Monomer adsorption, metal surface

Michaelides A, Ranea VA, de Andres PL, King DA. 2003b. General model for water monomer adsorption on close-packed transition and noble metal surfaces. Phys Rev Lett 90 216102. 

Figure 5-11 shows a simple model of the compact double layer on metal electrodes. The electrode interface adsorbs water molecules to form the first mono-molecular adsorption layer about 0.2 nm thick next, the second adsorption layer is formed consisting of water molecules and hydrated ions these two layers constitute a compact electric double layer about 0.3 to 0.5 nm thick. Since adsorbed water molecules in the compact layer are partially bound with the electrode interface, the permittivity of the compact layer becomes smaller than that of free water molecules in aqueous solution, being in the range from 5 to 6 compared with 80 of bulk water in the relative scale of dielectric constant. In general, water molecules are adsorbed as monomers on the surface of metals on which the affinity for adsorption of water is great (e.g. d-metals) whereas, water molecules are adsorbed as clusters in addition to monomers on the surface of metals on which the affinity for adsorption of water is relatively small (e.g. sp-metals). 

In the adsorption of water molecules on metal electrodes in aqueous solutions, unpaired electrons in the frontier orbital of oi en atoms in water molecules form covalent bonds with surface metal atoms. Then, the adsorbate water molecules act as a Lewis base (covalent-electron providers) and the adsorbent surface metal atoms act as a Lewis acid (covalent-electron receivers). Since the bond energy (0.4 to 0.7 eV) of water molecules with the surface metal atoms is close to the energy of hydrogen bond (0.2 to 0.4 eV) between water molecules, the adsorbed water molecule is combined not only with the metallic surface atoms but also with the acijacent water molecules to form a bi-molecular layer rather than a monomer layer as shown in Fig. 5-31. 

Further evidence has been forthcoming on metallic sodium as an initiator. This is widely employed to form the tetrameric dianion of a-methylstyrene, which in turn is a highly convenient initiator. Recent work has shown that monomer adsorption onto the metal surface to produce a rotationally mobile species almost certainly plays an important role, and the ubiquitous dianion probably has the structure (16) rather than those previously suggested. 

Further evidence of monomer adsorption on a metal surface is provided by the extensive studies of Richards and his co-workers (14). As is well known, alkyl bromides in tetrahydrofuran vigorously react with alkali metals, say lithium, yielding the Wurtz coupling products. The violent reaction slows down on addition of aromatic monomers like styrene, and the nature of... 

The interaction of An, Py, and Th derivatives with metal or metal oxide surfaces may also lead to chemical reactions that differ from the simple anchoring of the single ring to the electrode surface [76], As an example, the formation of oligomeric and polymeric species from the relevant monomers, as a consequence of the adsorption on the metal surface, has been reported (Fig. 5.16a) [45, 90]. Surface polymerization may involve the reduction of the metal substrate with concomitant oxidation of the monomer and consequent oUgo- or polymerization reaction. The addition of a proper oxidizing species has also been experienced [91]. Unfortunately, the relevant mechanism is stUl unknown. 

This rule of competitive adsorption to the metal was illustrated by electroinitiating the EA polymerization in ACN/DMF mixtures of various compositions. It must be noted that density functional theory (DFT) calculations ranked the affinity of EA ACN, and DMF toward Ni as follows ACN > EA> DMF. Figure 10 shows the progressive increase of the peak I intensity when the relative content of ACN in the solvent mixture is increased, consistent with the increasingly more effective displacement of the monomer from the metal surface by ACN. 

Crispin X, Lazzaroni R, Geskin VM, Baute N, Dubois P, Jerome R, Bredas JL (1999) Controlling the electrografting of polymers onto transition metal surfaces through solvent versus monomer adsorption. J Am Chem Soc 121 176-187... 

Kinetic models referred to as adsorption models have been proposed, especially for olefin polymerisation with highly active supported Ziegler-Natta catalysts, e.g. MgCl2/ethyl benzoate/TiCU AIR3. These models include reversible processes of adsorption of the monomer (olefin coordination at the transition metal) and adsorption of the activator (complexation via briding bonds formation). There are a variety of kinetic models of this type, most of them considering the actual monomer and activator concentrations at the catalyst surface, m and a respectively, described by Langmuir-Hinshelwood isotherms. It is to be emphasised that M and a must not be the same as the respective bulk concentrations [M] and [A] in solution. Therefore, fractions of surface centres complexed by the monomer and the activator, but not bulk concentrations in solution, are assumed to represent the actual monomer and activator concentrations respectively. This means that the polymerisation rate equation based on the simple polymerisation model should take into account the... 

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