Amphoteric materials

Oxides and hydroxides of Al, Fe, Mn, and Si may exist in the subsurface mainly as a mixture (known also as a solid solution) rather than as pure mineral phases. They are considered amphoteric materials, characterized by no permanent surface charge. Their cation and anion exchange capacities reflect adsorption of potentialdetermining ions such as H+ and OH". Different surfaces have a diverse affinity for H+ and OH" ions and thus exhibit various points of zero charge (PZC). Details of various models for variable charge minerals may be found in the extensive review of McBride (1989). 

In spite of the fact that theoretically the approach of Drago is more sound, the use of the AN and DN numbers of Guttman is more accepted. These numbers can also characterize amphoteric materials and the available data base is much larger for this theory. Schreiber [30], for example, lists these parameters for a large number of polymers and other components used in polymer systems. 

A glance at the pKa values in Table 3.1 reveals that many classes of compounds can act either as acids or as bases, depending on the reaction environment. Such materials are termed amphoteric. They must have an acidic proton (i.e., a proton attached to an electronegative element or group) and unshared pairs of electrons that can be donated to a proton. For example, water, alcohols, and other hydroxylic compounds as well as amines and amides are all amphoteric materials. Comparing the p.Ka s of these materials permits an assessment of the predominant behavior in a given environment. For example, if an amine is dissolved in water, it could function as an acid or a base. To determine which behavior will predominate, the position of the equilibrium can be determined for each process. Comparison of these values will indicate which will be the principal behavior. Thus, as an acid, the amine would donate a proton to water to give an amide anion and the hydronium ion. 

Zinc oxide, as an amphoteric material, reacts with acids to form zinc salts and with strong alkali to form zincates. In the vulcanization of rubber, the chemical role of zinc oxide is complex and the free oxide is required, probably as an activator. 

These TLC plates have cation- or anion-exchange resins bonded to their surfaces. The resins are materials such as styrene-divinylbenzene copolymers having either quaternary ammonium or sulphonic acid groupings for ion exchange. They are particularly usefril for substmices of high molecular weight and for amphoteric materials. Strong acids or alkalis are usually used as mobile phases. 

Gelatin is an amphoteric material and will react with both acids and bases. It is also a protein and thus exhibits chemical properties characteristic of such materials for example, gelatin may be hydrolyzed by most proteolytic systems to yield its amino acid components. 

Concrete is an alkaline environment where iron passivates and the amphoteric materials, aluminum and zinc, react with fresh concrete by evolving hydrogen. In the case of zinc, the reaction can be quenched by addition of chromates to the canent or prepassivating the zinc used as a protective layer for reinforcing iron bars in concrete. Lead in concrete, in conditions of high humidity, corrodes, and the concrete prevents the formation of the protective layers of basic lead carbonate, which would be formed in its absence. 

Cationic surfactants represent one of the smaller classes of surfactants, with a consumption estimated to be 700000 tons per year. Typically, reviews and market studies include in this class of materials all amine-based surfactants, whether they be charged or uncharged. In this present chapter we will use the same definition, but exclude amphoteric materials, which will be covered in the next chapter in this volume. 

Acidity in solid materials, surface acidity, condensation Acidity-basicity effects must always be taken into account to explain the properties of oxygenated material. Thus, in strong acids, proton transfer to water is complete but this is not so for weaker acids and amphoteric materials. 

Now let us examine again the band structures of solids in Fig. 3. In Fig. 3a, the structure of metals with zero or low Eg signifies that all metals are "soft." However, they do not appear to be as "soft" as the q values calculated from the A-I data (Table 2). Indeed, metals have been classified as amphoteric materials. Most of the metals are "soft" acids, and some of them "soft" bases. When two metals are brought together into close contact, one of them assumes to be an acid while the other a base. In a recent paper by Cain et al. the interaction at the Cu/Cr interface has been treated as an acid-base Interaction. In this case, a soft base (Cu) and a soft acid (Cr) reacts preferentially. In fact, Cr (q = 3.1 eV) is very slightly softer than Cu (q= 3.3 eV) (See Table 2). 

One advantage of using these membranes over AAO is that they can be removed by organic solvents such as methylene chloride, dichloromethane, or chloroform that can provide protection for amphoteric materials or metals that can, respectively, dissolve or react in the high or low pH solvent that is needed for AAO removal. 

An acid (Lewis acid) is an electron acceptor while a base (Lewis base) is an electron donor. The degree of acidity or basity is dependent on the materials in contact. An acidic surface will be wetted by a basic fluid while a basic surface will be wetted by an acidic fluid. A basic fluid will not wet or adhere well to a basic surface and vice versa. An amphoteric material is one that can act as either an acid or a base in a chemical reaction, depending on the nature of the other material. An example of an amphoteric material is aluminum. 

Amphoteric material A material that can either gain or lose an electron (i.e. act as either an acid or a base) in a chemical reaction. Example Aluminum can form AI2CU or AI2O3. 

Intermetallic compound A chemical compound composed of two metals, one of which is an amphoteric material. Example AI2CU where aluminum is the amphoteric material. See also Amphoteric material. 

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