Emulsions

35


Emulsions




Study Points



image The uses of pharmaceutical emulsions


image The different types of emulsion and their identification


image The formulation of emulsions


image Selection of emulsifying agents and other ingredients


image The dispensing processes for emulsions



Introduction


An emulsion consists of two immiscible liquids, one of which is uniformly dispersed throughout the other as fine droplets normally of diameter 0.1–100 μm. To prepare a stable emulsion, a third ingredient, an emulsifying agent, is required. Oral emulsions are stabilized oil-in-water dispersions that may contain one or more active ingredients. They are a useful way of presenting oils and fats in a palatable form. Emulsions for external use are known as lotions, applications or liniments if liquid, or creams if semi-solid in nature. Some parenteral products may also be formulated as emulsions. Most important of these is total parenteral nutrition (see Ch. 44). Pharmaceutically, the term ‘emulsion’, when no other qualification is used, is taken to mean an oil-in-water preparation for internal use.



Pharmaceutical applications of emulsions


Emulsions have a wide range of uses, including:



Examples of traditionally used emulsions for oral use are cod liver oil emulsion (see Example 35.1), Liquid Paraffin Oral Emulsion BP (see Example 35.2). An example of emulsion for external use is Oily Calamine Lotion BP (see Example 35.3).



Example 35.1


Prepare 200 mL cod liver oil emulsion to the following formula:













Cod-liver oil 60 mL
Chloroform 0.4 mL
Cinnamon water to 200 mL


Example 35.2


Prepare 100 mL Liquid Paraffin Oral Emulsion BP 1968.

























Liquid paraffin 50 mL
Vanillin 50 mg
Chloroform 0.25 mL
Benzoic acid solution 2 mL
Methylcellulose 20 2 g
Saccharin sodium 5 mg
Water to 100 mL


Example 35.3


Prepare 100 mL Oily Calamine Lotion BP 1980.



















Calamine 5 g
Wool fat 1 g
Oleic acid 0.5 mL
Arachis oil 50 mL
Calcium hydroxide solution to 100 mL


Emulsion types


Emulsions may be oil-in-water (o/w) emulsions, where oil is the disperse phase in a continuous phase of water, or water-in-oil (w/o) emulsions, where water is the disperse phase in a continuous phase of oil. It is also possible to form a multiple emulsion, e.g. a water droplet enclosed in an oil droplet, which is itself dispersed in water–a w/o/w emulsion. Multiple emulsions are increasingly used in manufactured pharmaceutical products and are used for delayed action drug delivery systems.


If the emulsion is for oral or intravenous administration it will always be oil-in-water. Intramuscular injections may be water-in-oil for depot therapy. When selecting emulsion type for preparations for external use, the therapeutic use, texture and patient acceptability will be taken into account. Oil-in-water emulsions are less greasy, easily washed off the skin and more cosmetically acceptable than water-in-oil emulsions. They have an occlusive effect, which hydrates the upper layers of the skin (called an emollient, see Ch. 36). Water-in-oil emulsions rub in more easily.



Identification of emulsion type


There is a range of tests available to identify the emulsion type. Some of the tests that can be used are outlined below.


Miscibility test. An emulsion will only mix with a liquid that is miscible with its continuous phase. Therefore an o/w emulsion is miscible with water, a w/o emulsion with an oil.


Conductivity measurement. Systems with an aqueous continuous phase will conduct electricity, while systems with an oily continuous phase will not.


Staining test. A dry filter paper impregnated with cobalt chloride turns from blue to pink on exposure to stable o/w emulsions.


Dye test. If an oil-soluble dye is used, o/w emulsions are paler in colour than w/o emulsions. If examined microscopically, an o/w emulsion will appear as coloured globules on a colourless background while a w/o emulsion will appear as colourless globules against a coloured background.



Formulation of emulsions


An ideal emulsion has globules of disperse phase that retain their initial character, that is the mean globule size does not change and the globules remain evenly distributed. The formulation of emulsions involves the prevention of coalescence of the disperse phase (often called ‘cracking’) and reducing the rate of creaming.



Emulsifying agents


Emulsifying agents help the production of a stable emulsion by reducing interfacial tension and then maintaining the separation of the droplets by forming a barrier at the interface. Most emulsifying agents are surface-active agents. Emulsion type is determined mainly by the solubility of the emulsifying agent. If the emulsifying agent is more soluble in water (i.e. hydrophilic), then water will be the continuous phase and an o/w emulsion will be formed. If the emulsifying agent is more soluble in oil (i.e. lipophilic), oil will be the continuous phase and a w/o emulsion will be formed. If a substance is added which alters the solubility of the emulsifying agent, this balance may be altered and the emulsion may change type. The process is called phase inversion. The ideal emulsifying agent is colourless, odourless, tasteless, non-toxic, non-irritant and able to produce stable emulsions at low concentrations.


Emulsifying agents can be classed into three groups: naturally occurring, synthetic surfactants and finely divided solids.



Naturally occurring emulsifying agents


These agents come from vegetable or animal sources. Therefore, the quality may vary from batch to batch and they are susceptible to microbial contamination and degradation.


Polysaccharides. Acacia is the best emulsifying agent for extemporaneously prepared oral emulsions as it forms a thick film at the oil–water interface to act as a barrier to coalescence. It is too sticky for external use. Tragacanth is used to increase the viscosity of an emulsion and prevent creaming. Other polysaccharides, such as starch, pectin and carrageenan, are used to stabilize an emulsion.


Semi-synthetic polysaccharides. Low-viscosity grades of methylcellulose (see Example 35.2) and carboxymethylcellulose will form o/w emulsions.


Sterol-containing substances. These agents act as water-in-oil emulsifying agents. Examples include beeswax, wool fat and wool alcohols (see Ch. 36).



Synthetic surfactants


These agents are classified according to their ionic characteristics as anionic, cationic, non-ionic and ampholytic. The latter are used in detergents and soaps but are not widely used in pharmacy.


Anionic surfactants. These are organic salts which, in water, have a surface-active anion. They are incompatible with some organic and inorganic cations and with large organic cations such as cetrimide. They are widely used in external preparations as o/w emulsifying agents. They must be in their ionized form to be effective and emulsions made with anionic surfactants are generally stable at more alkaline pH.


Some pharmaceutical examples of anionic surfactants include:



Cationic surfactants. These are usually quaternary ammonium compounds which have a surface-active cation and so are sensitive to anionic surfactants and drugs. They are used in the preparation of o/w emulsions for external use and must be in their ionized form to be effective. Emulsions formed by a cationic surfactant are generally stable at acidic pH. The cationic surfactants also have antimicrobial activity. Examples include cetrimide and benzalkonium chloride.


Non-ionic surfactants. These are synthetic materials and make up the largest group of surfactants. They are used to produce either o/w or w/o emulsions for both external and internal use. The non-ionic surfactants are compatible with both anionic and cationic substances and are highly resistant to pH change. The type of emulsion formed depends on the balance between hydrophilic and lipophilic groups which is expressed as the HLB (hydrophilic–lipophilic balance) number (see below). Examples of the main types include glycol and glycerol esters, macrogol ethers and esters, sorbitan esters and polysorbates.


The HLB (hydrophilic–lipophilic balance) system. An HLB number, usually between 1 and 20, is allocated to an emulsifying agent and represents the relative proportions of the lipophilic and hydrophilic parts of the molecule. The lower the number, the more oil soluble the emulsifying agent. Higher numbers (8–18) indicate a hydrophilic molecule which produces an o/w emulsion. Low numbers (3–6) indicate a lipophilic molecule which produces a w/o emulsion. Oils and waxy materials have a ‘required HLB number’ which helps in the selection of appropriate emulsifying agents when formulating emulsions. Liquid paraffin, for example, has a required HLB value of 4 to obtain a w/o emulsion and 12 for an o/w emulsion. Two or more surfactants can be combined to achieve a suitable HLB value and often give better results than one surfactant alone. HLB values of some commonly used emulsifying agents are given in Table 35.1.


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Jun 24, 2016 | Posted by in PHARMACY | Comments Off on Emulsions

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