Electric-Field-Induced Transitions of Amphiphilic Layers on Mercury Electrodes

There are numerous examples in the literature of amphiphilic molecules which, when adsorbed onto mercury electrodes, undergo electric-field-induced transitions between different molecular conformations. In general, very sharp and reversible voltammetric features associated with these transitions are observed when the electrode potential is scanned in the negative direction, typically over the range of -0.30 to 1.50 V vs SCE, although no redox center is active in these molecular assemblies within this potential range. Using simple electrostatic and thermodynamic arguments, an analytical expression is derived that allows the voltammetric response to be computed in terms of possible molecular conformational changes of the monolayer. The magnitude, shape, and potential of the voltammetric wave are dependent upon molecular parameters (e.g., charge distribution, dimensions, and dielectric properties of the amphiphile), surface coverage, and nonelectrostatic energy contributions. A peak-shaped voltammetric response is shown to be consistent with the redistribution of charged sites within the amphiphilic layer in response to the surface electric field. Numerical results are in qualitative agreement with voltammetric data for dioleoylphosphatidylcholine (DOPC) adsorbed onto mercury electrodes.