The transfer coefficient α is a quantity that is commonly employed in the kinetic investigation of electrode processes. In the 3rd edition of the IUPAC Green Book, the cathodic transfer coefficient αc is defined as –(RT/nF)(dlnkc/dE), where kc is the electroreduction rate constant, E is the applied potential, and R, T, and F have their usual significance. This definition is equivalent to the other, -(RT/nF)(dln|jc|/dE), where jc is the cathodic current density corrected for any changes in the reactant concentration at the electrode surface with respect to its bulk value. The anodic transfer coefficient αa is defined similarly, by simply replacing jc with the anodic current density ja and the minus sign with the plus sign. It is shown that this definition applies only to an electrode reaction that consists of a single elementary step involving the simultaneous uptake of n electrons from the electrode in the case of αc, or their release to the electrode in the case of αa. However, an elementary step involving the simultaneous release or uptake of more than one electron is regarded as highly improbable in view of the absolute rate theory of electron transfer of Marcus; the hardly satisfiable requirements for the occurrence of such an event are examined. Moreover, the majority of electrode reactions do not consist of a single elementary step; rather, they are multistep, multi-electron processes. The uncritical application of the above definitions of αc and αa has led researchers to provide unwarranted mechanistic interpretations of electrode reactions. In fact, the only directly measurable experimental quantity is dln|j|/dE, which can be made dimensionless upon multiplication by RT/F, yielding (RT/F)(dln|j|/dE). One common source of misinterpretation consists in setting this experimental quantity equal to αn, according to the above definition of the transfer coefficient, and in trying to estimate n from αn, upon ascribing an arbitrary value to α, often close to 0.5. The resulting n value is then identified with the number of electrons involved in a hypothetical rate-determining step or with that involved in the overall electrode reaction. A few examples of these unwarranted mechanistic interpretations are reported. In view of the above considerations, it is proposed to define the cathodic and anodic transfer coefficients by the quantities αc = –(RT/F)(dln|jc|/dE) and αa = (RT/F)(dlnja/dE), which are independent of any mechanistic consideration.

Combined Langmuir-Blodgett vertical withdrawing and Langmuir-Schaefer horizontal touch (LB-LS) methods were employed to transfer DMPC bilayers onto a Au(111) electrode surface. Charge density measurements and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate electric field induced changes in the structure of the bilayer. The results show that the physical state and the molecular arrangement found in the monolayer at the air-water interface is to a large extent preserved in the bilayer formed by the LB-LS method. This approach provides an opportunity to produce supported bilayers with a well-designed architecture. The properties of the bilayer formed by the LB-LS method were compared to the properties of the bilayer produced by spontaneous fusion of unilamellar vesicles investigated in an earlier study (Bin, X.; Zawisza, I.; Lipkowski, J. Langmuir 2005, 21, 330-347). The tilt angles of the acyl chains are much smaller in the bilayer formed by the LB-LS method and are closer to the angles observed for vesicles and stacked hydrated bilayers. The tilt angles of the phosphate and choline groups are also smaller and are characteristic of an orientation in which the area per DMPC molecule is small. The electric field induced changes of these angles are also less pronounced in the bilayer formed by the LB-LS method. We have shown that these differences are a result of the higher packing density of the phospholipid molecules in the bilayer formed by the LB-LS method.

Electrochemical measurements, atomic force microscopy, and scanning tunneling microscopy have been combined to present the first direct images of the potential-controlled phase transition between the hemimicellar and condensed states of a dodecyl sulfate (SDS) film at the Au(111) electrode surface. The adsorbed SDS forms stripe-shaped hemimicellar aggregates at small or moderate charge densities at the electrode. High-resolution STM images of these aggregates revealed that adsorbed SDS molecules are ordered and form a long-range two-dimensional lattice. A unit cell of this lattice consists of two vectors that are 4.4 and 0.5 nm long and are oriented at an angle of 70°. We propose that each unit cell contains two flat-laying SDS molecules stretched out along the longer axis of the cell with the hydrocarbon tails directed toward the interior of the cell. The remaining SDS molecules in the hemimicelle assume a tilted orientation. This long-range structure is stabilized by the interactions of sulfate groups belonging to the adjacent cells. The sulfate groups of the flat-laying SDS molecules are arranged into a characteristic (√3 × √7) structure in which the sulfate groups along the √7 direction are bridged by hydrogen-bonded water molecules. When the positive charge on the metal either becomes equal to or exceeds the charge of adsorbed surfactant, the surface aggregates melt to form a condensed film. The transition between the hemimicellar and condensed states of the film is reversible. The hemimicellar aggregates may be re-formed by decreasing the charge density at the electrode surface. The charging and discharging of the gold electrode can be easily controlled by a proper variation of the electrode potential.

A mixed bilayer of cholesterol and dimyristoylphosphatidylcholine has been formed on a gold-coated block of quartz by fusion of small unilamellar vesicles. The formation of this bilayer lipid membrane on a conductive surface allowed us to study the influence of the support's surface charge on the structure and hydration of the bilayer lipid membrane. We have employed electrochemical measurements and the specular reflection of neutrons to measure the thickness and water content in the bilayer lipid membrane as a function of the charge on the support's surface. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative than -8 micro C cm(-2), the bilayer starts to detach from the metal surface. However, it remains in a close proximity to the metal electrode, being suspended on a thin cushion of the electrolyte. The field-driven transformations of the bilayer lead to significant changes in the film thicknesses. At charge densities more negative than -20 micro C cm(-2), the bilayer is approximately 37 A thick and this number is comparable to the thickness determined for hydrated multilayers of dimyristoylphosphatidylcholine from x-ray diffraction experiments. The thickness of the bilayer decreases at smaller charge densities to become equal to approximately 26 A at zero charge. This result indicates that the tilt of the acyl chains with respect to the bilayer normal changes from approximately 35 degrees to 59 degrees by moving from high negative charges (and potentials) to zero charge on the metal.

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The adsorption of hydroxide ions at a Au(111) single-crystal electrode has been investigated quantitatively using chronocoulometry and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). By thermodynamic analysis of the charge density data, the Gibbs excess, Gibbs energy of adsorption, and number of electrons flowing to the interface per one adsorbed hydroxide ion at a constant electrode potential (electrosorption valency) were determined. The electrosorption data indicate that the adsorption of OH -has a three-state character. The adsorbed OH -forms quite a polar surface bond at a negatively charged surface, while the polarity of the surface bond is significantly decreased at positive charge densities. Oxide formation begins at higher charge densities. Infrared spectroscopy shows that oxide formation takes place when the surface concentration of hydroxide ions exceeds one-third of a monolayer. The integrated infrared intensity of the O-H stretching band correlates very well with the Gibbs excess of hydroxide determined by chronocoulometry.

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