(1) Preparation. Emulsion bases are made by melting the greasy and oily
materials together in one container and heating the water and water-soluble materials in
another container. At the temperature of 750 C, they are mixed together until a smooth
cream results. While the mixture is still warm and thin, it may be passed through a
homo-genizer to improve the appearance and quality of the base. The mixture is then
stirred until it congeals.
(2) Summary of emulsion bases. We can sum up the important aspects of
emulsion bases as follows:
(a) Properties. The w/o emulsion bases are insoluble in water and are
not washable; the o/w emulsion bases are washable and nongreasy.
(b) Example. Lanolin, USP (w/o); Hydrophilic Ointment, USP (o/w);
vanishing creams (o/w).
(c)
Advantages. Washable and nongreasy if oil-in-water (o/w).
(d) Disadvantages. Subject to water loss if o/w, greasy and
unwashable if water-in-oil (w/o), unless, a preservative is added, the emulsion bases
are subject to mold growth.
d. Water-Soluble Bases. The polyethylene glycol polymers, or Carbowaxes,
are of great importance in ointments. The names of the Carbowaxes include numbers
that roughly indicate their average molecular weight. Carbowaxes with a molecular
weight in the area of 1,000 are soft, ointment-like substances. As the molecular weight
increases, they become harder and they finally become waxes. They are water-soluble,
nonvolatile, and do not deteriorate or support mold growth.
(1)
Formulations.
(a) The most suitable Carbowax ointment bases are formulations of
heavy and light molecular-weight polyethylene glycols, such as the formula below for
Polyethylene Glycol Ointment, USP:
Polyethylene Glycol 4,000
40%
Polyethylene Glycol 400
60%
(b) This base is so water-soluble that not more than 5 percent water
can be added in making ointments. When greater volumes of water must be added to
the ointment, the following formulation is recommended:
Polyethylene Glycol 4,000
47.5%
Polyethylene Glycol 400
47.5%
Cetyle Alcohol
5.0%
MD0809
5-5