Liquid Dosage Forms - Disperse Systems: Emulsions Flashcards
Definition of emulsions
Thermodynamically unstable, heterogeneous systems consisting of at least one immiscible liquid intimately dispersed in another in the form of droplets/globules
What are the 2 liquid phase systems in emulsions
- Internal/dispersed/discontinuous phase
2. External/dispersion medium/continuous phase
Types of emulsions
- o/w - miscible with water, water washable, absorb water, nonocclusive, nongreasy
- w/o - insoluble in water, not water washable, absorb water, occlusive, may be greasy
- multiple emulsions: o/w/o, w/o/w
- special names: creams, lotions, microemulsions, fluid emulsions
How to determine emulsion types
- By phase ratio between oil and water phases
- By order of addition
- By type of emulsifier (phase that the emulsifier is soluble will most probably be the continuous phase)
- Experimentally - dilution test, electrical conductivity test
Applications of emulsions
- Preparation of a relatively stable and homogeneous mixture of 2 immiscible liquids
- Administration of an oil as a microscopic droplets rather than in bulk -> more palatable administration
- Taste or smell-masking of oils or oil-soluble drugs
- Dispersion into microscopic globules to render an oil more readily digestible and absorbed
- Parenteral nutrition: intravenous administration of an oil
- Dermatologicals: external application as ointment, cream, lotion, etc.
What are the theories of emulsification? What does emulsification do?
- Decrease surface tension (surface tension allows for formation of internal phase droplets where surfactants film prevent coalescence of the dispersed phase) https://i.stack.imgur.com/dLpVC.jpg
- Oriented wedge: monomolecular layers of emulsifier curves around droplet of internal phase (-> particles separate from each other -> wont come tgt)
Main ingredients of emulsion
- Drug - can be dissolved/dispersed in the oil/aqueous phase, can be the oil phase itself
- Oil phase - can be vegetable/mineral oil based on the application, can be altered using other miscible materials
- Aqueous phase - water soluble: preservatives, flavors, colors, buffers, thickening agents
- Emulsifiers - substances that concentrate at the surface of a liquid or interface of 2 liquids, decreasing surface tension and forming a protective barrier around the dispersed droplets; have both a hydrophilic and lipophilic portion; some agents enhance stability by imparting a charge on the droplet surface, thus decreasing physical contact between the droplets and minimizing potential for coalescence
Type of oil can affect
viscosity, spreading, film forming and drug transport
Types of emulsifiers
- Natural products - polysaccharides (produce o/w) (e.g. Acacia, tragacanth, agar and pectin), sterols (mainly dissolved in the oil phase) (e.g. cholesterols - produce w/o), phospholipids (e.g. lecithin - produce o/w)
- Surfactants - consist of 2 distinct parts, a hydrophobic tail and hydrophilic head group
- Finely divided solids - function as emulsifiers because of small particle size; concentrate at the liquid-liquid interface and form a particulate film around the dispersed droplets; particles wetted by aqueous phase -> o/w, wetted by oil phase -> w/o (e.g. colloidal clays, bentonite, Mg(OH)2, Al(OH)3)
Properties of emulsifying agents
- Surface active and reduce surface tension to below 10 dyn/cm
- Be absorbed quickly around the dispersed drops as a condensed, nonadherent film to prevent coalescence
- Increase viscosity of the emulsion
- Impart to the droplets an adequate electric potential -> mutual repulsion -> prevent droplets from coming together
- Promote optimum emulsification with min. amount
Pharmaceutical requirements of emulsifying agents
- Nontoxic
- Stable and not deteriorate
- Possess little odor, taste or color
- Compatible with other ingredients
- Does not interfere with the stability and efficacy of the drug
- Promote and maintain emulsification
How to decide on the amounts and types of surfactants needed in stable products
Use the hydrophile-lipophile balance system (HLB).
Low HLB -> low number of hydrophilic groups -> w/o emulsion.
High HLB -> large number of hydrophilic groups -> o/w emulsion
Methods of emulsion preparation
- Continental (dry gum, or 4:2:1) method
- English (wet gum) method
- Bottle (Forbes bottle) method
Methods of emulsion preparation: Describe Continental (dry gum, or 4:2:1) method
- Primary emulsion: 4 parts oil, 2 parts water, 1 part emulsifier
- In a mortar, 1 part gum is levigated with 4 parts oil until the powder is thoroughly wetted then 2 parts of water is added all at once and mixed vigorously and continually triturated until the primary emulsion is creamy white and produces a “crackling” sound as it is triturated. Additional water or aqueous solutions may be added after that.
- Solid substances are generally dissolved and added as a solution and oil soluble substances, in small amounts, may be incorporated directly into the primary emulsion
- Any substance that might reduce the physical stability of the emulsion should be added near the end of the process
- When all agents have been added, the emulsion should be transferred to a calibrated vessel where it is brought to final volume with water then blended to form uniform distribution of ingredients
Methods of emulsion preparation: Describe English (wet gum) method
1 part of gum triturated with 2 parts of water to form a mucilage -> 4 parts oil added slowly in portions while triturating -> mixture triturated for several mins to form the primary emulsion
Other ingredients may be added.
More difficult to perform successfully, esp with more viscous oils
Advantages: may give a more stable emulsion
