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Pharmaceutical Emulsion - Types, Advantages, Disadvantages and Its Identification

Pharmaceutical Emulsion

The pharmaceutical emulsion may be defined as a thermodynamically unstable biphasic system consists of two immiscible liquids, one of which is finely and uniformly dispersed (the dispersed phase)as globules throughout the second phase (the continuous phase). As emulsions are a thermodynamically an unstable system, a third agent, the emulsifier/emulsifying agent is added to stabilize the system.
It is a biphasic liquid dosage form in which two immiscible liquids phase change into a stable uniform system with the help of an emulsifying agent. One phase is dispersed as droplets in the dispersion medium. Globule diameter in the range of 0.1-100µm. Emulsion also called a heterogeneous system or biphasic system.
Natural emulsions – milk, rubber latex, crude oil, etc.
O/W type emulsion – Dispersed phase à Oil, dispersion media/continuous phase a water
W/O type emulsion – Dispersed phase à water, dispersion media/ continuous phase a oil
The ratio of disperse phase volume to the total volume is known as phase volume or phase volume ration.
Poly disperse system (emulsion within emulsion) à with more than one discontinuous phase may be possible like double and multiple emulsion like O/W/O, W/O/W, They used for sustained drug release dosage form.

Microemulsion – Globule size is 0.01 – 0.1 µm and transparent.
Macroemulsion/conventional emulsion/coarse emulsion – Globule size is 0.1 – 100 µm, opaque in appearance. (Submicron emulsion = 0.1 – 1 µm)
O/W emulsion used for internal and external use while W/O used only externally as cream and lotion for emollient use.

Types of emulsion

Oil in water emulsion

If the oil droplets are dispersed in the aqueous phase, the emulsion is termed as oil-in-water (O/W) emulsion. They are non-greasy. They are easily removable from the skin surface and they are used externally to provide a cooling effect and internally to mask the bitter taste of the oil. O/W emulsion gives a positive conductivity test as water, the external phase is a good conductor of electricity.

Water in oil emulsion

A system in which the water is dispersed as globules in the oil continuous phase is termed as water-in-oil (W/O)emulsion. They are greasy. They are not water washable and are used externally to prevent evaporation of the moisture from the surface of skin e.g. cold cream.
W/O emulsion is not gives a positive conductivity tests, because oil is the external phase which is a poor conductor of electricity.

Multiple emulsions

Multiple emulsions are complex systems.  They can be considered as emulsions of emulsions.
Their pharmaceutical applications include taste masking, adjuvant vaccines, immobilization of enzymes and sorbent reservoir of overdose treatments, and sometimes for the augmentation of external skin or dermal absorption.

Advantages of Pharmaceutical emulsion

  • Unpalatable oils can be administered in a palatable form.
  • Unpalatable oil-soluble drugs can be administered in a palatable form.
  • The aqueous phase is easily flavored.
  • The oily sensation is easily removed.
  • The rate of absorption is increased.
  • It is possible to include two incompatible ingredients, one in each phase of the emulsion.
  •  Mask the unpleasant taste – Ex. Laxative, phenolphthalein, vit. A.
  • Improved bioavailability – Griseofulvin, insulin and heparin.
  • Sustained release medication – Water-soluble antigenic materials are dispersed in mineral oil and are given as an intramuscular injection (depot) and release antigen over a long period from depot.
  • Nutritional supplement – Fats dissolved in the oil phase and water-soluble nutrients are incorporated in the aqueous phase. This used for malnourished and stressed patients.
  • Diagnostic purposes – Radio opaque emulsions are used as diagnostic material in x-ray examination.
  • Topical use – Cream, lotions, liniment and aerosol.
  • Emulsified perfluorocarbon formulations have been considered for use as oxygen-transport fluids or blood substituents.

Disadvantages of Pharmaceutical emulsion

  • It needs to be shaken well before use.
  • A measuring device is needed for administration.
  • A degree of technical accuracy is needed to measure a dose.
  • Storage conditions may affect stability.
  • Bulky, difficult to transport, and prone to container breakages.
  • Liable to microbial contamination which can lead to cracking.
Emulsifying agent for O/W – gum acacia, tween, veegum, tragacanth, methylcellulose, saponins, and soaps formed from monovalent bases like Na+, K+ and NH4+.
Emulsifying agent for W/O – Wool fat, resins, beeswax, and soaps formed from divalent bases like Ca+, Mg+2, and Zn+2. (Bentonite used for both emulsions).
The emulsion has white color due to light refraction.

Tests for Identifying Emulsion Types

Dilution test/miscibility test 

Miscibility test involves the addition of continuous phase, e.g.  In case of O/W emulsion; the emulsion remains stable upon unlimited addition of water but will be unstable upon unlimited addition of oil, that is, the oil will separate. Vice versa is the case with W/O emulsion.

Electrical conductivity test

Water is a good conductor of electricity; hence, an emulsion with the water continuous phase will readily conduct electricity while that with oil continuous phase will not.

Staining test/dye-solubility test

In this test, a small amount of water-soluble dye, such as methylene blue is added to the emulsion, if the water is the continuous phase (O/W emulsion), the dye will dissolve uniformly throughout the system. If the oil is the continuous phase (W/O emulsion), the dye will remain as a cluster on the surface of the system

Identification test for emulsion

  • Cobalt chloride test – Color changed blue to pink with o/w emulsion but not changed with w/o emulsion.
  • Conductivity test – o/w emulsion than bulb glow because more water has conductivity.
  • w/o emulsion than bulb not glow because more oil has not conductivity.
  • Dilution test – Phase inversion.
  • Dye test – Water solution dye (amaranth) mix to unknown emulsion, if droplets are colored than emulsion is w/o, if dispersion media colored than emulsion is o/w type.
  • Direction of creaming – Downward – w/o type emulsion, upward – o/w type emulsion.
  • Always oil moves upward and water more downward because oil<water density.
  • Filter paper test – o/w emulsion spread rapidly on filter paper while w/o spread or migrate slowly.
  • Fluorescence test – Oil has fluorescence character so that if dispersion media fluorescent – w/o emulsion droplets fluorescent – o/w emulsion.