Electrode surface, adsorbate molecular orientation

Adsorbate Molecular Orientation at Electrode Surface. Adsorption of some molecules from solution produces an oriented adsorbed layer. For example, nicotinic acid (NA, or 3-pyridinecarboxylic acid, niacin, or vitamin B3) is attached to a Pt(lll) surface primarily or even exclusively through the N atom with the ring in a (nearly) vertical orientation (12) (Fig. 10.5a). 

Figure 10.5. Adsorbate molecular orientation at the electrode surface d) nicotinic acid (Z ) benzoic acid (c) 2,6-pyridinedicarboxylic acid. (From Ref. 12, with permission from the American Chemical Society.)...

The electrochemistry of redox proteins is characterized by a strong dependence on the nature of the electrode surface. Extensive studies by Hill etal. (cited in Refs. 31-33) show that provided the electrode surface is modified to be compatible with the redox protein, direct electrochemistry can be rapid. Their studies have emphasized the importance of the orientation of the protein at the electrode surface so that the distance over which the electron must transfer is not excessive. This is important because redox sites in these proteins are generally located toward one side of the protein, and the exponential dependence of the electron transmission coefficient/ei on distance means that the rate of electron transfer drops rapidly as distance increases (Fig. 9.11). Most of this work has used low molecular weight modifiers adsorbed at the electrode surface, " although similar effects should be possible at polymer-modified electrode surfaces. 

Surface SHG [4.307] produces frequency-doubled radiation from a single pulsed laser beam. Intensity, polarization dependence, and rotational anisotropy of the SHG provide information about the surface concentration and orientation of adsorbed molecules and on the symmetry of surface structures. SHG has been successfully used for analysis of adsorption kinetics and ordering effects at surfaces and interfaces, reconstruction of solid surfaces and other surface phase transitions, and potential-induced phenomena at electrode surfaces. For example, orientation measurements were used to probe the intermolecular structure at air-methanol, air-water, and alkane-water interfaces and within mono- and multilayer molecular films. Time-resolved investigations have revealed the orientational dynamics at liquid-liquid, liquid-solid, liquid-air, and air-solid interfaces [4.307]. 

The features of the electro-oxidative polymerization can he explained as follows. The molecular weight of the obtained polymer stayed constant during the polymerization, because the polymerization proceeds heterogeneously in the diffusion layer of electrode. The C-0 coupling reaction is predominant, probably because the phenol is adsorbed and oriented on the electrode surface. The polymerization started from the dimer is much suppressed, because the dimer diffuses from the bulk phase into the diffusion layer very slowly. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

A subsequent description by Bockris and associates drew attention to further complexities as shown in Figure 15. The metal surface now is covered by combinations of oriented structured water dipoles, specifically adsorbed anions, followed by secondary water dipoles along with the hydrated cation structures. This model serves to bring attention to the dynamic situation in which changes in potential involve sequential as well as simultaneous responses of molecular and atomic systems at and near an electrode surface. Changes in potential distribution involve interactions extending from atom polarizability, through dipole orientation, to ion movements. The electrical field effects are complex in this ideal polarized electrode model. 

Recent decades have witnessed spectacular developments in in-situ diffraction and spectroscopic methods in electrochemistry. The synchrotron-based X-ray diffraction technique unraveled the structure of the electrode surface and the structure of adsorbed layers with unprecedented precision. In-situ IR spectroscopy became a powerfiil tool to study the orientation and conformation of adsorbed ions and molecules, to identify products and intermediates of electrode processes, and to investigate the kinetics of fast electrode reactions. UV-visible reflectance spectroscopy and epifluorescence measurements have provided a mass of new molecular-level information about thin organic films at electrode surfaces. Finally, new non-hnear spectroscopies such as second harmonics generation, sum frequency generation, and surface-enhanced Raman spectroscopy introduced unique surface specificity to electrochemical studies. 

Furthermore, a modifier may alter the electronic nature of the electrode. By changing the electric field at the surface, a modifier may affect the reactant-substrate interactions. A change in reactant-substrate interactions may be manifested, for instance, in a change in molecular orientation of the reactant molecule adsorbed on the surface. Clear evidence does exist for the influence of surface electronic properties on catalytic reactions. It is apparent that a modifier which acts duough an electronic effect could influence both reaction kinetics and the tendency to poison. 

---