Crystalline alkali-cellulose

Sarko, A., Nishimura, H., and Okano, T., Crystalline alkali-cellulose complexes as intermediates during mercerization, in The Structures of Cellulose, Atalla, R.H., Ed., ACS Symposium Series, No. 340, American Chemical Society, Washington, D.C., 1987, p. 169. 

Crystalline Alkali-Cellulose Complexes as Intermediates During Mercerization... 

It was observed In earlier studies of controlled alkall-mercerlzatlon of ramie cellulose that the crystal structure of native cellulose Is transformed to cellulose II through a series of crystalline alkali-cellulose complexes (1,2). The relationships between these "Na-celluloses" and their pathways of transformation are Illustrated In Fig. 1. It has further been observed that all of the transformations are crystal-to-crystal phase changes, not Involving Intermediate amorphous phases. All of the experimental evidence has suggested... 

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... 

The most important alternative crystalline form is cellulose II. This form can result from treatment of cellulose in concentrated alkali, such as 23% NaOH, followed by rinsing in water. This is also the main form that results from crystallization of dissolved cellulose, such as regeneration of rayon. Supercritical water can also effect the transformation [216]. The treatment of cotton in milder alkali, for industrial mercerization, amounts mainly to disruption and decrystallization rather than transformation to crystalline II. Cellulose II can occur as the native state when the normal biosynthesis and subsequent crystallization is disrupted [217-219]. 

The effect and action of enzymes seems to be very limited because ol the stronger conditions of alkali of mercerizing strength. Enzymatic hydrolysis is accelerated when mercerization is carried out without tension [44]. The greater accessibility and lower crystallinity of cellulose mercerized without tension is a decisive factor in the enzymatic hydrolysis process. Mercerized cotton is generally more prone to enzymatic modification than untreated cotton. 

There has been another proposal that plane-structures consisting of cellulose molecules in the lOl plane of native cellulose are held together by hydrophobic interactions even in the presence of alkali, and that hydrophilic surfaces of the 101 plane-structures are solvated with alkali and water (24). However, if such planar structures were solvated with aqueous alkali, they would be expected to result in the formation of a dispersion of micelles. It seems to us more likely that some strong or sterically protected inter-molecular hydrogen bonds of native cellulose survive even in alkali cellulose. On the other hand, since some hydrogen bonds are cleaved by NaOH and water which penetrate into the crystalline lattice of cellulose, new lattice planes can be formed as, for example, in Na-Cellulose I or other soda celluloses. 

The pretreatment of any lignocellulosic biomass is cmcial before enzymatic hydrolysis. The objective of pretreatment is to decrease the crystallinity of cellulose which enhances the hydrolysis of cellulose by cellulases (17). Various pretreatment options are available to fractionate, solubilize, hydrolyze and separate cellulose, hemicellulose and lignin components (1,18-20). These include concentrated acid (27), dilute acid (22), SOj (25), alkali (24, 25), hydrogen peroxide (26), wet-oxidation (27), steam explosion (autohydrolysis) (28), ammonia fiber explosion (AFEX) (29), CO2 explosion (30), liquid hot water (31) and organic solvent treatments (52). In each option, the biomass is reduced in size and its physical structure is opened. Some methods of pretreatment of Lignocellulose is given in Table I. 

As a result of sodium hydroxide penetration into crystalline regions of parent cellulose (cellulose I), alkali cellulose is formed. Then, after washing out unreacted NaOH, the formation of regenerated cellulose (cellulose II) takes place. 

When crystalline cellulose I is treated with aqueous alkali solutions of sufficient strength, a process known as mercerization takes place. As a result of it, cellulose I is converted to cellulose II, the most stable or the four crystalline cellulose polymorphs. The conversion proceeds in the solid state, without apparent destruction or change in the fibrous morphology of the cellulose. As our diffraction analysis indicates, however, it is accompanied by a reversal of the chain packing polarity—from the parallel-chain cellulose I to the... 

Strong basic solutions, such as sodium hydroxide, penetrate the crystalline lattice of a-cellulose producing an alkoxide called alkali or soda cellulose. Mercerized cotton is produced by aqueous extraction of the sodium hydroxide. 

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. 

Two mannans may be isolated from ivory nuts. Mannan A, which is extracted with alkali, occurs in granular form,94 96 and x-ray diffraction photographs of both the native and the extracted polysaccharide show distinct crystalline patterns.96 Mannan B cannot, however, be extracted directly, and it is separated from cellulose by precipitation from cupram-monium solution.91 In the plant, mannan B is built up of microfibrils analogous to those of cellulose, but the extracted polysaccharide shows no tendency to crystallize on precipitation.95... 

---