a marked frequency dependence for higher frequencies. Both effects can be explained on the basis of a new model, which is able to fit the frequency dependence of the impedance accurately. At the adsorption potential, the increase of the high frequency resistance can be related to the formation of a coherent condensed film of the lipid, which strongly decreases the mobility of electrolyte ions near the interface. At desorption potentials, the lipid film, held by adhesive forces near to the electrode surface, remains stable and shows the same decrease in electrolyte ion mobility. The frequency dependence of the double layer capacitance at desorption potentials can be explained through a decrease in the exchange rate of the adsorbed water molecules due to the presence of the desorbed lipid film. In the equivalent circuit representation, the double layer capacitance has then to be replaced by a high frequency capacitance in parallel with a series connection of an adsorption resistance and an adsorption capacitance. The product of adsorption capacitance and adsorption resistance gives the adsorption relaxation time. The adsorption relaxation time of water was estimated to be 10(-4) s for the interfacial region modified by the desorbed lipid. The influence of Ca2+ ions added to the electrolyte on the high frequency resistance and capacitance is described. The implications of these results on the classical picture of organic adsorption/desorption introduced by Frumkin are also discussed. (C) 2003 Elsevier B.V. All rights reserved.