The above figures have a purely empirical value, since they represent a complicated mixture of various residues derived from the celluloses and compound celluloses. This mixture may be further resolved, and by special quantitative methods the proportions of actual cellulose, ligno-cellulose and cuto-celluloses estimated (J. König, Ber., 1906, 39, p. 3564). The figures are taken as an inverse measure of digestibility; at the same time it has been established that this group of relatively indigestible food constituents are more or less digestible and assimilable as flesh and fat producers. The percentage or coefficient of digestibility of the celluloses of the more important food-stuffs—green fodder, hay, straw and grains—varies from 20 to 75%. It has also been established that their physiological efficiency is, under certain conditions, quite equal to that of starch.
It must also be borne in mind that the indigestible food residues, as finally voided by the animal, have played an important mechanical part as an aid to digestion of those constituents more readily attacked in the digestive tract of animals. They are further an important factor of the agricultural cycle. Returned to the soil as “farm-yard manure,” mixed with other cellulosic matter which has served as litter, they add “fibre” to the soil and, as a mechanical diluent of the mineral soil components, maintain this in a more open condition, penetrable by the atmospheric gases, and promoting distribution of moisture. Further by breaking down, with production of “humus,” a complex of colloidal “unsaturated” bodies of acid function, they fulfil important chemical functions by interaction with the mineral soil constituents.
Chemistry of Cellulose.—Purified cotton cellulose, which is the definitive prototype of the cellulose group or series, is a complex of monoses or their “residues.” It is resolved by solution in sulphuric acid and subsequent hydrolysis of the esters thus produced into dextrose. This fundamental fact with its elementary composition, most simply expressed by the formula C6H10O5, has caused it to be regarded as a polyanhydride of dextrose. Forming, as it does, simple esters in the ratio of the reacting hydroxyls 3OH: C6H10O5, and taking into account its direct converson into [omega]-brom-methyl furfural (Fenton) a constitutional formula has been proposed by A. G. Green (Zeit. Farb. Textil Chem. 3, pp. 97 and 309 (1904)), which is a useful generalization of its reactions, and its ultimate relations to the simpler carbohydrates, viz.,
Green considers, moreover, that a group thus formulated may consistently represent the actual dimensions of the reacting unit, but that unit of larger dimensions, if postulated, is easily derived from the above by oxygen linkings.
From another point of view the unit group has been formulated as
the main linking of such units in the complex taking place as between their respective CO and CH2 groups in the alternative enolic form CH—C(OH). This view gives expression to the genetic relations of the celluloses to the ligno-celluloses, to the tendency to carbon condensation as in the formation of coals, and pseudo-carbons, to the relative resistance of cellulose to hydrolysis, and its other points of differentiation from starch, and more particularly to the ketonic character of its carbonyl (CO) groups, which is also more in harmony with the experimental facts established by Fenton as to the production of methyl furfural.
The probability, however, is that no simple molecular formula adequately represents the constitution of cellulose as it actually exists or indeed reacts. On the other hand, it has been suggested that cellulose is to be regarded as representing a condition of matter analogous to that of a saline electrolyte in solution, i.e. as a complex of molecular aggregates, and of residues (of monose groups) having distinct and opposite polarities; such a complex is essentially labile and its configuration will change progressively under reaction. The exposition of this view is the subject of a publication by Cross and Bevan (Researches on Cellulose, ii. 1906). The main purpose is to give full effect to the colloidal characteristics of cellulose and its derivatives, with reference to the modern theory of the colloidal state as involving a particular internal equilibrium of amphoteric electrolytes.
The typical cellulose is a white fibrous substance familiar to us in the various forms of bleached cotton. Other fibrous celluloses are equally characteristic as to form and appearance, e.g. bleached flax, hemp, ramie. It is hygroscopic, absorbing 6 to 7% its weight of moisture from the air. When dry, it is an electrical insulator, and has a specific inductive capacity of about 7: when wetted it is a conductor, and manifests electrolytic phenomena. It is insoluble in water and in the ordinary solvents; it dissolves, however, in a 40–50% solution of zinc chloride, and in ammoniacal solutions of copper oxide (3% CuO, 15% NH3): from these solutions it is obtained as a highly hydrated, gelatinous precipitate, from the former by dilution or addition of alcohol, from the latter by acidification; these solutions have important industrial application. Projected or drawn into a precipitating solution they may be solidified continuously to threads of various, but controlled dimensions: the regenerated cellulose, now amorphous, in its finer dimensions is known as artificial silk or lustra-cellulose. These forms of cellulose retain the general characters of the original fibrous and “natural” celluloses. In composition they differ somewhat by combination with water (of hydration), which they retain in the air-dry condition. They also further combine with an increased proportion of atmospheric moisture, viz. up to 10–11% of their weight.
Derivatives.—Important derivatives are the esters or ethereal salts of both inorganic and organic acids, cellulose behaving as an alcohol, the highest esters indicating that it reacts as a trihydric alcohol of the formula n[C6H7O2(OH)3]. The nitrates result by the action of concentrated nitric acid, either alone or in the presence of sulphuric acid: the normal dinitrate represents a definite stage in the series of nitrates, and the ester at this point manifests the important property of solubility in various alcoholic solvents, notably ether-alcohol. Such nitrates are the basis of collodion, of artificial silk by the processes of Chardonnet and Lehner, and of celluloid or xylonite. Higher nitrates are also obtainable up to the limit of the trinitrate, which is insoluble in ether or alcohol, but is soluble in nitroglycerin, nitrobenzene and other solvents. These higher nitrates are the basis of the most important modern explosives.
Cellulose reacts directly with acetic anhydride to form low esters; in the presence of sulphuric acid the reaction proceeds to higher limits; the triacetate is soluble in chloroform. The acid sulphuric ester, C6H8O3(SO4H)2, is obtained by the action of sulphuric acid, but its relation to the original cellulose is doubtful. The monobenzoate and dibenzoate are formed by benzoyl chloride reacting on alkali-cellulose (see below). Cellulose xanthates are obtained from carbon bisulphide and alkali-cellulose; these are water soluble derivatives and the basis of “viscose,” and of important industries. Mixed esters—-aceto-sulphate, aceto-benzoate, nitrobenzoyl nitrates, aceto-nitro-sulphates—have also been investigated.
Cellulose (cotton), when treated with a 15–20% caustic soda solution, gives the compound C6H10O5·H2O·2NaOH, alkali-cellulose, the original riband-like form with reticulated walls of the cellulose being transformed into a smooth-walled cylinder. The structural changes in the ultimate fibre determine very considerable changes in the dimensions of fabrics so treated. The reactions and structural changes were investigated by J. Mercer, and are known generally as “mercerization.” In recent years a very large industry in “mercerized” fabrics (cotton) has resulted from the observation that if the shrinkages of the yarns and fabrics be antagonized by mechanical means, a very high lustre is developed.
Similar, but less definite compounds, are formed with the oxides of lead, manganese, barium, iron, aluminium and chromium. These derivatives, which also find industrial applications in the dyeing and printing of fabrics, differ but little in
- C. F. Cross and E. J. Bevan, Jour. Chem. Soc., 1895, 67, p. 449; C. R. Darling, Jour. Faraday Soc. 1904; A. Campbell, Trans. Roy. Soc. 1906.