Carboxymethyl cellulose

Carboxymethyl cellulose
Other names Carboxymethylcellulose; carmellose; E466
Identifiers
CAS number	9000-11-7 Y
	Properties
Molecular formula	variable
Molar mass	variable
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Carboxymethyl cellulose (CMC) or cellulose gum^[1] is a cellulose derivative with carboxymethyl groups (-CH₂-COOH) bound to some of the hydroxyl groups of the glucopyranose monomers that make up the cellulose backbone. It is often used as its sodium salt, sodium carboxymethyl cellulose.

Preparation

It is synthesized by the alkali-catalyzed reaction of cellulose with chloroacetic acid. The polar (organic acid) carboxyl groups render the cellulose soluble and chemically reactive.

The functional properties of CMC depend on the degree of substitution of the cellulose structure (i.e., how many of the hydroxyl groups have taken part in the substitution reaction), as well as the chain length of the cellulose backbone structure and the degree of clustering of the carboxymethyl substituents.

Uses

CMC is used in food science as a viscosity modifier or thickener, and to stabilize emulsions in various products including ice cream. As a food additive, it has E number E466. It is also a constituent of many non-food products, such as personal lubricants, toothpaste, laxatives, diet pills, water-based paints, detergents, textile sizing, and various paper products. It is used primarily because it has high viscosity, is nontoxic, and is generally considered to be hypoallergenic as the major source fiber is either softwood pulp or cotton linter.^[2][3] CMC is used extensively in gluten free ^[4] and reduced fat food products.^[5] In laundry detergents, it is used as a soil suspension polymer designed to deposit onto cotton and other cellulosic fabrics, creating a negatively charged barrier to soils in the wash solution. CMC is used as a lubricant in nonvolatile eye drops (artificial tears). Sometimes, methyl cellulose (MC) is used, but its nonpolar methyl groups (-CH₃) do not add any solubility or chemical reactivity to the base cellulose.

Following the initial reaction, the resultant mixture produces about 60% CMC plus 40% salts (sodium chloride and sodium glycolate). This product is the so-called technical CMC which is used in detergents. A further purification process is used to remove these salts to produce the pure CMC used for food, pharmaceutical, and dentifrice (toothpaste) applications. An intermediate "semipurified" grade is also produced, typically used in paper applications.

CMC is also used in pharmaceuticals as a thickening agent, and in the oil-drilling industry as an ingredient of drilling mud, where it acts as a viscosity modifier and water retention agent. Polyanionic cellulose (PAC) derived from cellulose is also used in oilfield practice. CMC is both chemically and physically distinguished from polyanionic cellulose. CMC is definitely a carboxylic acid, where PAC is an ether. Although CMC and PAC are manufactured from the same raw materials, by adjusting the type of cellulose and stoichiometry of the reactants, different final products may be produced. The primary difference between the CMC and PAC production processes is in the radicalization step.^{[citation needed]}

Insoluble microgranular CMC is used as a cation-exchange resin in ion-exchange chromatography for purification of proteins.^[6] Presumably, the level of derivatization is much lower, so the solubility properties of microgranular cellulose are retained, while adding sufficient negatively charged carboxylate groups to bind to positively charged proteins.

CMC is also used in ice packs to form a eutectic mixture resulting in a lower freezing point, and therefore more cooling capacity than ice.^[7]

Aqueous solutions of CMC have also been used to disperse carbon nanotubes. The long CMC molecules are thought to wrap around the nanotubes, allowing them to be dispersed in water. In conservation-restoration, it is used as an adhesive or fixative (commercial name Klucel).

CMC is used to achieve tartrate or cold stability in wine. This innovation may save megawatts of electricity used to chill wine in warm climates. It is more stable than metatartaric acid and is very effective in inhibiting tartrate precipitation. It is reported (GERBAUX 1996) that KHT crystals, in presence of CMC, grow slower and change their morphology. Their shape becomes flatter because they lose 2 of the 7 faces, changing their dimensions. CMC molecules, negatively charged at wine pH, interact with the electropositive surface of the crystals, where potassium ions are accumulated. The slower growth of the crystals and the modification of their shape are caused by the competition between CMC molecules and bitartrate ions for binding to the KHT crystals (CRACHERAU et al. 2001).

Enzymology

CMC has also been used extensively to characterize enzyme activity from endoglucanases (part of the cellulase complex). CMC is a highly specific substrate for endo-acting cellulases, as its structure has been engineered to decrystallize cellulose and create amorphous sites that are ideal for endoglucanase action. CMC is desirable because the catalysis product (glucose) is easily measured using a reducing sugar assay, such as 3,5-dinitrosalicylic acid. Using CMC in enzyme assays is especially important in regard to screening for cellulase enzymes that are needed for more efficient cellulosic ethanol conversion. However, CMC has also been misused in earlier work with cellulase enzymes, as many had associated whole cellulase activity with CMC hydrolysis. As the mechanism of cellulose depolymerization has become more understood, exocellulases are shown to be dominant in the degradation of crystalline (e.g. Avicel) and not soluble (e.g. CMC) cellulose.

See also

• Hydroxypropyl cellulose
• Methyl cellulose
• Croscarmellose sodium

References

External links

• CMC chemical structure and properties
• MC and CMC: commercial preparations and various uses, including paper conservation; bibliography