Pharmaceutical Manufacturing Handbook: Production and

298 SEMISOLID DOSAGES: OINTMENTS, CREAMS, AND GELS
respectively. Some of the enzymes secreted by microbes decrease the viscosity of
HEC dispersions. The presence of higher levels of electrolytes may also destabilize
HEC dispersions. Inclusion of a suitable antimicrobial preservative is essential to
retain the viscosity of HEC gels [31] .
Hydroxyethylmethyl Cellulose ( HEMC) HEMC is a partially o - methylated and
o - (2 - hydroxyethylated) cellulose. It is available as powder or granules which are
white, grayish white, or yellowish white in color. Various viscosity grades of HEMC
are commercially available, and form viscous colloidal dispersions or gels in cold
water which has a pH in the range of 5.5 – 8 [6] .
Hydroxypropyl Cellulose ( HPC) HPC is a partially substituted poly(hydroxypropyl)
ether of cellulose. It is obtained by treating alkali cellulose with propylene oxide at
higher temperatures. It is available as tasteless and odorless powder which is yellowish
or white in color. Different viscosity grades are commercially available.
Gradual addition of HPC powder into vigorously stirred water yields clear viscous
dispersions or gels below 38 ° C. Increase in temperature destabilizes the dispersion
and leads to precipitation. The viscosity of dispersions can be increased by increasing
the concentration of HPC or by using high - molecular - weight grades. Inclusion
of a cosolvent such as dichloromethane or methane produces viscous dispersion or
gels with modifi ed texture. The viscosity of HPC dispersions can be increased by
mixing with an anionic polymer. High concentrations of electrolytes destabilize
HPC dispersions. HPC dispersions are neutral in pH (6 – 8). They undergo acid
hydrolysis at lower pH and oxidation at higher pH. Both processes decrease the
dispersion viscosity. In addition, certain enzymes produced by microbes degrade
HPC and reduce its viscosity. Addition of an antimicrobial preservative is therefore
recommended for HPC gels. Preservation of these gels from light can further
improve its physical stability [25] .
Hydroxypropylmethyl Cellulose ( HPMC) HPMC is a partly o - methylated and
o - (2 - hydroxypropylated) cellulose obtained by treating alkali cellulose with chloromethane
and propylene oxide. It is available as odorless and tasteless granular or
fi brous powder which is creamy white or white in color. HPMC is soluble in cold
water. Aqueous dispersions are prepared by dispersing material in about 25% hot
water (80 ° C) under vigorous stirring. On complete hydration of HPMC, a suffi cient
quantity of cold water is added and mixed. The gel point of HPMC dispersions varies
from 50 to 90 ° C. Gels are stable over a wide pH range (3 – 11). The viscosity HPMC
dispersions depends on the concentration of material used, its molecular weight,
vehicle composition, presence of preservatives, and so on. Viscous gels can be prepared
using high concentrations of high - molecular - weight grades. Inclusion of
organic solvents such as ethanol or dichloromethane improves the viscosity. Addition
of an antimicrobial preservative (e.g., benzalkonium chloride) minimizes microbial
spoilage of HPMC gels [25] .
Glyceryl Behenate Glyceryl behenate is a mixture of glycerides of fatty acids
which is obtained by esterifi cation of glycerin with behenic acid. It may also contain
arachidic acid, stearic acid, erucic acid, lignoceric acid, and palmitic acid. It is available
as a waxy mass or powder, possesses a faint odor, and is white in color. It is