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.)...

Immediately it was noticed that the electrochemical reactivities of the adsorbed and unadsorbed forms of these compounds are very different, as illustrated by Fig. 26. This separability of adsorbed and unadsorbed reactivity is useful for the measurement of packing densities (T, mol cm 2) from which adsorbate molecular orientation and certain other structural characteristics can be obtained. 

There is, of course, a mass of rather direct evidence on orientation at the liquid-vapor interface, much of which is at least implicit in this chapter and in Chapter IV. The methods of statistical mechanics are applicable to the calculation of surface orientation of assymmetric molecules, usually by introducing an angular dependence to the inter-molecular potential function (see Refs. 67, 68, 77 as examples). Widom has applied a mean-held approximation to a lattice model to predict the tendency of AB molecules to adsorb and orient perpendicular to the interface between phases of AA and BB [78]. In the case of water, a molecular dynamics calculation concluded that the surface dipole density corresponded to a tendency for surface-OH groups to point toward the vapor phase [79]. 

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

In order to describe the second-order nonlinear response from the interface of two centrosynnnetric media, the material system may be divided into tlnee regions the interface and the two bulk media. The interface is defined to be the transitional zone where the material properties—such as the electronic structure or molecular orientation of adsorbates—or the electromagnetic fields differ appreciably from the two bulk media. For most systems, this region occurs over a length scale of only a few Angstroms. With respect to the optical radiation, we can thus treat the nonlinearity of the interface as localized to a sheet of polarization. Fonnally, we can describe this sheet by a nonlinear dipole moment per unit area, -P ", which is related to a second-order bulk polarization by hy P - lx, y,r) = y. Flere z is the surface nonnal direction, and the... 

Heinz T F, Tom H W K and Shen Y R 1983 Determination of molecular orientation of monolayer adsorbates by optical second-harmonic generation Phys. Rev. A 28 1883-5... 

The ordered structure and molecule orientation in the monolayers, as suggested by the Hardy model, have been studied by various means. Electron diffraction techniques, for example, including both reflection and transmission, have been employed to examine the molecular orientation of adsorbed monolayers or surface hlms. The observations from these studies can be summarized as follows [3]. 

In summary, impressive progress has been achieved to understand the role of adsorbed layers in boundary lubrication, but the effect of molecular orientation on tribology performance and the shear strength of adsorbed monolayers are the issues that remain to be clarihed in a long future study of boundary lubrication. 

Analyzing orientational structures of adsorbates, assume that the molecular centers of mass are rigidly fixed by an adsorption potential to form a two-dimensional lattice, molecular orientations being either unrestricted (in the limit of a weak angular dependence of the adsorption potential) or reduced to several symmetric (equivalent) directions in the absence of lateral interactions. In turn, lateral interactions should be substantially anisotropic. 

As seen from Chapter 2, adsorbed molecules often form monolayers with chain orientational structures in which the chains with identically oriented molecules alternate (Fig. 2.4). Consider the Davydov splitting of vibrational spectral lines in such systems. Let molecular orientations be specified by the angles 6> and spherical coordinate system with the z-axis perpendicular to the lattice plane ... 

As a main point of this subsection,186 it is shown that due to sufficiently strong lateral interactions of adsorbed molecules on a two-dimensional triangular lattice, the spectral line for collective vibrations manifests some characteristic peculiar relationships between its dephasing-induced broadening and the resonance width 77 for the low-frequency mode The line width changes as Tj n(l/rf) for surface-normal and as rjm for surface-parallel molecular orientations, and takes nonzero values (independent of 77) for inclined molecules with the inclination angle ranging from 47° to 90°. 

Atomic force microscopy (AFM) can be used to obtain high-resolution imagery of molecular orientation and ordering for materials adsorbed onto substrates. Early AFM studies on gluconamides were hampered by the tendency of the fibers to unravel on substrates forming bilayer sheets.41 These layers showed the head-to-tail packing of a monolayer which is similar to the crystal structure reported for anhydrous gluconamides.38 A procedure to retain the fiber networks on surfaces with the addition of a small fraction of... 

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

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. 

In the same study, the SERS spectrum of adsorbed PhNC shows a v(N=C) peak at 2180cm, which is shifted 55cm" to higher values than that of free PhNC [31]. These results are in good agreement with other studies of PhNC adsorphon on gold [41] and indicate a molecule bonded to one Au atom in an on-top (t) ) position. The authors state, however, that a theorehcal calculation of molecular orientation that considers the adsorption of only one molecule is not entirely appropriate because it does not take into account intermolecular interactions among the adsorbates. 

The absorption modes of (S)-3-phenyl-2-hydroxypropionic acid, (S)-3-phenyl-2-aminopropionic acid, and (S)-alanine adsorbed on a nickel plate or RNi were studied by Suetaka s group (71, 72). From the measurement of infrared (IR) dichroism in the reflection spectrum, the molecular orientation of the modifying reagent was deduced. Figures 19-21 show molecular orientations of (S)-2-hydroxy-3-phenylpropionic acid on a nickel plate and (R)-alanines on RNis modified at 5° and 100°C, respectively. 

With the foregoing ideas in mind, one characteristic of the adsorbed monolayer becomes apparent molecular orientation at surfaces. For a film of RX on water, the picture that emerges is one in which the polar groups are incorporated into the aqueous phase with the hydrocarbon part of the molecule oriented away from the water. Such details as the depth of immersion of the tail and the configuration of the alkyl group are best approached by considering how the properties of the monolayer depend on the physical variables. 

When the model does apply, the experimental value of m permits Asp to be evaluated if a0 is known, or a° to be evaluated if Asp is known. It is often difficult to decide what value of a0 best characterizes the adsorbed molecules at a solid surface. Sometimes, therefore, this method for determining Asp is calibrated by measuring ct° for the adsorbed molecules on a solid of known area, rather than relying on some assumed model for molecular orientation and cross section. 

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