Emulsion

PHRM 210

Emulsion

• An emulsion is a thermodynamically unstable system consisting of at least two immiscible liquid phases, one of which is dispersed as globules (dispersed phase) in the other liquid phase (continuous phase), stabilized by the presence of an emulsifying agent.

• Emulsified systems range from lotions of relatively low viscosity, to ointments & creams, which are semisolid in nature.

Fig: Good emulsion

Types of Emulsion

• One liquid phase in an emulsion is essentially polar (eg, aqueous), while the other is relatively nonpolar (eg, an oil).

Oil-in-water (o/w) emulsion: • When the oil phase is dispersed as globules

throughout an aqueous continuous phase, the system is referred to as an oil-in- water (o/w) emulsion.

Types of EmulsionWater-in-oil (w/o) emulsion:• When the oil phase serves as the continuous

phase, the emulsion is termed an water-in-oil (w/o) emulsion.

Multiple (w/o/w or o/w/o) emulsions: • These are emulsions whose dispersed phase

contains droplets of another phase. Multiple emulsions are of interest as delayed-action drug delivery systems.

Why emulsions? Or Advantages

• Permits administration of liquid drug in the form of tiny globules rather than in bulk.

• o/w emulsion can be developed to mask the offensive taste of oil.

• Irritating medicinal agents to be applied externally onto the skin (i.e. lotion or cream)

Why emulsions? or Advantages

o/w vs. w/o for topical preparations-• w/o emulsions- spreads more evenly on

damaged skin to protect• o/w emulsions- easily removed from skin

Identification of emulsion

• Miscibility tests with oil or water- the emulsion will only be miscible with liquids that are miscible with its continuous phase.

• Conductivity measurements- systems with aqueous continuous phases will readily conduct electricity whilst systems with oily continuous phases will not.

Identification of emulsion

• Staining test- Water-soluble and oil soluble dyes are used one of which will dissolve in and color the continuous phase.

Emulsifying agents

• To prevent coalescence (when small droplets recombine to form bigger ones), it is necessary to introduce an emulsifying agent that forms a film around the dispersed globules.

Emulsifying agents: Properties

• Stable, nontoxic, nonirritating and inert• Should be surface active• Should prevent coalescence• Should impart an adequate electrical

potential on the droplets of internal phase• Should be effective in low concentration• Should be odorless, tasteless and colorless

Classification

Emulsifying agents may be divided into three major groups:

• Surface active agents• Hydrophilic colloids• Finely divided solid particles

Surface active agents

• The name surfactant is short for “surface active agents”. Surfactants are adsorbed at oil-water interfaces to form monomolecular films and reduce interfacial tensions.

• Often a mixture of surfactants is used: one with hydrophilic character and the other with hydrophobic character. A hydrophilic emulsifying agent is needed for the aqueous phase, and a hydrophobic emulsifying agent is needed for the oil phase.

Surface active agents

• Nonionic surfactants are widely used in the production of stable emulsions. They are less toxic than ionic surfactants and are less sensitive to electrolytes and pH variation.

• Examples include sorbitan esters, polysorbates.

Fig: The action of surfactants creating the oil/water interface

Hydrophilic colloids

• A number of hydrophilic colloids are used as emulsifying agents. These include gelatin, casein, acacia, cellulose derivatives, and alginates.

• These materials adsorb at the oil-water interface and form multilayer films around the dispersed droplets of oil in an o/w emulsion.

Finely divided solid particles

• Finely divided solid particles are adsorbed at the interface between two immiscible liquid phases and form a film of particles around the dispersed globules.

• Finely divided solid particles that are wetted to some degree by both oil and water can act as emulsifying agents.

Finely divided solid particles

• For external use- bentonite and aluminum magnesium silicate.

• For internal use- aluminum and magnesium hydroxide.

HLB (Hydrophile-Lipophile balance) systems:

• Griffin’s method of selecting emulsifying agents is based on the balance between the hydrophilic and lipophilic portions of the emulsifying agent; this is now widely known as the hydrophile-lipophile balance (HLB) system.

HLB System

• The higher the HLB value of an emulsifying agent, the more hydrophilic it is. The emulsifying agents with lower HLB values are less polar and more lipophilic.

• The Spans, i.e., sorbitan esters, are lipophilic and have low HLB values (1.8-8.6); the Tweens, polyoxyethylene derivatives of the Spans, are hydrophilic and have high HLB values (9.6-16.7).

HLB System

• The HLB of an emulsifier or a combination of emulsifiers determines whether an o/w or w/o emulsion results. In general, a value from-

- 3 to 6 indicates a w/o emulsifier - 7 to 9 indicates a wetting agent - 9 to 12 indicates an o/w emulsifier - 12 to 15 indicates a detergents - 15 to 20 indicates a solubilizer

Emulsion stability

Stability of emulsion means it should retain its original characters, it depends on- » Size of particles » Uniform distribution

Four major changes associated with physical stability are:- Flocculation - Creaming- Coalescence & - Breaking

Flocculation

• Particles within the emulsion form large aggregates, which can be easily redispersed upon shaking.

• It is a precursor of coalescence.

Fig: Flocculated emulsion

Creaming

• It occurs when the disperse droplets or floccules separate from the disperse medium.

• Generally a creamed emulsion can be restored to its original state by gentle agitation.

• Reducing the droplet sizes and thickening the continuous phase can minimize the creaming.

Fig: Creamed emulsion

Coalescence

• It is a much more serious type of instability. • It occurs when the mechanical or electrical

barrier is insufficient to prevent the formation of progressively large droplets.

Fig: Coalescence

Breaking

• Due to coalescence and creaming combined, one phase separates completely from the other phase so that it floats at the top in a single, continuous layer.

Fig: Breaking

o o

Flocculation

o o

Coalescence

o

w

w

Figure: Schematic presentation of flocculation and coalescence of emulsion droplet

Preservation of emulsions

Growth of microorganisms in emulsion causes:• Physical phase separation: Partitioning • Discoloration: Turns white or brown• Gas emission (possibly)• Odor formation• Changes in rheological properties

Preservation of emulsions

• Microorganisms degrade the emulsifying agent.

• Use of preservatives is essential, for example- Methylparaben and Propylparaben may be used.

Phase inversion

• The relative volume of internal and external phases of an emulsion is important.

• Increase internal phase concentration: Increase viscosity up to a certain point.

Phase inversion

• Viscosity will decrease after that point. At this point the emulsion has undergone inversion i.e. it has changed from an o/w to a w/o, or vice versa.

• In practice, emulsions may be prepared without inversion with as much as about 75% of the volume of the product being internal phase.