Amphoteric Character of Carbons

It is not our intention to present an exhaustive review of this important subject. Up-to-date reviews are provided elsewhere [38,37]. The classic review by Garten and Weiss [41 ] offers an excellent historical perspective. We do need to summarize here the issues that are essential for understanding the aqueous-phase adsorption phenomena. The main features of carbon surface chemistry are presented first and the con.sequent acid/base behavior of carbons is briefly discus.sed to illustrate their amphoteric character. In Section III it is shown that these phenomena often govern the adsorption of most inorganic compounds. In Section IV we argue that these phenomena can be dominant in the adsorption of organic compounds as well, but they are more often only a part of the whole story. 

The essential features of the surface chemistry of carbons, relevant to their behavior in aqueous solution, are summarized in Figs. 2 and 3. These are discussed in more detail elsewhere [37[, and the reader will appreciate from such discussions that the features presented here are perhaps gross (over)simplifica-tions we shall argue here that they are also very convenient and useful simplifi- 

2 Macroscopic representation of the features of carbon surface chemistry that are thought to be sufficient for understanding aqueous-phase adsorption phenomena. 

4 Electrophoretic mobility vs. pH for a range of amphoteric solids, including alumina [777], a furnace carbon black [56] and chemically modilied carbon blacks and activated carbons [82], 

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Among nanomateiials, carbon surfaces represent very attractive materials for electrochemical studies, such as biosensor applications, due to their different allotropes (graphite, diamond and fullerenes/nanotubes). Carbon electrodes are well polarizable. However, their electrical conductivity strongly depends on the thermal treatment, microtexture, hybridization and content of heteroatoms. Additionally, the amphoteric character of carbon allows use of the rich electrochemical propierties of this element from donor to acceptor state. Application of recently developed carbon materials include. 

The point of interest is the "amphoteric" character of the allyl anion in this complex. On the one hand it may react as an anion, but on the other hand it is susceptible to nucleophilic attack by, for example, carbon centred anions. This has found widespread use in organic synthesis. The reaction with the anion releases a palladium zero complex and in this manner palladium can be employed as a catalyst. 

The historically popular concept of hydrolytic adsorption, which in fact obscures the key role of carbon surface chemistry, has often been used to account for the pH effects. Thus, for example, Rosene and Manes [622] used a Polanyi-based model of competitive adsorption between benzoic acid and sodium benzoate. They criticize the approach taken by Ward and Getzen [677], especially with regard to anion interaction with a positively charged surface, and essentially ignore the amphoteric character of the activated carbon. 

Solutions of any desired pH may be obtained simply by mixing a weak acid or base with one of its salts in various proportions. It is evident from equation (48) that even small amounts of strong acids and bases have only a slight effect on the pH of such mixtures. Certainly the small quantities of alkali from glass and carbon dioxide from the atmosphere can exert no perceptible influence. Such mixtures which are resistant to a change in reaction were called Buffer Mixtures by S. P. L. SOkensen. L. Michaelis coined the term Regulators. They may also be referred to as Ampholytes because of the amphoteric character of such mixtures. AU mixtures of weak adds and their salts,... 

Leon and Radovic (1994) and Wunder et al. (1993) carried out extensive work to understand the amphoteric character of the carbon surface and to use such surfaces to optimize the preparation of Mo/C and Pd/C catalysts. In their study of Pd/C catalysts (Leon y Leon et al., 1992) they concluded that maximum catalyst dispersion is favored when the entire carbon surface is chemically accessible that is when there is electrostatic attraction between the positively charged surface (below pHjEp) and the catalyst precursor anions or between the negatively charged surface (above pH, p) and the catalyst precursor cations . 

The elements show increasing metallic character down the group (Table 14.6). Carbon has definite nonmetallic properties it forms covalent compounds with nonmetals and ionic compounds with metals. The oxides of carbon and silicon are acidic. Germanium is a typical metalloid in that it exhibits metallic or nonmetallic properties according to the other element present in the compound. Tin and, even more so, lead have definite metallic properties. However, even though tin is classified as a metal, it is not far from the metalloids in the periodic table, and it does have some amphoteric properties. For example, tin reacts with both hot concentrated hydrochloric acid and hot alkali ... 

Whereas KF can be considered as a weak base, when impregnated on alumina, it becomes very strong and is able to ionize extremely weak carbon acids (up to pKa 30) [27]. This enhancement in basicity results from ionic dissociation K+F K+-i-F on the surface of alumina due to its amphoteric character (Scheme 6). 

In an attempt to get a better knowledge of tire surface chemistry of carbons, a combination of theories and data obtained from different techniques should be considered, so as to get an accurate or at least coherent picture of the carbon surface. A brief review of the most common available techniques for the characterization of the carbon surface is gathered in the following sections. In this section, the emphasis is placed on the different surface functionalities that can be found on carbons and their effect on the amphoteric character (acidic or basic nature) of the carbons. 

Insertion of sulfur dioxide (SO2) into the metal-carbon bond of transition metal alkyl and aryl complexes has also been studied extensively. SO2 shows several binding modes to transition metals as shown in Scheme 7.15 because it is amphoteric, behaving as a Lewis acid and a Lewis base. The Lewis base character of SO2 provides the structural types r/ -planar (3) or (S,0) (4) where SO2 donates a pair of electrons to the metal accompanied by rr back-bonding from filled d orbitals of the metal atom. The Lewis acid behavior of SO2 as a ligand affords an 17 -pyramidal bonding mode (5) where SO2 accepts a pair of electrons from the metal. As ligands tike olefins or carbon dioxide generally tend to prefer... 

Amphoteric surfactants have a special application profile which favours their use mainly in cosmetics. In recent years, they have also found increasing application in the development of dishwashing agents or household cleaners. Compared to the amounts produced world-wide of anionic and nonionic surfactants, the volume of amphoterics is still relatively small. For cosmetic products, their consumption in Europe in 1992 was about 15 000 t. The main carbon chain includes the Cg-Cig range. The zwitterionic character of amphoterics strongly influences their particular behaviour, i.e. both anionic and cationic... 

Carbon surfaces can display both acidic and basic properties. This amphoteric character influences the behavior of carbonaceous materials to a great extent in most of their applications. This fact accounts for the impressive literature devoted to the analytical methods of characterization of acidic and basic surface groups developed for many years, especially in the last decade. The important recent studies will be reviewed here, which will emphasize the current trend of using a battery of complementary methods rather than only one of them. This is justified on the one hand by the complexity of carbonaceous raw materials and their products (obtained by thermal or chemical processes such as oxidation or functionalization) and on the other hand by the great variety of applications. 

The acid-base character of the oxides progresses from the acidic carbon and silicon dioxides to the amphoteric germanium and tin oxides to the more basic but still amphoteric lead oxide. Silicon and carbon dioxides are exceedingly different compounds, again due to the inability of silicon to form pir-pTT bonds. The oxides also demonstrate the trend toward the stability of the +2 oxidation state in the heavier congeners. 

Germanium, tin, and lead all form compounds representing oxidation state j-4, which is shown also by carbon and silicon. They form a second series of compounds representing oxidation state -f2, which is the most important oxidation state for lead, and is less important for germanium than the higher oxidation state. The hydroxides of these elements tend to be amphoteric. The acidic character is more pronounced for the quadripositive state than for the bipositive state, and is most pronounced for germanium, decreasing to tie and lead. 

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