Total and free charge densities on mercury coated with self-assembled phosphatidylcholine and octadecanethiol monolayers and octadecanethiol/phosphatidylcholine bilayers

The thermodynamic 'total' charge density is the charge to be supplied to th e electrode to keep the applied potential constant when the electrode surfa ce is increased by unity, while the extrathermodynamic 'free' charge densit y is the charge actually experienced by the diffuse layer ions. The total c harge density at dioleoylphosphatidylcholine (DOPC) and octadecanethiol (OD T) monolayers and mixed ODT/DOPC bilayers self-assembled on mercury from aq ueous solutions was determined from chronocoulometric single potential step s to a final potential negative enough to cause complete desorption of the film. The effect of different alkali metal ions and of tetramethylammonium on DOPC desorption was examined. The total charge for ODT monolayers and OD T/DOPC bilayers, + 56 +/- 3 muC cm(-2), agrees with the value obtained by i ntegration of the current under the reductive desorption voltammetric peaks , only provided the scan rate is higher than 100 mV s(-1). An approximate m odel of the interface of the ODT-coated electrode, which accounts for parti al charge transfer from sulfur to mercury and for the degree of dissociatio n of the sulfhydryl group upon self-assembly, was employed to estimate the free charge density. (C) 2001 Elsevier Science B.V. All rights reserved.