Carboxylic adds strength

Carboxylic adds. Even in quite dilute solution in non-polar solvents, acids exist essentially as dimeric species, which may be readily explained on the basis of the electronic structure of the carboxyl group. Powerful hydrogen bonding between the molecules, and the strength of these bonds, has been accounted for on the basis of a large contribution of an ionic resonance structure. 

Compounds that contain carboxyl groups are called acids (or carboxylic adds). As illustrated in Figure l.l, a carboxyl group in aqueous solution is partially ionized to the carboxylate anion. The degree of ionization depends on the dissociation constant of the acid and the initial pH of the solution. The strength of these acids varies son ewhat depending on the attached R gfoup. Esters are formed by reac-... 

Thus all successful chlor-alkali membranes currently employ a perfluorocarboxylate polymer to lower the rate of hydroxide ion transport. The sulfonate portion of some of these membranes is present mainly to add strength to the thinner carboxylate barrier layer. Fabric backing is also used in some cases to improve physical strength. 

Donnet and coworkers [89-91] have studied adhesion between epoxy resins and carbon acid. The number of add groups on the fiber surface was used to neutralize strong addic groups such as carbo l and phenol. Sodium ethoxide was used to neutralize weaker addic groups such as hydro l and carbonyl. A direct correlation was found between the number of carboxylic add groups and the interlaminar shear strength of the composite (Figure 13). 

Throughout the chapter it is important to keep in mind that carboxylic acids are, clearly, acidic. Generally, carboxyhc acids are weak acids in that proton donation to water is incomplete and thus carboxylic acids are not completely ionized. So, as pointed out earlier (Chapter 5), the add strength or addity is referred to as the position of equilibrium in the reaction of the acid with a base. Thus, for the generalized acid RCO2H ionizing in water (H2O), Equation 9.75 can be written ... 

Cation exchange Strong (sulfonic add) or weak (carboxylic acid) Positively charged functional groups, such as amines Aqueous, low ionic strength Buffers such as acetate, citrate and phosphate... 

The most important inference is that Chemisorption is a direct response to carboxyl group concentration indicated by the XPS photopeak component at 288.7 eV. It seems likely that weak add functionality is of minor import to applications for surface treatments, while interfacial phenomena such as practical adhesion may be sensitive to small concentrations of very high site energies. Interphase modification in epoxy resins, for example, can occur by direct reaction of epoxide groups with surface carboxyls (17), or by accelerated cure chemistry near the surface (39). Carboxyl groups on carbon surfaces may interact with basic moieties in polymers such as polycarbonate or poly(ethylene)oxide (40=42), or promote interfacial crystallinity that improves impact strength and other aspects of composite performance (43, M)-... 

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