There are four types of base that are used to formulate pharmaceutical ointments and pastes: (1) hydrocarbon; (2) absorption; (3) water-miscible/removable; and (4) water-soluble.
Hydrocarbon bases are non-aqueous formulations, based on various paraffins that have the following properties:
- emollient, thereby restricting water loss from the site of application due to the formation of an occlusive film
- excellent retention on the skin
- predominantly hydrophobic, and therefore difficult to remove from the skin by washing and difficult to apply to (spread-over) wet surfaces (e.g. mucous membranes, wet skin)
- only a low concentration (5%) of water may being corporated into hydrocarbon bases (with careful mixing)
- Chemically inert.
Hydrocarbon bases frequently contain the following components: (1) Hard paraffin; (2) white/yellow soft paraffin; (3) liquid paraffin (mineral oil); and (4) microcrystalline wax.
This is a mixture of solid saturated hydrocarbons that are derived from petroleum or shale oil. Hard paraffin is a colourless or white wax-like material that is physically composed of a mixture of microcrystals. The melting temperature of hard paraffin is between 47 and 65°C and, when solid, it is used to enhance the rheological properties of ointment bases.
White/yellow soft paraffin
This is a purified mixture of semisolid hydrocarbons (containing branched, linear and cyclic chains) that are derived from petroleum. White/yellow soft paraffin consists of microcrystals embedded in a gel composed of liquid and amorphous hydrocarbons that are themselves dispersed in a gel phase containing liquid and amorphous hydrocarbons. The melting range of the soft paraffins is between 38 and 60°C. White soft paraffin and yellow soft paraffin (the former being a bleached form of yellow soft paraffin) may be used as an ointment base without the need for additional components, although it may be combined with liquid paraffin
Liquid paraffin (mineral oil)
This is a mixture of saturated aliphatic (C14–C18) and cyclic hydrocarbons that have been refined from petroleum. It is usually formulated with white/yellow soft paraffin to achieve the required viscosity for application to the required site.Formulations containing liquid paraffin require the incorporation of an antioxidant due to the ability of this material to undergo oxidation.
This is a solid mixture of saturated alkanes (both linear and branched) with a defined range of carbon chain lengths (C41–C57). This excipient is used to enhance the viscosity of ointments (and creams). One of the advantages of microcrystalline wax is the greater physical stability provided to formulations containing liquid paraffin (reduced bleeding of the liquid component).
Unlike hydrocarbon bases, absorption bases may be formulated to contain significant amounts of an aqueous phase. These may be either non-aqueous formulations to which an aqueous phase may be added to produce a water in oil emulsion (termed non-emulsified bases) or water in oil emulsions that can facilitate the incorporation of an aqueous phase (without phase inversion orcracking). Although absorption bases can accommodate a larger volume of aqueous phase than hydrophobic bases, they are still difficult to remove from the site of application by washing. This is due to the predominantly hydrophobic properties of this formulation class.
These are water-miscible bases that are used to form oil in water emulsions for topical applications. The use of these bases offers a number of advantages, including: ·
They are able to accommodate large volumes of water, e.g.aqueous solutions of drug, excess moisture at the site of application, e.g. exudate from abrasions and wounds. ·
They are not occlusive. ·
They may be easily washed from the skin and from clothing. Furthermore, they may be readily applied to (and removed from) hair. ·
They are aesthetically pleasing.
The British Pharmacopoeia describes three water-miscible/removable bases: 1. Emulsifying ointment 2. cetrimide emulsifying ointment 3. cetomacrogol emulsifying ointment.
Each of these contains:liquid paraffin 20% w/w, white soft paraffin 50% w/w, anionic, cationic or non-ionic emulsifying wax 30% w/w.As may be observed, an important component of this ointment-base is emulsifying wax, of which there are three types: (1)anionic; (2) non-ionic; and (3) cationic.
The reader will have observed that the three previous ointment bases are predominantly hydrophobic, are hydrophobic with added surface-active agents or are water-miscible, containing both water soluble and insoluble components. In contrast, water-soluble bases are composed entirely of water-soluble ingredients.
The advantages of the use of these bases include:
They are non-greasy and may be easily removed by washing.
They are miscible with exudates from inflamed sites.
They are generally compatible with the vast majority of therapeutic agents.