Emulsifying agents

                      Emulsifying agents

nIf oil and water are agitated, two phases will produced due to the interfacial tension, where the cohesive force between one phase is greater than the adhesive force between the two phases.

nThe cohesive force of each phase is manifested as an interfacial tension at the boundary between the liquids ( the greater the interfacial tension the greater the immiscibility between the two liquids.

nReduction of the interfacial tension makes a good emulsion.

nThe work done to bring two phases together (d W);

d W = ץ d S

where

ץ = interfacial tension

d S = change in the surface area

•The reduction of ץ usually carried out by using emulsifying agents.
nEmulsion type is determined 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, then oil will be the continuous phase and a w/o emulsion will be formed.
Classification of emulsifying agents:

Emulsifying agents can be classed into three groups:

nNaturally occurring

nSurfactants

nfinely divided solids.

A-Naturally occurring emulsifying agents:

•From animal or vegetable sources, the quality may vary from batch to batch and they are susceptible to microbial contamination.

1-Polysaccharides:

nAcacia

•used to prepared oral emulsions.

•Not used externally as it is too sticky.

nTragacanth

•is used to increase the viscosity of an emulsion and prevent creaming.

nOther polysaccharides, such as starch and pectin are use to stabilize an emulsion.
2-Semi-synthetic polysaccharides;

nLow viscosity grades of methylcellulose and carboxymethylcellulose will form o/w emulsions.

3-Sterol-containing substances:

nE.g., beeswax, woolfat and wool alcohols, produce w/o emulsions.

4-Proteins:

E.g., gelatin, which produces o/w emulsions. ,

5-Phospholipids :

nE.g., lecithin, which produces o/w emulsions.

B-Surfactants:

•These agents contain both hydrophilic and lipophilic regions in the molecule. They are classified according to their ionic characteristics as anionic, cationic nonionic and ampholytic. The latter are used as detergents and soaps but are not widely used in pharmacy.

•1-Anionic surfactants:

nIn water (ionized) have a surface active anion.

nThey are sensitive to cationic surfactants such as cetrimide.

nUsed in preparations of o/w emulsion for external use.

nEmulsions made with anionic surfactants are generally stable at alkaline pH.. Some examples include:

•Alkali metal and ammonium soaps such as sodium stearate (o/w).

•Soaps of divalent and trivalent metals such as calcium oleate (w/o).

•Amine soaps such as triethanolamine oleate (o/w).

•Alkyl sulphates such as sodium lauryl sulphate (o/w).
•2-Cationic surfactants:

nIn water (ionized) have a surface active cation.

nThey are sensitive to anionic surfactants such as the soaps

nUsed in preparations of o/w emulsion for external use.

nEmulsions formed by a cationic surfactant are generally stable at acidic pH. The cationic surfactants also, have anti-microbialactivity.

•These are usually quaternary ammonium compounds

Examples include

•cetrimide and

•bezalkoniumchloride.
3-Non-ionic 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 given by the HLB (hydrophilic lipophilic balance) number.

nHigh HLB numbers (8-18) indicate a hydrophilic molecule, and produce an o/w emulsion.

nLow HLB numbers (3-6) indicate a lipophilic molecule and produce a w/o emulsion. Examples include;

nTween 80, has HLB number of 15 and is more soluble in water to give o/w emulsion and

nSpan 80, has HLB number of 4.3 and is more soluble in oil to give w/o emulsion.

•C-Finely divided solids:

nThey can be adsorbed at the oil-water interface to form a coherent film that prevents coalescence of the dispersed globules.

nIf the particles are preferentially wetted by oil, a w/o emulsion is formed. Conversely, if the particles are preferentially wetted by water, an o/w emulsion is formed.

nThey form emulsions with good stability, which are less liable to microbial contamination than those formed with other naturally derived agents.

Examples are

•Bentonite, aluminium magnesium silicate are used for external preparations.

•Colloidal aluminium and magnesium hydroxides are used for internal preparations.

Mechanism of action of emulsifying agents:

•Emulsifying agents may be classified in accordance ,with the type of film they form at the interface between the two phases into;

I -Monomolecular films;

oForm a monolayer of adsorbed molecules or ions at the oil water interface and are capable of stabilizing the emulsion.

oE.g., potassium laurate and Tween 80.

2 -Multimolecular films;

oHydrated lyophilic colloids form multimolecular films around droplets of dispersed oil. It forms strong rigid films which produce o/w emulsion.

ohydrophilic colloids are adsorbed at an interface to form strong, coherent multimolecular films;

oExamples, acacia and gelatin.
3 -Solid particles films.

nThey form film of solid particles that are small in size compared to the droplet of dispersed phase, Particles must be wetted by both phases to some extent in order to remain at the interface and form a stable film.

–They can form either o/w or w/o emulsions, depending on method of preparation.
–Examples, bentonite, graphite, and magnesium hydroxide.