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 gene ral, very sharp and reversible voltammetric features associated with t hese 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 assemb lies within this potential range. Using simple electrostatic and therm odynamic arguments, an analytical expression is derived that allows th e voltammetric response to be computed in terms of possible molecular conformational changes of the monolayer. The magnitude, shape, and pot ential of the voltammetric wave are dependent upon molecular parameter s (e.g., charge distribution, dimensions, and dielectric properties of the amphiphile), surface coverage, and nonelectrostatic energy contri butions. 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 q ualitative agreement with voltammetric data for dioleoylphosphatidylch oline (DOPC) adsorbed onto mercury electrodes.