Hydroxypropyl cellulose

Hydroxypropyl cellulose (HPC) (cellulose, 2-hydroxypropyl ether) is a derivative of cellulose with both water solubility and organic solubility.

Chemistry
Hydroxypropyl cellulose is an ether of cellulose where some of the hydroxyl groups of the cellulose have been hydroxypropylated forming -OCH2CH(OH)CH3 groups. The average number of hydroxyl groups in the glucose ring substituted is referred to as the degree of substitution (DS). Complete substitution would provide a DS of 3.0. Because the hydroxypropyl group added contains a hydroxyl group, this can also be etherified during preparation of hydroxypropyl cellulose. When this occurs, the number of moles of hydroxypropyl groups per glucose ring (moles of substitution - MS) can be higher than 3. Because of the fact that cellulose is very crystalline HPC must have an MS about 4 in order to reach a good solubility in water. HPC has a combination of hydrophobic and hydrophilic groups, so it has a Lower Critical Solution Temperature (LCST) at 45°C: under the LCST HPC is readily soluble in water, over 45°C HPC is not soluble. An interesting property of HPC is the fact that forms liquid crystals and many mesophases according to its concentration in water. Such mesophases include isotropic, anisotropic, nematic and cholesteric (cholesteric liquid crystal). The last one gives many colors, like violet, green and red.

The CAS registry number of hydroxypropyl cellulose is 9004-64-2.

Uses
Lacrisert, manufactured by Aton Pharma, Inc., is a formulation of it used for artificial tears. It is used in syndromes of insufficient tear production (keratoconjunctivitis sicca), recurrent corneal erosions, decreased corneal sensitivity, exposure and neuroparalytic keratitis, and as a lubricant for artificial eyes.

As a food additive, hydroxypropyl cellulose is used as a thickener and as an emulsion stabiliser with E number E463.