Essay on the Classification of Ointment Bases

Although ointment bases can be classified in many ways, the simplest one based on composition is as follows:

A. Oleaginous bases.

B. Absorption bases.

C. Emulsion bases.

D. Water-soluble (hydrophilic) bases.

A. Oleaginous bases

Salient characteristics of these bases are:

i) they are anhydrous,

ii) they are hydrophobic (do not absorb water readily),

iii) they are insoluble in water, and

iv) they are not removable by water.

These are the earliest ointment bases which consisted of vegetable and animal fats as well as petroleum hydrocarbons. A list of commonly used materials has been given here although in practice still a larger number of materials are used.

Oleaginous bases are expected to provide a film, which resists soap and water yet readily removable by solutions of surfactants. This can possibly be achieved by emulsifying the silicones or other hydrophobic film formers so that when the protective ointment is properly applied, an invisible protective film is left on the skin. Film forming agents are exemplified by polyvinylpyrollidone, polyvinyl alcohol and the cellulose derivatives.

B. Absorption bases

The term absorption as applied here implies the hydrophilic or water absorbing properties of the base and not the absorption of medicaments from the bases. These bases are generally anhydrous but capable of absorbing several times their own weight of water ultimately forming w/o type of emulsions.

Absorption bases vary in their composition and are usually mixtures of animal sterols with petrolatum. Eucerin and Aquaphor are the commercial bases consisting of combination of cholesterol and/or other suitable lanolin fraction with white petrolatum. Anhydrous absorption bases can also be formulated by the addition of lipophilic surfactants to
petrolatum and anhydrous water-removable bases can be formed by the addition of hydrophilic surfactants to petrolatum. Some important formulae of absorption ointment bases are given below.

Absorption bases were primarily developed so as to have a product to which water or an aqueous solution of medicinal substances could be easily added. These bases are usually highly compatible with the majority of drugs used topically. The limited popularity of absorption bases however is attributed to their greasiness.

C. Emulsion bases

These may be either o/w or w/o type emulsions.

(a) Water-in-oil type emulsion (Hydrophobic ointment):

The w/o type emulsion bases such as lanolin and cold cream are used as emollients. The aqueous phase hydrates the skin and the oily phase forms an occlusive covering which prevents loss of water by evaporation. Emulsion bases also serve as vehicle for medicaments such as sulphur, ammoniated mercury, balsam of peru, zinc oxide etc. The main drawback of w/o emulsion bases is their greasy and sticky nature and therefore they are less popular than o/w type of bases.

Earlier cold creams consisted of oil (40 to 70%), wax or spermaceti (5 to 15%), and water (20 to 35%). Thus a large proportion of water was loosely held in the water- in-oil mixture. More stable creams were later formulated employing borax which formed the sodium soap by reacting with the fatty acids present in the beeswax.

Presently cold creams are also formulated by employing non-ionic surfactants alone or in combination with beeswax. When cold creams are applied, slow evaporation of the water causes a pleasant cooling sensation and hence these creams are given the name ‘cold creams’. A simple formula for borax-beeswax cold cream is as follows.

(b) Oil-in-water ointment (Hydrophilic Ointment):

Oil in water type emulsion bases are used as vehicles for medicinal agents. Water being in the external phase they are easily removed with water alone from skin and linen. They are non-grease and non-sticky. Vanishing creams are often used as cosmetics.

The vanishing type of cream bases contain a large proportion of water which may be as high as 80% and this seems to account for the high release of medicaments from such bases. Vanishing creams essentially contain about 20% stearic acid (triple pressed) and a part of it is reacted with alkali to form soap in-situ. About 5 to 10% glycerin is also included in the formulation as humectant which can be substituted either partially or wholly by propylene glycol. The typical sheen of vanishing cream is due to stearic acid.

Truly speaking, vanishing cream should be regarded as dispersion and not an emulsion. Two simple formulae for vanishing cream are given below.

D. Water-soluble bases:

These bases are prepared from mixture of low and high molecular weight polyethylene glycols which range in their consistency from liquids to solids. Their water solubility is due to the presence of many polar groups and other linkages. They are non-volatile, unctuous, inert and possess the ability to form an emollient surface. They neither hydrolyse and deteriorate nor support mould growth.

Medicaments like benzoic and salicylic acids, phenol, tannic acid, bacitracin etc. have a solubilizing effect on bases consisting of high molecular weight polyethylene glycols. Although the diffusion of medicaments through such bases occurs readily yet the percutaneous absorption is very little. Some formulae for water-washable ointments containing polyethylene glycols are given below.

Example

PEG 400 monostearate 26.0 PEG 4000 42.5

PEG 400 37.0 PEG 400 37.5

PEG 4000 37.0 1, 2, 5-Hexanetriol 20.0

Some of the water-soluble bases are also prepared by employing glyceryl monostearate (GMS), cellulose derivative, sodium alginate, bentonite, colloidal magnesium aluminium silicate, and Carbopol 934. Carbopol 934 and acid polymer disperses readily in water to yield an acid solution of low viscosity. It is physiologically inert, non- irritating and non-sensitizer. It exhibits excellent compatibility with materials frequently incorporated in ointment formulations. Some examples of ointment bases employing materials like GMS, hydrocolloids and Carbopol 934 etc. are given below.

Example 01

Calcium citrate 0.05 g

Sodium alginate 3.0 g

Methyl paraben 0.20g

Glycerin 45.0 g

Purified water to make 100 g

Example 02

Petrolatum 32.0 g

Bentonite 13.0 g

Sodium lauryl sulphate 0.5 g

Purified water

Methyl paraben 54 g

0.1 g

Example 03

Mineral oil 10

White petroleum 30

GMS 10

Cetyl alcohol 05

Glycerin 05

Purified water 40

Example 04

Methocel 90 HC 4000 1.0g

Carbopol 934 0.3g

Propylene glycol 20.0ml

Methyl paraben 0.15g

Purified water q.s

Sodium hydroxide q.s pH 7.0 100ml

Each ointment base type has different physical characteristics and therapeutic uses based upon the nature of its components. The following table summarizes the composition, properties, and common uses of each of the five types of bases.