Carboxyl Groups in Cellulose

The main causes for the formation of carbonyl and carboxyl groups in cellulose are isolation and purification procedures besides natural aging. This applies in particular to cellulosic pulps from wood, which has undergone a number of processing steps to be freed from lignin, hemicelluloses, and extractives. 

Only second to carbonyl groups, carboxyls are a very important oxidized function in celluloses (Scheme 7). Whereas hemicelluloses inherently contain a high number of different acid groups, carboxyl groups in cellulose are artifi-... 

Table 2 Classical methods for determination of carboxyl groups in cellulose...

For a recent review on a critical evaluation on classical methods see Fardim et al. [123]. In addition to the wet chemical methods, FTIR spectroscopy and ESCA [124,125] techniques have been applied to quantify carboxyl groups in cellulose and pulp. In case of FTIR, satisfactory results in comparison to wet chemical analysis could only be obtained with carboxyl-rich samples [ 126— 128],... 

It has been realized for many years that excessive acidity is the single most serious cause of degradative effects on paper (I, 2, 5, 6). Although acids may be present from many sources (carboxyl groups in cellulose resulting from oxidation and the presence of acidic hemicellu-loses, bleach residues, some wet-strength resins, etc.), the most important... 

The addition of excess calcium acetate to purified cellulosic material suspended in water, followed by titration of the liberated acetic acid, was suggested by L(idtke. From the 0.1% carboxyl content obtained, a D.P. value of 277 was calculated for the cellulosic material employed. This method has been studied further and comparative data with the earlier conductometric titration procedure have been presented by Heymann and Rabinov, by Kenyon and coworkers, and by Sookne and Harris. The latter workers report that electrodialyzed depectin-ized cotton has a carboxyl content of 0.045% or a D.P, of 616. An iodometric titration procedure for estimating carboxyl groups in cellulosic materials has also been described by Ltidtke. The procedure involves a thiosulfate titration of the iodine liberated from a mixture of potassium iodate and potassium iodide by the carboxyl groups of the cellulosic material. The carboxyl content determined by this method agreed well with that obtained by the calcium acetate method. 

O. Ant-Wuorinen, Determination of Carboxyl Groups in Cellulose Report 96, The State Institute for Technical Research, p. 48, Helsinki, Finland (1951). 

Kauss (39) has shown that the esterification of the carboxyl groups in the D-galacturonic acid chain takes place by a transfer of the methyl groups from S-adenosyl-L-methionine, analogous to the case in which the 4-methyl ether groups are transferred to D-glucuronic acid of hemi-cellulose (40). 

The use of nitrogen dioxide for the selective oxidation of polysaccharides to polyuronic acids was introduced by Kenyon and his coworkers13,63 in 1941. By this means extensive oxidation of the primary alcohol groups in cellulose was obtained, through the mechanism of preferential nitration followed by decomposition of the nitric acid ester with carboxyl forma-tion.68(0< > Apparently some undissociated nitration products also were formed, since infrared absorption studies54 indicated the presence of nitrate radicals in the polyuronic acid. Side reactions produced carboxyl,... 

Additional sources of acid contamination include carboxyl group in the cellulose, acidic carbohydrate gums, bleach and pulping chemical residues, and some constituents of coating colors. Acids may be introduced after the paper is manufactured by atmospheric contamination, particularly sulfur dioxide which is absorbed by the paper and, in the presence of moisture, will generate sulfuric acid. 

The reaction appears to be well suited for selective conversion of biomass carbohydrates into their corresponding oxidized derivatives. This system serves to oxidize several polymeric carbohydrates including starches and pullulan.445 49 More recent work has described the TEMPO-catalyzed introduction of carboxyl groups in native cellulose and its different morphological forms,45(M52 cellulose derivatives,453 and the surface of cellulose nanocrystals.454 The related biopolymer, chitin, also is oxidized under these conditions.455... 

Properties of Carboxyl Groups in Wood Cellulose and Derivatives.. .. 315... 

Recently, Hiller and Pacsu have shown that accurate results may be obtained in the absence of sodium chloride by a direct alkali titration of the carboxyl groups in purified cotton. The carboxyl group content found was 0.02%, but after oxidation with potassium permanganate solution, 0.126% was found. From these data it may be seen that both carboxyl and aldehydic groups are present in the original cellulosic... 

The affinity of variously oxidized cellulosic materials for certain basic dyes, notably methylene blue, has long been known. Kenyon and coworkers have recently shown that in the case of nitrogen dioxide oxycelluloses, the methylene blue absorption is a function of the carboxyl group content. Application of the reversible methylene blue method to the determination of carboxyl groups in purified cellulose has been made by Davidson and by Weber, who was able to show good agreement with the cation-exchange methods previously considered as well as with viscosity and osmotic pressure measurements. 

Figure 8. Incorporation of oxidized groups in cellulose at 190 °C. The numbers of lactone and carboxyl groups formed in nitrogen at 190 °C are represented by -V-.

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