Terminal group adsorption

Self-assembled monolayers (SAMs) provide a versatile system for control of the interfacial properties of inorganic compounds. SAMs result from the spontaneous adsorption and assembly of molecular compounds into crystalline structures. Commonly used molecules consist of a head group with a specific affinity for a particular material, and a terminal group containing a chemical function that becomes exposed... 

The presence of many different C- and N-terminal groups in low concentration in wool protein preparations may be due to the presence of many protein components, the rupture of different peptide bonds in a small number of different protein molecular species, the adsorption of amino acids or peptides on the proteins, or a combination of these possibilities. 

It is not necessary that these three types of influence be exerted through point defects in the intrinsic crystal lattice of the catalyst. Species adsorbed on the surface or more permanently attached as terminating groups of the lattice may in their original state or after radiation-decomposition provide suitable electronic or geometrical conditions for adsorption or catalysis (E). Irradiation may then destroy or create catalytic sites in the adsorbed layer, and these sites have to be considered in fixing the responsibility for an observed radiation effect. At least conceptually, they can operate in any of the three ways just described and, in addition, can block active surface sites on the intrinsic lattice. 

Table 3 lists some ionization properties of functionalized gold-thiol monolayers and relevant alkylsiloxane monolayers together with the appropriate bulk values. Monolayers with carboxylate terminal groups show abnormal wetting behaviour, which makes it difficult to determine accurately their surface pKa values308. Apart from contact angle titration, other methods were also used to study proton transfer equilibria at the mono-layer surfaces, such as quartz crystal microbalance (Table 3, line 1), measurements of the adhesion force between the monolayer deposited at the surface of an AFM tip and the same monolayer deposited on the substrate (chemical force microscopy, Table 3, lines 3, 4, 15), FT-IR spectroscopy (Table 3, line 7), adsorption of polyelectrolytes (Table 3, line 5) and differential capacitance measurements (Table 3, lines 12, 13). 

Recently, Tops e et al. have reconsidered the same system in a quite detailed study. Ammonia and pyridine adsorption have, in particular, been studied. The most active acidic site, characterized by the band at 3600 cm , are thought to be at the channel intersection. Weaker acidic sites, characterized by the band at 3720 cm" are, in contrast with previous assignments, related to terminal groups on the external surface of zeolite and possibly to non-zeolitic impurities. 

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