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What is emulsion

An emulsion is a system consisting of two immiscible liquid phases, one of which is dispersed as fine globules throughout the other.
This system is stabilized by the addition of an emulsifying agent


Advantage of Emulsion

Can vary components to give varying properties (flexibility) = viscosity and greasiness

Emulsion tend to look more elegant (cosmetic)

Mask bad taste; incorporate drug into dispersed phase - further mask by adding flavor into continuous phase

Slow release: drug must partition from disperse phase to continuous phase then to cell (longer time to diffuse)


Types of emulsion

Simple emulsion
Multiple emulsion
Microemulsion (10-75nm; transparent to the naked eye)

Micellar emulsion
They consist of a relatively high concentration of surfactant and a small proportion of disperse phase which is solubilized by the surfactant.
The surfactant exists as micelles (5-20 nm) which are extremely small. Hence, these emulsions appear transparent to the naked eye.

Two types:
Oil-in-water micellar emulsion
This consists of oil which is found in the non-polar interior of the micelles which are present in an aqueous continuous phase.

Reverse micellar emulsion This consists of water which is found in the polar interior of the micelles which are present in an oily continuous phase.


why is there a need to add emulsifying agent

When oil and water are mixed and agitated droplets of varying sizes are produced. A tension exists at the interface because the 2 immiscible phase tends to have different attractive forces for a molecule at the interface.The system is thermodynamically unstable. Therefore respective droplets tend to coalesce together causing phase separation which is more stable.

Hence to prevent this we have to add an emulsifying agent.


How emulsifying agents stabilize an emusion

3 theories

1) Formation of a rigid interfacial film
A tightly packed film contributes to the stability of emulsion.
Cholesterol + Sodium cetyl sulphate
Closely packed condensed complex --> Stable emulsion

Oleyl alcohol + Sodium cetyl sulphate
Loosely packed condensed complex --> Poor emulsion

Cetyl alcohol + Sodium oleate
Fairly closely packed but no complex formation
Fairly poor emulsion

to fulfilled this 2 criteria must be met
- 2 types of surfactant must interact w one another to form a complex
- they must form a tightly packed film

(2) Formation of an electric double layer
This applies to ionic surfactants.
The electric double layer serves as an electrical barrier to approach of droplets.
(prevent globules from coming together)

3) Increase viscosity of the continuous phase
Gums and waxes (hydrophilic colloid) are commonly used to increase the viscosity of water and oil respectively.
A more viscous emulsion is generally more stable.


formation of rigid interfacial film critria

o fulfilled this 2 criteria must be met
- 2 types of surfactant must interact w one another to form a complex
- they must form a tightly packed film


General theory of emulsion forming w/o thermodynamic factor and the specific theories of forming emulsion

A number of simultaneous process have to be considered as to o/w or w/o emulsion will be form
1) droplet formation
2) aggregation
3) coalescence of droplets
4) interfacial film formation

On shaking together oil and water, both phases initially formed droplets.
The phase that persists in droplets form the longest should become the disperse phase and surrounded by continuous phase formed from the more rapidly coalescing droplets.

The Phase volumes and interfacial tensions will determine the relative number of droplets produced and hence the probability of collision.

eg. The interfacial film produced by adsorption of emulsifier at the o/w interface can alter the rate of coalescence by acting as a physical barrier to coalescence.

The type of emulsion formed depend on the polar/non-polar characteristic of the emulsifying agent. The phase in which it is more soluble being the continuous phase.

Specific theory
1) bancroft theory
2) Harkin Oriented wedge theory
3) angle of contact theory


Explain bancroft theory

A few theories have been proposed to explain the formation of o/w and w/o emulsions.

The adsorption of surfactants at the oil/water interface gives rise to an interfacial film.
This film experiences two interfacial tensions,one between the film and the aqueous phase and the other between the film and the oil phase.
The film will curve in the direction of the greater interfacial tension.Thus, the disperse phase is on the side of the film with the higher interfacial tension.

IF(film/oil) > IF (film/water)
o/w emulsion

IF (film/oil) = IF (film/water) no emulsion

IF (film/oil) < IF (film/water)
w/o emulsion



Oriented wedge theory explain


such as sodium oleate(soap of monovalent base) and magnesium oleate(soap of divalent base).

Surfactant molecules (e.g. soaps) will orientate at the oil/water interface such that a tightly packed film is formed.
The relative location of the polar heads of the soap molecules will determine the type of emulsion produced.

The sodium oleate molecule, which is composed of one polar head and one non-polar tail, can be looked upon as wedge-shaped.
These molecules will orientate with their polar heads on the external side of the droplet to allow more molecules to pack at the interface.
As the polar heads are also hydrophilic, this results in the formation of an o/w emulsion.

The magnesium oleate molecule is composed of one polar head and two non-polar tails.
These molecules will orientate with their polar heads on the internal side of the droplet to allow more molecules to pack at the interface.
As the polar heads are also hydrophilic, this results in the formation of a w/o emulsion.


Theory based on angle of contact explain

This applies to emulsifying agents which are FINELY DIVIDED SOLID, with the following properties:
 Insoluble in both aqueous and oily phases. Preferentially wetted by one of the phases.
Able to form a thin interfacial film.
Of colloidal size.

Such solid particles are attracted to the interface between the two immiscible liquids.
The angle of contact formed by the solid particle at the interface determines the type of emulsion produced

If angle is more than 90 = w/o emulsion
angle = 90 = no emulsion
angle = less than 90 = o/w emulsion


what is needed to form emulsion

Oil phase
emulsifying agent


Type of oil phase
and the properties to consider

 A wide variety of lipids or lipophilic materials such as mineral oils, vegetable oils, silicones and waxes maybe used.

Mineral oil are more stable than vegetable oil coz of unsaturated fatty acid in VO

The following properties of the oil phase are important as they affect the performance of the emulsion:
“Feel” or tactile characteristic
Drug solubility


Function of emulsifying agent

Its function is to stabilize the emulsion


class of emulsifying agent

surfactants, hydrophilic colloids and finely divided solids.


Factor affect the selection of emulsifying agent

Factors affecting the selection of emulsifying agents: Type of emulsion (o/w or w/o)
Compatibility with other components
Toxicity of emulsifying agent
Cost of emulsifying agent


what is surfactants and the groups

These compounds have a hydrophilic group and a lipophilic group in their molecular structure.
This amphipathic nature causes the molecules to become attached to interfaces, thereby lowering interfacial tension.
They are divided into four major groups: anionic, cationic, amphoteric and nonionic.


Anionic surfactant
for what use
and types

Generally employed for external preparations. Incompatible with cationic compounds, low pH and high concentration of electrolytes.
Effectiveness enhanced by nonionic surfactants Different types:
1) Soaps of monovalent bases (o/w)
2) Soaps of polyvalent bases (w/o)
3) Amine soaps (o/w)
4) Sulphated and sulphonated fatty acids and alcohols (o/w)
5) Quillaia saponins (o/w)

many drug are anionic so normally we use anionic surfactant


Soap of monovalent base properties

Sodium stearate Potassium stearate Ammonium stearate

form o/w emulsion
- Presence of polyvalent cations will cause phase inversion.
Sodium and potassium soaps have high pH and are unsuitable for emulsions where a high pH cannot be tolerated.


Soap of polyvalent bases properties

Calcium oleate Zinc oleate
w/o emulsion
Presence of monovalent cations will cause phase inversion.


amines soaps properties

Triethanolamine stearate
o/w emulsion
Suitable for o/w emulsions where a high pH cannot be tolerated.


Sulphated and sulphonated fatty acids and alcohols

Sodium lauryl sulphate Sodium cetyl sulphate
o/w emulsion

Generally more effective than other types but strongly alkaline.
SLS + CSA -->Emulsifying Wax


Quillaia saponins

for internal use

o/w emulsion
Glycosides from Quillaja saponaria
Produce o/w emulsions of low viscosity.
Can be employed for oral preparations


Cationic surfactants properties

Possess emulsifying and antiseptic properties. Incompatible with anionic compounds.
Examples include quaternary ammonium compounds, such as cetrimide,cetyl pyridinium chloride and benzalkonium chloride.
Promote the formation of o/w emulsions



Amphoteric surfactant properties

These surfactants are cationic at low pH and anionic at high pH.
Not widely used.

E.g.lecithinfor I/V fat emulsions.


Non ionic surfactant properties

Low toxicity and irritancy.
Less sensitive to pH changes and addition of electrolytes and polyvalent ions
For external as well as internal preparations.


Types of non-ionic surfactant

Sorbitan esters and polyoxyethylene sorbitan esters (DEPENDS ON HLB)
Glycol and glycerol esters (O/w)
Fatty acid polyglycol esters (o/w)
Fatty alcohol polyglycol ethers (Macrogol ethers) (depend on HLB)
Higher fatty alcohols


sorbitan esters and polyoxyethylene sorbitan esters

Depending on the HLB of the blend, it may promote the formation of o/w or w/o emulsions.

NONionic surfactant

span (sorbitan) and tween (POE)

POE increase hydrophilicity

20,40,60 etc = mono
65,85 = tri
number increase hydrocarbon chain increase so HLB drop

HLB higher = more hydrophilic


glycol and glycerol esters properties

glyceryl monostearate

Promote formation of o/w emulsions.
Effectiveness enhanced by soaps of monovalent bases and amine soaps.


Fatty acid polyglycol esters (POE fatty acid esters)

E.g. POE(40) stearate
Promote formation of o/w emulsions.
Effectiveness enhanced by stearyl alcohol.


Fatty alcohol polyglycol ethers (POE fatty ethers) (Macrogol ethers) properties

E.g. Cetomacrogol 1000
Blends of hydrophilic and lipophilic members are usually employed.
Depending on HLB of the blend, it may promote the formation of o/w or w/o emulsions.

Cetomacrogol 1000 + Cetostearyl alcohol ---> Cetomacrogol emulsifying wax