Water site headerMasthead Island, Great Barrier Reef Print-me keygo to Water Visitor Book contributions
Go to my page Water Structure and Science
Cotton plant

Cellulose

Cellulose is a water-binding agent.

 

V Sources for cellulose
V Structural unit
V Molecular structure
V Microcrystalline cellulose

V Functionality

Sources for cellulose

Cellulose is a prominent scaffolding polysaccharide found in plants as microfibrils [1598] (2-20 nm diameter and 100 - 40 000 nm long). These form the structurally strong  framework in the cell walls. Cellulose (E460) is mostly prepared from wood pulp. Cellulose is also produced in a highly hydrated form by some bacteria (for example, Acetobacter xylinum).

Structural unit

cellulose structure

 

Cellulose is a linear polymer of β-(1->4)-D-glucopyranose units in 4C1 conformation. The fully equatorial conformation of β-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility (for example, relative to the slightly more flexible α-linked glucopyranose residues in amylose). Cellulose preparations may contain trace amounts (~0.3%) of arabinoxylans.a  [Back to Top to top of page]

Molecular structure

Cellulose is an insoluble molecule consisting of about 1000 or so residues with processed or crystalline preparations being somewhat shorter. There are four crystalline forms, Iα, Iβ, II and III.

 

Cellulose;

data from [2552]

Weight-averaged number of residues water content; moles/mole glucose
Iβ, 430 0.46
II 740 0.89
III 290 0.86

 

The stabilities of these crystalline forms are similar(<10 J ˣ g-1 ), may depend on the temperature and have been proposed with cellulose I as the least stable [2552]. It forms crystals (cellulose Iα) where intra-molecular (O3-H->O5' and O6->H-O2') and intra-strand (O6-H->O3') hydrogen bonds holds the network flat allowing the more hydrophobic ribbon faces to stack. Weak C6-H->O2' hydrogen bonds may also make some contribution to the crystal stability. Each residue is oriented 180° to the next with the chain synthesized two residues at a time. Although individual strand of cellulose are intrinsically no less hydrophilic, or no more hydrophobic, than some other soluble polysaccharides (such as amylose) this tendency to form crystals utilizing extensive hydrophobic interactions [1645] in addition to intra- and intermolecular hydrogen bonding makes it completely insoluble in normal aqueous solutions (although it is soluble in more exotic solvents [2055, 2905] such as aqueous N-methylmorpholine-N-oxide N-methylmorpholine-N-oxide (NMNO, ~0.8 mol water/mol, then up to 30% by wt cellulose at 100 °C [1060]) [2938], CdO/ethylene-diamine (cadoxen), LiCl/N,N'-dimethyl-acetamide, concentrated NaOH [2497], near-supercritical water [1070] or aqueous tetrabutyl-ammonium hydroxide [3027]). Cellulose partially ionizes and swells in concentrated sodium hydroxide solutions that also cause a small amount of dissolution [3017]. It is thought that water molecules catalyze the formation of the natural cellulose crystals by helping to align the chains through hydrogen-bonded bridging. At the interface with water, the intra- and inter chain hydrogen bonding breaks up the hydrophobic surface such that these surfaces do not behave like macroscopic hydrophobic planes but more like hydrophobic strips like fatty acid chains [2269]. Water is excluded, however, from the region between the interior hydrophobic surfaces.

 

Crystalline cellulose Iβ and cellulose II both show negative value of Poisson’s ratio (ν, the signed ratio of transverse strain to axial strain, also known as auxetic behavior), where there is transverse expansion upon axial stretching of the material. Twisting, due to the presence of inter-chain hydrogen bonding, seems to be the cause of this perpendicular expansion [2825]..

 

The hydrogen bond linkages in cellulose is shown below. Donated hydrogen bonds from HO(2) and HO(6) hydroxyl groups

to receptor oxygen atoms O6 and O3 respectively can exchange with water but the HO(3) groups exclusively donates to the intra-chain neighboring O(5) and does not exchange with local water molecules, if present [2717].

inter and intra-molecular hydrogen bonding links in cellulose

 

Part of a cellulose preparation is amorphous between these crystalline sections. The overall structure is of aggregated particles with extensive pores capable of holding relatively large amounts of water by capillarity. The natural crystal is made up from metastable Cellulose I with all the cellulose strands parallel and no inter-sheet hydrogen bonding. This cellulose I (that is, natural cellulose) contains two coexisting phases cellulose Iα (triclinic) and cellulose Iβ (monoclinic) in varying proportions dependent on its origin; Iα being found more in algae and bacteria whilst Iβ is the major form in higher plants.

 

The two conformationally distinct cellulose layers in cellulose 1b

Cellulose Iα and cellulose Iβ have the same fibre repeat distance (1.043 nm for the repeat dimer interior to the crystal, 1.029 nm on the surface [721]) but differing displacements of the sheets relative to one another. The neighboring sheets of cellulose Iα (consisting of identical chains with two alternating glucose conformers -A-B-) are regularly displaced from each other in the same direction whereas sheets of cellulose Iβ (consisting of two conformationally distinct alternating sheets, (as shown right where the 2-OH and 6-OH groups both change orientations so altering the hydrogen bonding pattern) each made up of crystallographically identical glucose conformers) are staggered [559]. It has been found that cellulose (Iβ) significantly alters the water structuring at its surface out to about 10 Å, which may affect its enzymatic digestion [905].

 

Cellulose Iα and cellulose Iβ are interconverted by bending during microfibril formation [418] and metastable cellulose Iα converts to cellulose Iβ on annealing.

 

Interactive structures are available (Jmol).

 

If it can be recrystallized (for example, from base or CS2) cellulose I gives the thermodynamically more stable monoclinic Cellulose II structure with an antiparallel arrangement of the strands and intermolecular and both intra- and inter- sheet hydrogen-bonding.

 

cellulose II crystal, from [2039]This crystalline form of cellulose II, shown left, may also be prepared by deacetylation of cellulose acetate [2039]. Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures; the chains consisting of -A-A- or -B-B- repeat units [627], shown alternating from the top in the x-axis view. Both have intra-chain hydrogen bonding but only the -A-A- one has inter-chain hydrogen bonding. There are also hydrogen bonds between the A and B sheets such that an additional sheet structure consists of alternant A and B strands, with these sheets joined by hydrogen bonds, shown as diagonal sheets in the z-axis view. In the x-axis view the cellulose molecules go across the view (6 molecules shown) whereas in the z-axis view they disappear into the cartoon (18 molecules shown).

 

 

The interactive structure of cellulose II is available (Jmol).

 

Cellulose II hydrate may be prepared by immersion of mercerized cellulose II in anhydrous hydrazine followed by washing with water [1736]. it is similar to Cellulose II but has a larger unit cell containing the extra water molecules between the layers.

 

Cellulose III is formed from cellulose mercerized in ammonia and is similar cellulose II but with the chains parallel, as in cellulose Iα and cellulose Iβ [753]. For a review of cellulose structure, see [288]. Microcrystalline cellulose gives needle-shaped nanocrystals (microfibrillated cellulose [1902]) of a relative uniform size with length of 90 ± 50 nm and width of 10 ± 4 nm [1690].  

 

Alternative cellulose II and IIII structures have been suggested using molecular dynamics [2459]. The simulated neutron fiber diffraction pattern of cellulose IIII supported the new structure while that of cellulose II showed discrepancies with the reported experimental diffraction pattern.

Microcrystalline cellulose

Microcrystalline cellulose (MCC) [3005] is pure partially depolymerized α-cellulose made from any of a number of sources, including hardwood, softwood, and agricultural residues. Any amorphous cellulose present is destroyed in its preparation, using acid or enzymes for example, leaving just the remaining crystalline cellulose. The cellulose particles are small (~100 µ, with surface area ~100 m2 ˣ kg-1) and able to form a colloidal suspension in excess water. Different samples from different sources and preparation methodologies show differences at the particle level [3003].

 

Although MCC does not dissolve in water, water strongly binds to the surface of its cellulose crystallites in saturated
water vapor [3002]. A solid hydrate shell may form at higher water contents that affects the trans-gauche to trans-trans equilibrium of the surface hydroxymethyl groups [3002].

[Back to Top to top of page]

Functionality

Cellulose has many uses as an anticake agent, emulsifier, stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water. Microcrystalline cellulose is among the most commonly used cellulose derivatives in the food industry [3004] and is an important excipient due to its exceptional binding and tableting properties, characterized by its plasticity and cohesiveness when wet. Water cannot penetrate crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible. Some of this water is non-freezing but most is simply trapped. Less water is bound by direct hydrogen bonding if the cellulose has high crystallinity but some fibrous cellulose products can hold on to considerable water in pores and its typically straw-like cavities; water holding ability correlating well with the amorphous (surface area effect) and void fraction (that is, the porosity). As such water is supercoolable, this effect may protect against ice damage. Cellulose can give improved volume and texture particularly as a fat replacer in sauces and dressings but its insolubility means that all products will be cloudy.

 

Swelled bacterial cellulose (ex. Acetobacter xylinum), in its never-dried state with much smaller fibrils (~1%) than from plants, exhibits pseudoplastic viscosity like xanthan gels but this viscosity is not lost at high temperatures and low shear rates as the cellulose can retain its structure. Where individual cellulose strands are surrounded by water they are flexible and do not present contiguous hydrophobic surfaces. Bacterial cells may be removed by hot alkali and the clean wet cellulose used as a substrate for immobilizing biomolecules [843] or for covering wounds [844]. On drying the properties of bacterial cellulose (as those of other never-dried celluloses such as cotton [2242]) irreversibly lose their hydrated properties and tend to those of plant cellulose.

 

About a third of the world's production of purified cellulose is used as the base material for a number of water-soluble derivatives with pre-designed and wide-ranging properties dependent on groups involved and the degree of derivatization (for an extensive review see [287]). Derivatizing cellulose interferes with the orderly crystal-forming hydrogen bonding, described above, so that even hydrophobic derivatives may increase the apparent solubility in water. Methyl cellulose (E461) [231] (made by methylating about 30% of the hydroxyl groups) is thermogelling, forming gels above a critical temperature due to hydrophobic interactions between high-substituted regions and consequentially stabilized intermolecular hydrogen bonding. Such gels break down on cooling In a manner similar to that causing the solubility minimum for non-polar gases; hydrophobic saccharides becoming less soluble as the temperature increases [187]. This property is useful in forming films as barriers to water loss and for holding on to small gas bubbles.

 

Hydroxypropylmethylcellulose (HPMC, E464) has similar properties and uses but with added water interaction and surface activity [1292]. Both methylcellulose and HPMC may be used in gluten-free bakery products as gluten substitutes. Hydroxypropyl cellulose (E463) possesses good surface activity but does not gel as it forms open helical coils. It is a water-soluble thickener, emulsifier and film-former often used in tablet coating. Another important derivative of cellulose is carboxymethylcellulose.

 

Interactive structures are available (Jmol).  [Back to Top to top of page]


Footnotes

a Cellulose biosynthesis [1465] and breakdown [2620] have been reviewed. [Back]

 

 

Home | Site Index | Hydrocolloids | Polysaccharide hydration | Hydrogen bonding | LSBU | Top

 

This page was established in 2002 and last updated by Martin Chaplin on 8 September, 2017


Creative Commons License
This work is licensed under a Creative Commons Attribution
-Noncommercial-No Derivative Works 2.0 UK: England & Wales License