Pharmaceutical emulsion

 Formulation of emulsion

PART II

Before jumping into actual concept of emulsion formation, it is very necessary to make up our minds about it.

Basically emulsification is the process of making a metastable mixture like water in oil, which eventually separates into two phases in equilibrium.

Here we have to create a optimist scenario about emulsification.

v  Surface active agents:-

The compound due to their chemical structure, have tendency to accumulate at the boundary between two phases are known as surface active agents, Or surfactant Or amphiphiles.

Due to adsorption at the interfaces it lowers the interfacial tension between oil and water results in emulsion.

Surfactant are classified into three categories-

A] hydrophobic ( water hating) surfactant

B] hydrophilic ( water loving) surfactant

C] amphipathic surfactant

The organic compounds like saturated or unsaturated alkanes, alkenes, alkynes heterocyclic Or aromatic compounds are hydrophobic compounds.

The hydrophilic surfactant are generally cationic, anionic, Or Non-ionic.

v               Surface activity :-

Amphipathic molecules due to their unique features have surface activity.

Their adsorption at the interface escapes into atmosphere causes increase in vapour pressure.

According to surface tension theory that we will discuss later, the molecules at the surface if the liquid are surrounded partially as compared to molecules of liquid in the bulk.

It results into net inward force of contraction which makes the surface contract.

The surface contraction represents low free energy.

So surfactant orient themselves in such a way that it eliminates hydrophobic groups and achieves minimum free energy.

These causes replacement of water molecules with non polar groups, which emphasizes lower force of attraction.

Addition of surfactant in the mixture of two immiscible liquid causes reduction in interfacial tension and leads to miscibility.

v                   Micelle formation :-

The surface tension of any solution in which surfactant is mixed is known as surfactant solution.

The surfactant solution in which two immiscible phases are present is inversely proportional to surfactant molecules present in the system.

So,

Surface tension of surfactant molecules is inversely proportional to number of surfactant molecules present in the system.

After attaining a desired layer a saturation is achieved which initiates the alternative means of shielding with hydrophobic compounds with aqueous environment of colloidal dimension known as micelles.

The micelles are formed of hydrophobic chain surrounded by hydrophilic shell which maintains stability.

The saturation point of surfactant molecule is known as critical micelles concentration (CMC).

The physical properties like surface tension, scattering of light, temperature, pressure can be detected by graph.

The CMC decreases with increase in hydrophobic chain.

 

A typical micelle is spherical structure composed of 50-100 surfactant molecules.

The radius is 2.5nm with interior hydrophobic core having properties of liquid hydrocarbon.

For ionic micelle counter ion is present which reduces the charge.

The compact layer around the core of ionic micelle, which contains the head group and bounded counter ion is called STERN LAYER.

The outer surface of stern layer is shear surface of micelle, a diffused layer known as GOUY – CHAPMAN ELECTRICAL DOUBLE LAYER.

This double layer neutralizes the core of micelle.

Non- ionic micelle have a hydrophobic core are surrounded by polyoxyethylene chain which is termed as PALISADE LAYER.

Properties of surfactant on emulsification:-

A surfactant is a chemical substance that alters the interfacial properties by absorbing the boundary between two immiscible phases.

Basically it consists of hydrophilic lipophilic groups the lipophilic groups tends to repel away from water molecule is known as hydrophobic interaction.

A surfactant molecule spontaneously absorb the boundary and self assemble itself forming a micelles beyond critical concentration.

The hydrophobic molecule forms a hydrophobic compartment surrounded by hydrophilic shell.

The fundamental properties of surfactant is absorption of micellization, lowering the interfacial tension.

Surfactant absorbed to interface is able to reduce excess free energy at oil water interface. This excess energy is known as interfacial energy.

Emulsifying agent:-

Emulsifying agent can be defined as a compound that lowers is surface tension and forms a film at the interface of two  immiscible liquid making them miscible.

Types of emulsifying agent-

True agents that can form and stabilize emulsion by themselves.

Auxiliary agents which do not form fine emulsion but assist true emulsifying agent.

Classification of Emulsifying agent:-

 

A)       Natural Emulsifying agent from vegetable sources –

These are carbohydrates which are generally gums and mucilage.

They are anionic in nature and produces O/W emulsion.

The emulsion prepared from this Emulsifying  agent need suitable preservative to preserve them because carbohydrate present in them act as a medium for bacterial growth.

I)                    Acacia – these are better Emulsifying agent for extemporaneous preparation, because they are attractive in appearance, quite palatable and relatively stable for wide range if pH  .

The emulsion prepared from these Emulsifying agent have low viscosity before creaming takes place quite rapidly.

II) Tragacanth – tragacanth alone is rarely used as Emulsifying agent because it creates very thick and coarse emulsion.

The appearance and stability of the emulsion can be improved by passing the emulsion through homogenizer.

A stable emulsion can also be produced if tragacanth is used with gum Acacia as Emulsifying agent. 

III) Agar – it is not a good Emulsifying agent as it forms a very thick and coarse emulsion.

It is used as Emulsifying agent by preparing 2% mucilage, by dissolving in boiling water and cooled at 45oC

IV) Starch – starch mucilage is very less used because it forms very coarse emulsion.

It is generally used in preparing enemas.

V) Pectin- it is used as Emulsifying agent in 1% water.

It is incompatible with alcohol, alkalis, tannic acid and salicylic acid.

Natural Emulsifying agent from animal sources :-

I) Wool fat – It is used in emulsion which are meant for external use.

It produces oil in water type of emulsion and can absorb about 50% of water.

II) Egg yolk – It is use in extemporaneous preparation meant for internal use because it gets spoiled during transportation.

The emulsion prepared with egg yolk require proper preservation and storage in a refrigerator.

It is used as emulsifying agent in concentration of 15% .

III) Gelatin – it is used in the concentration of 1% Emulsifying agent.

It is mainly used for emulsification of liquid paraffin.

The colour is white and have agreeable taste.

It needs proper preservation because emulsion are prone to bacterial contamination. 

Semi synthetic polysaccharides-

I) Methyl cellulose – it is synthetic derivative of cellulose.

It is used as Emulsifying agent in the concentration of 2% .

II) Sodium carboxy methyl cellulose – it is used as a emulsion stabilizer in the concentration of 0.5-1 %.

It is soluble in both hot water and cold water.

D) Synthetic Emulsifying agent:-

I) Anionic – various alkali soaps, sulphated alcohol and sulphonates, are used as anionic Emulsifying agent.

SLS are are used as Emulsifying agent among sulphated alcohols.

It is used as o/w emulsion.

II) Cationic – the quaternary ammonium compounds such as benzalkonium chloride, cetrimide.

They bear positive charge on them.

They are used to prepare external preparation.

III) Non ionic – the glyceryl esters, like glyceryl monostearate, sorbitan fatty acids esters like sorbitan monopalmitate are commonly used as non ionic surfactant.

They are used in preparation of emulsion because emulsion prepared from non ionic surfactant remains for longer time .

 Mechanism of emulsifying agent:-

There are three types of films formed around at dispersed phase.

Mechanism of emulsifying agent depends upon formation of film they form at interface of two phases.

i. Monomolecular film:-

Emulsifying agent with stabilizing action forms monolayer at oil water interphase. This monolayer prevents coalescence of droplet.

ii. Multimolecular films :-

Multimolecular films forms a film around the droplet of dispersed phase are formed by hydrophilic colloids.

They forms a coat making the highly resistant to coalescence , in fact having ability to swell and increase velocity.

iii. Solid particle films:-

The solid particle which get wetted by oil and water produce a particulate film around a dispersed droplet and avoid coalescence. Eg – veegum, hectronite, bentonite.

 Emulsification equipments:-

The equipment used on laboratory and industrial level are just to break the dispersed phase into continuous phase, so that the droplet are quite small enough.

A] Small scale processing-

On laboratory basis, most frequently used equipment are Wedgwood, or porcelain morter and pestle.

1. Shaker mixers/ agitators:-

In shaker mixer, the material in the container is agitated by oscillator, whereas on large scale production the material in the container is agitated by rotatory movement similar to ball mill.

2. Propeller mixer:-

It is used on small and large scale production.

If preheating is required, the mixing container are made steam jacketed.

It is used for preparing for liw viscosity emulsion at the speed of 8000 RPM.

The drawbacks are it tends to vortex formation and air entrapment.

3. Turbine mixers:-

They are used to prepare high velocity emulsion. They are generally in circular disc impeller attached to short straight and curved blades.

4. Homogenizers:-

This are the most advanced technology to prepare emulsion.

Among all the emulsification equipments the homogenizers are the most widely used continuous operation used machine.

High pressure homogenizers are provided with high pressure pump raises the pressure upto 50 to 500 bar.

During emulsification, the dispersion of two liquids is carried out by forcing them through a fine orifice at high pressure.

The assembly of homogenizer is consisting of rotor with blades and stator with fine opening.

As the rotor rotates a fine vaccum is created which pulls the liquid in and out of assembly, resulting into liquid circulation.

 The size of dispersed phase is reduced due to –

a) Due to mechanical collision against the walls of homogenizer because of high liquid acceleration.

b) Due to shear force occurring within the gap between rotor and stator.

5. Ultrasonifiers:-

Ultrasonifiers, the transduced piezoelectric instrument are used for laboratory scale emulsion production, they have limited output and are expensive.

Their mechanism of action is that the dispersion is forced through an orifice at the medium pressure of 150 to 350 psi and is allowed to collide up in a blade.

Ultrasonifiers provides an easy way of agitation for the laboratory scale preparation of constant and reproducible oil and water emulsion.

6. Colloid mills:-

Colloid mills are suitable for preparation of emulsion on continuous phase.

Due to intense shearing force, the emulsion produced by colloid mills are very small globules size

Colloid mills are generally used fir mulling if solids and for dispersion of very poorly wetable substances.

However they are very useful in preparation of high viscosity emulsion.

7. Whisks/ churns:-

These are used for emulsion preparation involving the agitation of emulsion ingredients for blending by blenders fitted in the containers.

Containers may also be fitted with steam jacketed if heating and cooling is required.

8. Silverson mixer homogenizers:-

Silverson mixer consists of an emulsifying head with blades which are surrounded by stainless steel fine mesh seives.

The emulsifying head is adjustable for insertion into the containers used for emulsification.

An electric motor is used for rotating the head. The liquid to be emulsified is sucked through fine mesh seives into the emulsifying head where they are subjected to intense mixing due to rotation of blades of emulsifying head.

9. Micro fluidizers:-

For the production of very fine particles, micro fluidizers are used.

It consists of interaction chambers and micro channels

During emulsion processing, the emulsion is subjected to high velocity through the micro channels of interaction chambers. Thus the particle are subjected to high shear, uproar, collision and cavitation.

• Foaming during agitation process:-

The reason of forming of foam is that during emulsification, the emulsifying agent also reduces the surface tension at the air water interface.

To avoid foam formation, the emulsification must be carried out in closed system or in vaccum.

In addition, continuous stirring during cooling stage of emulsion may also affect in preventing foam formation.

Part III will be released soon...