Pharmaceutical Emulsions | Types of pharmaceutical emulsions | Applications | Formulation | Instability pdf|

 

Emulsions are thermodynamically unstable heterogeneous biphasic systems consisting of at least one immiscible liquid, which is dispersed as globules (the interal, discontinuous or dispersed phase) in the other liquid phase (the external, continuous phase or dispersion medium), which is stabilised by the presence of an emulsifying agent. Internal phase droplets are generally in the range of diameter 0.1 mm 100 mm.

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1) Oil-in-water (o/w) emulsions: Emulsions are biphasic liquid systems, one phase of which is usually polar (i.e. aqueous), while the other is relatively non-polar (i.e. oil). When oil droplets are dispersed in the continuous aqueous phase, the emulsion is termed as oil-in-water (o/w) emulsion.

2) Water-in-oil (w/o) emulsions: When oil phase serves as the continuous phase and wate droplets as internal phase, the emulsion is water-in-oil (w/o) emulsion.

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3) Multiple emulsions: These are of two types, o/w/o and w/o/w type.
The oil-in-water-in-oil (o/w/o) emulsions are these emulsions in which the water droplets of the w/o emulsion enclose small oil droplets as internal phase in water-in-oil-in-water (w/o/w) emulsions, water droplets are enclosed in internal oil phase of alw emulsion.
4) Micro-emulsions: As per the globule size, emulsions can be classified into micro emulsions and macro-emulsions. When the globule diameter is as small as 1-500 mm, the emulsions are termed micro-emulsions or micellar emulsions and they appear as clear transparent solutions.

5) Macro-emulsions: Emulsions with globules of mean diameters between 0.2 to 10 mm are considered to be fine emulsions are referred to as coarse / macro emulsions Unike micro-emulsions, macro-emulsions are generally milky in appearance. This topic deals with micro emulsions.
6) Physical state: Emulsions may be in the liquid form for internal or extemal use or with macro-emulsions semisolid form for external application, The latter is termed creams. Most of the pharmaceutical emulsions designed for oral administrations, are of o/w type, whereas
Liniments, lotions and creams are either
o/w or w/o type.

☆ Pharmaceutical Applications of Emulsions

i)  Emodsions for oral use, help mask the taste of unpalatable oils/oil-soluble agents. For example, the external water phase in cod unpleasant taste of the oil. The external medicinal liver oil emulsion-masks the phase can also be suitably flavoured to enhance the masking effect.

ii) Emulsions permit the administration of a liquid drug in a subdivided form, which enhances the rate and extent of drug absorption. effect.

iii) Water sensitive drugs can be formulated as liquid dosage form in the form of liver oil emulsion-masks the phase can also be suitably flavoured to o/w emulsions.

iv) Oily emulsions provide unfavourable conditions for microbial growth.

v) For topical application, creams are preferable to ointments due to their spreading ability and easy washability, their less greasy nature and improved appearance.
vi) Parenterally, o/w emulsions containing fats, vitamins and carbohydrates are
administered W/o emulsions containing
intravenously to patients who cannot take food by mouth oil-soluble drug provide an effective sustained release mechanism. These are injected by intramuscular route.

Formation of emulsion

The emulsion, immiscible liquids are triturated to produce small droplets of both the phases. This step involves two processes, first, the dispersion of one liquid throughout another in the form of droplets and second, the coalescence (rejoining) of droplets of similar phases to form initial bulk liquids. The higher surface-free energy of droplets makes the system thermodynamically unstable. When two or more drops of the same liquid come in contact with one another, they tend to coalescence, making one larger drop having a lesser surface free energy than the total surface free energy of an individual drop.

The physically stable emulsion contains uniform distribution of droplets of either phase throughout the continuous medium. It means any one phase remains as droplets and the other phase undergoes coalescence to form a continuous medium. The third agent, an emulsifying agent, is required to maintain the integrity of the individual droplets of either of the phases, formed during compounding, by preventing coalescence.

 

☆Formulation of Emulsion:-

INSTABILITY OF EMULSIONS

Cracking (Coalescence)

When two or more globules of internal phase approach each other, they may undergo coalescence and separate out as a distinct layer.
1) Interfacial tension: Dispersion of immiscible liquids makes the system more energetic due to the increase in surface free energy (AG).

AG = YAA

Where. Y interfacial tension and AA increase in total surface area. Such
thermodynamically unstable emulsions tend to aggregate, to decrease the total energy, but it makes the emulsions physically unstable. Lowering interfacial tension can lower the increased energy, AG, and in turn the rate of coalescence. Surface-active agents reduce interfacial tension.

2) Interfacial film: Droplets come closer shaking or vibration, or which allows the content of the droplets to combine to form larger droplets. The flexible interfacial film of sufficient mechanical strength, formed by an emulsifier can minimise the coalescence.

3) Electrostatic forces: A system with higher inter-droplet electrostatic repulsion does not allow dispersed globules to coalesce and, this repulsion between droplets, results in a stable emulsion. Electrostatic forces can be adjusted by addition of electrolytes in optimum
concentration.

4) Physicochemical interaction: The emulsion containing incompatible ingredients could lead to cracking of an emulsion. For example,
Addition of polyvalent soaps, or use of hard water, to prepare emulsion-containing
monovalent soap as emulsifying agent.
1) Combination of cationic and anionic emulsifying agents will produce a cracked emulsion due to neutralisation.
Hydrophilic emulsifiers, such as acacia, tragacanth and gelatin precipitate by the
addition of organic solvent, like alcohol. Alkali soap emulsifying agents tend to decompose in acidic pH. Electrolytes, if used in other than optimum concentrations, change electrostatic forces

v) Dispersed phase and dispersed medium dissolve in a common solvent to form a emulsion.

5) Microbial degradation: Moulds, yeasts and bacteria can bring about the decomposition of system, i.e., Soap liniment. an emulsifying agent, producing a weak interfacial film. This can be avoided by addition of emulsion. 

6) Excess of dispersed phase: Chances of collision are of droplets more in concentrated ions, which in turn increase probability of coalescence.


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