Adsorbed organic molecules

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Fig. 4.56. Schematic diagram of a SERS-active substrate and the measurement arrangement. Alumina nanoparticles are deposited on a glass surface and produce the required roughness. A thin silver layer is evaporated on to the nanoparticles and serves for the enhancement. Organic molecules adsorbed on the silver surface can be detected by irradiation with a laser and collecting the Raman scattered light.

Reviewed here are surface electrochemical studies of organic molecules adsorbed at well-defined Pt(lll) electrode surfaces from aqueous solution. Emphasis is placed upon studies of nicotinic acid (NA), pyridine (PYR), and nine related pyridine carboxylic acids. 

Subtractively normalized interfacial Fourier transform infrared spectroscopy has been used to follow the reorientations of isoquinoline molecules adsorbed at a mercury electrode. Field induced infrared absorption is a major contribution to the intensities of the vibrational band structure of aromatic organic molecules adsorbed on mercury. Adsorbed isoquinoline was observed to go through an abrupt reorientation at potentials more negative than about -0.73 V vs SCE (the actual transition potential being dependent on the bulk solution concentration) to the vertical 6,7 position. 

Under certain conditions phosphorescence can be observed at room temperature from organic molecules adsorbed on solid supports such as filter paper, silica and other chromatographic supports. 

The STM has successfully resolved a number of organic molecules adsorbed on various conducting substrates (Sleator and Tycko, 1988 Ohtani et al., 1988 Foster and Frommer, 1988). A commonly used substrate is graphite, which is easy to prepare, defect free on large areas (several micrometers), and inert. Under favorable conditions, nearly atomic or atomic resolution can be achieved. In many cases, the organic molecules adsorbed on crystalline substrates form regular patterns, which are of scientific interest in and of themselves. 

In Section 6.93 it was pointed out that it has been found experimentally that the coverage of organic molecules adsorbing on an electrode follows aparabolic path with potential, and that this curve passes through a maximum that lies close to the pzc of the electrode. How can this behavior be explained ... 

Fig. P6.3. I, Voltammograms (current density vs. potential) and isoconcs (coverage vs. potential) diagrams for several organic molecules adsorbed on platinum. I, Solid line, pure electrolyte (0.01 MHCI broken line, addition of organic species. Sweep rate, 50 mV s"1 298 K. II. Measurements taken with radiotracer (RT) FTIR spectroscopy, and ellipsometric (ELIP) techniques. (Reprinted from J. O M. Bockris and K. T. Jeng, J. Electroanal. Chem. 330 54, copyright 1992, Figs. 5 and 6, with permission of Elsevier Science.)...

Although the basic principles behind this Intact ejection mechanism can be Illustrated with carbon monoxide, the extrapolation to large bloorganlc molecules Is not necessarily obvious. Calculations have been performed for a series of organic molecules adsorbed on a Ni(OOl) surface to understand the mechanisms of molecular ejection (8-12). The first molecules which have more than just a few atoms examined are benzene which it-bonds on a metal surface and pyridine which can either ir-bond or o-bond on a metal surface. Larger structures, whose sizes approach the diameter of bloorganlc molecules, are naphthalene, biphenyl and coronene whose adsorption structures are unknown. All the molecules except pyridine are assumed to ir-bond on the surface. 

Laser induced transient spectra and decay kinetics have been observed from a wide variety of samples including fractions of monolayers of organic molecules adsorbed on catalytic metal oxide surfaces(3, 4), and included within zeolites(5), from... 

Organic molecules adsorbed at the surface of Si02 are excited by light absorption and are enabled to undergo molecular rearrangement 64 ... 

Assuming large clusters may approximate the band structure of the crystal, what energy is required to excite electrons into the Fe(4s) conduction band in Fe2 bearing clays Can such electrons reduce organic molecules adsorbed in the surface or trapped in the interlayer ... 

The experimental studies of interactions of organic molecules adsorbed or intercalated in the interlayer space of clay minerals are very extensive. These experimental investigations were reviewed in several monographs [15-17]. In this part, we will review only experimental studies concerning systems of kaolinitic minerals (specifically dickite and kaolinite) with formamide (FA), N-methylformamide (MFA) and dimethylsulfoxide (DMSO) since theoretical studies of interactions of small organic molecules with clay minerals are devoted to intercalates and adsorbates of kaolinitic minerals with these organic molecules. This will allow to compare theoretical results with available experimental data. 

It is seen that the organic molecule adsorbs on the Fe in the same way with respect to potential as on Pt, there being a shift in the potential of the maximum corresponding to the difference of the pzc for Pt and Fe. Thus, it is correct to consider inhibitor adsorption in pH 4 as occurring on the metal of the pipeline rather than on an oxide or carbonate coating. It is possible that some oxide or carbonate exists on the metal surface in these situations of pH 4 (C02 saturated), but that the corrosion and its inhibition occurs through cracks in the film, which expose bare Fe to the solution. 

Johnston, L.J. (1991) Phototransformations of organic molecules adsorbed on silica and alumina, in Photochemistry in Confined Media (ed. V. Ramamurthy),... 

Metal ions and organic molecules adsorbed in the interlayer space and on the edges of clay minerals can transform to other chemical species via many different processes. The sorbed metal ions can undergo redox and hydrolytic reactions,... 

The light-induced charge transfer between a semiconductor electrode and a semiconductor particle adsorbed on the electrode surface, is also termed sensitization because the same type of effect occurs with organic molecules adsorbed onto a semiconductor electrode. These processes are described in Chapter 10. 

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