Cellulose, alkali structure

Aqueous solutions of guanidine cause a structural change in cellulose midway between that produced by aliphatic amines and by alkali hydroxides. The lattice change to a cellulose II structure, which occurs with aqueous alkali hydroxide above a certain concentration, does not occur with guanidine. This was attributed to N-H—O bridges, with the participation of guanidinium ions. 

H- 02 type was proposed for the cellulose II structures. The alkali solubility of cellulose was shown to have a higher correlation with the reduction in intramolecular hydrogen bonding than with the apparent amorphous content [16,71]. 

When the cell-wall materials are treated with dilute acid, alkali, or other suitable chemical compounds in order to remove the amorphous materials, the cellulosic fibrillar structure is exposed and may be examined by electron microscopy. " Microfibrils may be described as thin threads, the width of which has been variously estimated, according to the materials and methods used. Preston, working with algae, reported that microfibrils vary in width from 8.0 to 30.0 nm and are approximately half as thick as they are wide. - - Hodge and... 

In the present study, the role of cellulose physical structure in alkaline reactions was investigated by comparing the alkaline degradation of highly crystalline (cellulose I) fibrous hydrocellulose with that of amorphous (noncrystalline) hydrocellulose. The amorphous substrate was taken as a cellulose model the reactivity of which would most closely approximate that of alkali-soluble cellulose. The availablity of such an approximation to the inherent reactivity of cellulose allowed evaluation of the effects of the more highly ordered structure of the fibrous hydrocellulose. 

The x-ray diffractograms showed the presence of cellulose I structure in all raw fibers. In the treated banana fiber, cellulose II structure was dominant. While cellulose I ([002] peak) is found in all five raw fibers, the [002] peak cellulose II is found as a shoulder in kenaf and ramie. The measured crystallinity (x%) in raw and alkali-treated fibers are shown in the table below ... 

It has been established that changes can occur in the cellulose physical structure, using x-ray diffraction [132-134] and infrared spectroscopy [135,136] by several workers. Nine allotropes of alkali cellulose have been identified. Sobue [134] has described some of the possible transitions outlined in Figure 10.54 that are related to the alkalinity and temperature. 

Natural products are a mixture of these two modifications. Cellulose Ip is dominant in plants and wood, and the percentage of cellulose present varies depending on the species and the treatments. For example, the cellulose Ip content of cotton linter and ramie is 77%. Cellulose I transformed into cellulose Ip by a hydrothermal treatment in an alkali solution or by heat treatment at 280 °C in an inert gas. For example, Horii et al. reported that cellulose Ip content increases to 90% by heat treatment at 260°C in 0.1 M NaOH aqueous solution [21]. This indicates that cellulose Ip is thermodynamically more stable than cellulose Almost pure cellulose Ip is obtained from tunicates Halocynthia roretzi). Cellulose h is reported to be the major component of bacterial and algal cellulose. The structural difference between I and Ip is said to be brought about by shear stress during biosynthesis of cellulose microfibrils. 

Redox behavior of anthraquinone is shown in Scheme 4. The quinone moiety may be reduced to the hydroquinone form and converted to a leuco salt under alkali conditions. In general, the leuco salt has a strong affinity for cellulose and is soluble in water. The hydroquinone form is insoluble in water and has low affinity to cellulose. The preferred dyeing procedure depends on the structure and properties of the vat dye. The variables that are used to control the process include, e.g., strength and amount of alkali, reduction temperature, and the presence of salts. During the process of reduction, some side reactions, such as overreduction, hydrolysis,... 

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

From the variation of the dm spacing in alkali-celluloses I, II, and V, the number of water molecules has been calculated to be 3, 1, and 5, respectively. A value of c/,0i = 1.51 nm, which is the largest reported so far for alkali-celluloses, was obtained by mercerization of sulfite cellulose with 11% sodium hydroxide solution for one hour at 20°. Between six and eight water molecules are included in this structure. 

The resistance of polymers to corrosives is dependent on structure. Some polymers, such as cellulose acetate, do not have outstanding resistance to acids and alkalis. Mosf polymers are less brittle than the more chemically resistant ceramics and more resistant to corrosives than most metals. 

All of the likely conformations of cellobiose, cellulose, and xylan are explored systematically assuming the ring conformations and IC-D-O-IC-4 ) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The results lead to possible explanations of the known conformational stiffness of cellulose and Its solubility properties in alkali. The characteristics of xylan conformations are compared with cellulose. 

After devoting considerable thought to the problem of the chemical structure of alkali cellulose, Bleshinskil and Lositskaya 0 have suggested... 

Makolkin studied the rate of exchange of, 80 between labeled water and alkali cellulose, and between labeled water and the trisodium alcoholate of cellulose, and concluded that mercerization proceeds by reaction that is, alkali cellulose is an adduct. He based his condition on the fact that there is no measurable exchange between water and the alcoholate, whereas there is measurable exchange between alkali cellulose and water. His conclusion, which is based upon difference of rate, is, however, not necessarily valid. The trisodium alcoholate may not possess a structure that is as accessible to water as is that of alkali cellulose. 

Reaction of cellulose with alkalies) 34) T.Petitpas, MSCE 34, 139(1948)(Variation of structure of cellulose in alk solns) 35)A. 

The other carbohydrates in cane juice are the soluble polysaccharides vaguely classified under the terms "hemi-celluloses, soluble gums and pectins. It is possible that some of these polysaccharides may enter the juice during the milling of the cane as the plant cell structure is destroyed. A gummy product has been isolated from cane fiber by alkali extraction followed by alcohol precipitation. Acid hydrolysis of this substance yielded crystalline D-xylose and L-arabinose.10 Such gums in Trinidad cane juices were isolated by alcohol precipitation at suitable hydrogen ion concentration and assayed for pentose content by the Tollens 2-fural-dehyde assay the results showed an apparent pentosan content of 0.04-0.07%u of the Brix solids. 

Mintova et al. studied deposition of zeolite A on various cellulose fibers pretreated chemically and/or mechanically [151]. It was shown that the amount of zeolite deposited was controllable by suitable fiber pretreatment with ball-milling or with diethyl ether under ultrasonic action. The reactive high-concentration hydroxyl groups on the structurally loosened celluloses seem to interact with aluminosilicate species and thus promote the formation of nuclei for zeolite crystallization. Pretreatment of natural cellulose fibers with alkali provides another simple route for anchoring preformed zeolite crystallites onto the cellulose surface. 

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