Molecular surface composition

Analysis of Surface Molecular Composition. Information about the molecular composition of the surface or interface may also be of interest. A variety of methods for elucidating the nature of the molecules that exist on a surface or within an interface exist. Techniques based on vibrational spectroscopy of molecules are the most common and include the electron-based method of high resolution electron energy loss spectroscopy (hreels), and the optical methods of ftir and Raman spectroscopy. These tools are tremendously powerful methods of analysis because not only does a molecule possess vibrational modes which are signatures of that molecule, but the energies of molecular vibrations are extremely sensitive to the chemical environment in which a molecule is found. Thus, these methods direcdy provide information about the chemistry of the surface or interface through the vibrations of molecules contained on the surface or within the interface. 

Biocompatibility. The analysis of polymer implants has been employed using FTIR spectroscopy to elucidate the long-term biocompatibility and quality control of biomedical materials. This method of surface analysis allows the determination of the specific molecular composition and structures most appropriate for long-term compatibility in humans. 

It has long been realised that infrared (IR) spectroscopy would be an ideal tool if applied in situ since it can provide information on molecular composition and symmetry, bond lengths and force constants. In addition, it can be used to determine the orientation of adsorbed species by means of the surface selection rule described below. However, IR spectroscopy does not possess the spatial resolution of STM or STS, though it does supply the simplest means of obtaining the spatially averaged molecular information. 

Table I gives a compilation of the molecular composition of SFV grown in BHK-21 cells, based on the revised weight for the viral particle of 41-42 X 10 daltons (Jacrot et al, 1983). If one assumes that each phospholipid-cholesterol pair takes up a surface area of about 90-100 A (Israelachvili and Mitchell, 1975) and each glycolipid about 55 A (Pascher and Sundell, 1977), then about 80% of the surface area in the bilayer is occupied by the lipids, leaving about 20% for the spanning proteins. This is somewhat more than would be expected if 180 spike proteins span the bilayer, each having two transmembrane a helical segments.

Table 23.1 Bulk and Molecular Composition of DOM In the Surface (<100m) and Deep (>1000 m) Ocean. ...

Dendrimers, among other applications, are generating interest as soluble supports thanks to the following intrinsic characteristics (i) the well-defined molecular composition of a dendrimer provides a support with a precisely defined arrangement of the reactive sites, (ii) a high loading of reactive sites is achieved on the dendrimer surface and (iii) nanofiltration techniques are available to separate the dendritic support from products. Dendrimer 143, based on a carbosilane core, possesses 12 ester functionalities on... 

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

The surface chemical composition of InP as a function of thermal cleaning temperature was studied by Cheng, et al. (19), also using AES. They used an arsenic molecular beam and temperature of about 500 C to clean a freshly oxide passivated InP. The surface oxides are replaced by arsenic oxides which then vaporize at these temperatures. An atomically flat and carbon contamination free surface was obtained, as monitored in situ with AES and RHEED OJ). 

---