Methylcellulose

Methylcellulose (or methyl cellulose) is a chemical compound derived from cellulose. It is a hydrophilic white powder in pure form and dissolves in cold (but not in hot) water, forming a clear viscous solution or gel. It is sold under a variety of trade names and is used as a thickener and emulsifier in various food and cosmetic products, and also as a treatment of constipation. Like cellulose, it is not digestible, not toxic, and not allergenic.

Chemistry
Chemically, methylcellulose is a methyl ether of cellulose, arising from substituting the hydrogen atoms of some of cellulose's hydroxyl groups -OH with methyl groups -CH3, forming -OCH3 groups.

Different kinds of methylcellulose can be prepared depending on the number of hydroxyl groups so substituted. Cellulose is a polymer consisting of numerous linked glucose molecules, each of which exposes three hydroxyl groups. The Degree of Substitution (DS) of a given form of methylcellulose is defined as the average number of substituted hydroxyl groups per glucose. The theoretical maximum is thus a DS of 3.0, however more typical values are 1.3 - 2.6.

Different methylcellulose preparations can also differ in the average length of their polymer backbones.

Methylcellulose does not occur naturally and is synthetically produced by heating cellulose with caustic solution (e.g. a solution of sodium hydroxide) and treating it with methyl chloride.

The CAS number of methylcellulose is 9004-67-5.

Solubility and temperature
Methylcellulose dissolves in cold water. Higher DS-values result in lower solubility, because the polar hydroxyl groups are masked. The chemical is not soluble in hot water, which has the paradoxical effect that heating a saturated solution of methylcellulose will turn it solid, because methylcellulose will precipitate out. The temperature at which this occurs depends on DS-value, with higher DS-values giving lower precipitation temperatures.

Preparing a solution of methylcellulose with cold water is difficult however: as the powder comes into contact with water, a gluey layer forms around it, and the inside remains dry. A better way is to first mix the powder with hot water, so that the methylcellulose particles are well dispersed in the water, and cool down this dispersion while stirring, leading to the dissolution of those particles.

Uses
Methylcellulose has an extremely wide range of uses, of which several are described below.

Scientifically Advanced Cookery
Methylcellulose, as a gel, has the unique property of setting when hot and melting when cold. This technique is currently being developed at the University of Nottingham, in co-ordination with leading culinary alchemist Heston Blumenthal. Blumenthal's wishes were to "make a warm 'ice cream' or 'ice lolly' on a stick, which the customer will have to eat before it cools down and melts.".

Thickener and emulsifier
Methylcellulose is often added to hair shampoos, tooth pastes and liquid soaps, to generate their characteristic thick consistency. This it also done for foods, for example ice cream or whipped cream. Methylcellulose is also an important emulsifier, preventing the separation of two mixed liquids.

The E number of methylcellulose as food additive is E461.

Treatment of constipation
When eaten, methylcellulose is not absorbed by the intestines but passes through the digestive tract undisturbed. It attracts large amounts of water into the colon, producing a softer and bulkier stool. It is used to treat constipation, diverticulosis, hemorrhoids and irritable bowel syndrome. It should be taken with sufficient amounts of fluid to prevent dehydration.

Because it absorbs water and potentially toxic materials and increases viscosity, it can also be used to treat diarrhea.

A well-known trade name of methylcellulose when used as a drug is Citrucel by GlaxoSmithKline, but generic versions are also widely available.

Lubricant
Methylcellulose is used as a variable viscosity personal lubricant; it is the main ingredient in K-Y Jelly.

Artificial tears and saliva
Solutions containing methylcellulose or similar cellulose derivatives (see below) are used as substitute for tears or saliva if the natural production of these fluids is disturbed.

Paper and textile sizing
Methylcellulose is used as sizing in the production of papers and textiles. It protects the fibers from absorbing water or oil.

Glue and binder
Methylcellulose can be employed as a mild glue which can be washed away with water. This is used for example in the fixation of delicate pieces of art.

Methylcellulose is the main ingredient in many wallpaper pastes.

It is also used as a binder in pastel crayons.

Methylcellulose is used in book conservation to loosen and clean off old glue from spines and bookboards.

Construction materials
Methylcellulose finds a major application in construction materials. It is added to mortar dry mixes to improve the mortar's properties such as water retention, viscosity, adhesion to surfaces etc.

Cell culture/virology
Methylcellulose is also used in cell culture to study viral replication. Methylcellulose is dissolved in the same nutrient containing media that cells are normally grown in. A single layer of cells are grown on a flat surface, then infected with a virus for a short time. The strength of the viral sample used will determine how many cells get infected during this time. The thick methylcellulose media is then added on top of the cells in place of normal liquid media. As the viruses replicate in the infected cells they are able to spread between cells whose membrances touch each other, but are trapped when they enter the methylcellulose. Only cells closely neighboring an infected cell will become infected and die. This leaves small regions of dead cells called plaques in a larger background of living uninfected cells. The number of plaques formed is determined by the strength of the original sample.

Special effects
The slimy, gooey appearance of an appropriate preparation of methylcellulose with water, in addition to its non-toxic, non-allergenic, and edible properties, makes it popular for use in special effects for motion pictures and television wherever vile slimes must be simulated. In the film Ghostbusters, for example, the gooey substance that supernatural entities used to “slime” the Ghostbusters was mostly a thick water solution of methylcellulose.

Related compounds
Similar compounds derived from cellulose include carboxymethyl cellulose and hydroxypropyl cellulose. See: Category:Cellulose.