4.3 Compounding – Emulsions and Suspensions

Learning Outcomes

Be able to:

  • Define pharmaceutical emulsions and emulsifying agents, theory of emulsions.
  • Describe the pharmaceutical application of emulsions and suspensions
  • Identify the types of emulsions.
  • List the classifications of emulsions instability
  • Describe the HLB value and its application in emulsions
  • Summarise the steps in the preparation of emulsions and prepare
  • Describe the stability of emulsions.

An emulsion is a thermodynamically unstable dispersion of at least two immiscible (or partially miscible) liquids. IMMISCIBLE: DOES NOT MIX – INCAPABLE OF ATTAINING HOMOGENEITY

One liquid phase is uniformly dispersed within the other.

  • Dispersed phase (droplets) = internal phase
  • Continuous phase = external phase

The immiscible liquids are described as “oil” and “water” as one liquid is nonpolar (oil/wax/lipid) and the other is polar (water/aqueous solution)

Simple Emulsions take on the following forms:

Aqueous emulsion: Oil in water (O/W)

  • Water is external (continuous) phase.
  • Oil is internal (dispersed) phase.

Oily emulsion: water in oil (W/O)

  • Oil is external (continuous) phase.
  • Water is internal (dispersed) phase.

Multiple emulsions can be formed from oil and water by the re-emulsification of an emulsion to form 2 dispersed phases.

These are known as:

  • Oil-in-Water-in-Oil (O/W/O) or Water-in-Oil-in-Water (W/O/W) emulsions.
  • Oil-in-Water-in-Oil (O/W/O) are O/W emulsions dispersed in an oil phase.
  • Water-in-Oil-in-Water (W/O/W) are W/O emulsions dispersed in a water phase.
There are various advantages and disadvantages to emulsions
✔️Advantages:
❌Disadvantages:
  • Pharmaceutical emulsions may be used to deliver drugs that are poorly soluble in water but readily soluble in oils and vice versa.
  • Pharmaceutical emulsions may be used to mask the bitter taste and odour of drugs.
  • Provide protection to drugs that are susceptible to oxidation or hydrolysis.
  • Pharmaceutical emulsions are thermodynamically unstable, process to stabilise them is complex.
  • Bulky, difficult to transport, and prone to container breakages.
  • Liable to microbial contamination which can lead to cracking.
  • Uniform and accurate dose may not be achieved.
  • Separation of phases needs to be prevented by appropriate formulation

Where can we find emulsions?

Emulsions can be formulated for virtually all routes of administration. Most commercially available products are oral, parenteral, or topical.

  • Oral and IV emulsions are usually O/W.
  • Topical, SC and IM emulsions may be both O/W or W/O.

Oral-mineral oil emulsion

  • Rectal e.g. castor oil
  • Topical e.g. calamine lotion

Ophthalmic emulsions:

  • Cyclosporin and Difluprednate (an o/w emulsion for keratoconjunctivitis)

Injectable emulsions:

  • Total parenteral nutrition (Liposyn, 10% soy oil, 20% safloweroil, egg phospholipid, water).

In injectable emulsions, total amount of oil should not be more than 20-30%, due to limit of emulsifier (surfactant) concentration.

  • Unpalatable drugs may be administered in a palatable form eg cod liver oil emulsion
  • Increasing absorption of oil soluble drugs through intestinal wall (griseofulvin emulsion)
  • Intramuscular injection of some water-soluble vaccines provide slow release and greater antibody response while formulated as emulsions
  • Liquid dosage forms of incompatible ingredients may be formulated by dissolving or suspending each ingredient in one of the phases of an emulsion formulation.

Emulsifying Agents

The thermodynamically unstable system that is an emulsion, is stabilised by the addition of an emulsifying agent.

Surfactants/Emulgents/Emulsifying agents assist in the formation of emulsions. They adsorb at the interface between oil and water, creating droplets, also known as micelles, thereby decreasing the surface tension, and preventing coalescence. Thus, stabilising the emulsion.

Micelles are spherical structures that are formed when surfactants reach a certain concentration in water.

🎞️ Let’s watch a video the formation of a micelle:

Surfactants are classified into:

  • Anionic surfactants e.g. sodium lauryl sulphate, sodium stearate, calcium oleate
  • Cationic surfactants e.g. cetrimide, benzalkonium chloride
  • Non-Ionic surfactants e.g. polysorbates, glyceryl monostearate, cetomacrogol
  • What is surface tension and how do surfactants reduce surface tension?

🎞️ Let’s watch a video on Surfactants and Surface Tension ⏰ 7 Minutes

To ensure maximum stability

  • Mixtures of surfactants with high and low HLB are always better
  • The solubility of surfactants in both phases should be appropriate to maintain the stability of the product
  • Emulsifier(s) should produce an optimum HLB which is required to produce stable emulsions

🎞️ Let’s watch a video on Surfaces Presented by Dr Martina Mylrea ⏰ 16 Minutes

🎞️ Let’s watch a video on Emulsions 26 Minutes

Emulsion Formulation Excipients

When producing an emulsion, the following excipients would be considered:

  • Vehicles for w/o or o/w emulsions (suitable solvents)
  • Viscosity enhancer/Density modifiers: hydrophilic polymers
  • Buffers for pH adjustment
  • Humectants: glycerine
  • Antioxidants: ascorbic acid, propyl gallate
  • Flavours, colours for oral administration
  • Sweetening agents for oral administration
  • Preservatives: benzoic acid, phenol, benzalkonium chloride etc
  • To increase the viscosity, emulsion thickeners may be used

🎞️ Let’s watch a video on Emulsion Preparation:

 

Suspensions

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