Adsorption of DOPC onto Hg from the G|S interface and from a liposomal suspension

The adsorption of various physical forms of dioleoyl phosphatidylcholine (DOPC) onto a Hg electrode have been studied. The three systems investigated were adsorption of DOPC from the monolayer state present on the gas \ solution (G \ S) interface, the adsorption of 100 nm DOPC liposomes present in the electrolyte, and a mixed system consisting of the co-adsorption of the monolayer and liposomes. The adsorption was characterized by capacitance measurements, chronoamperometry and impedance spectroscopy. The adsorbed layer produced by liposome adsorption was distinctly different as compared to the layer adsorbed from the G \ S interface, whereas the co-adsorbed layer retained most of the liposome layer qualities. A low capacitance region, centered at - 0.40 V (SCE) with values of 1.85, and 1.45 muF cm(-2) for the G \ S and the liposome system were, respectively observed. This region was stable with respect to very negative potential cycling only for the liposome systems. Three phase transitions (denoted by pseudo-capacitance peaks) were noted for the three systems under study; two corresponding to a change in the adsorbed state, and one corresponding to the adsorption-desorption process. The kinetics of these changes were quite different and depended on the nature of the adsorbing species. The most significant differences were observed when measuring the capacity over a large range of perturbation frequency. At low frequencies, desorption of the DOPC layer was observed at very negative potentials for all the systems studied. As the perturbation frequency was increased, the capacitance measured for the G \ S system maintained this characteristic, but surprisingly this was not the case for the liposome systems. In both cases (liposome and co-adsorbed) the capacity, calculated assuming a series RC circuit, was found to be significantly lower than the capacity measured for the Hg \ 0.1 M KCl interface. Preliminary impedance spectra are presented illustrating the non-ideal behavior of the liposomal system. This phenomenon is undergoing further study. A possible mechanism for the adsorption of DOPC from the liposomal state is also presented. (C) 2001 Elsevier Science B.V. All rights reserved.