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Emulsions

We come across emulsions when two or more immiscible liquids combine intimately or disperse into each other in form of tiny droplets and moreover these are not homogeneous systems. Homogeneous system can be construed when ingredients are miscible with each other but in case of emulsions these are heterogeneous system where they contain ingredients which are miscible like water and oil. 

These droplet sizes can be vary from 0.1 to 10 micro meters in diameter and can remain in this dispersion form throughout.

 

Emulsions Definition

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Emulsions by definition are unstable dispersion and the system gets stabilized by the presence of one or more emulsifying agents. These agents are substances which help keeping the mixture of immiscible liquids dispersed into each other for quite a long time like in case of gelatine which help in reducing the tension between the two immiscible liquids. 

The choice of these agents is specifically governed by the composition of the emulsifying dispersed phase or the intended route of administration of the emulsion. Either of the continuous phase or dispersed phase of any emulsion may have the range of consistency from that of a free flowing mobile liquid to a range of a semi solid.

Emulsion in general are considered to be dispersions of oil and water. When an oily substance or an oil is the dispersed phase, the emulsion is called an oil in water type or when water is the dispersed phase then the emulsion is called water in oil emulsion. 

In recent times the origin is a third type of emulsion described as micro emulsion. A micro emulsion is also called a transparent emulsion because this possess the property of transparency due to the very small size of the dispersed droplets which are generally 0.05 micro meter or less.

The process of emulsion action or emulsification consist of dispersing one fluid into another or non-immiscible through an interface. The properties of such emulsion and uses are governed by temperature and composition along with size of droplets. The basic principles of phase inversion temperature method are presented and the influence of different parameters such as surfactant concentration and stirring intensity.

The mechanism of drop rupturing and the condition leading to mono-dispersity leads to new ideas of emulsification. These are all about emulsion stability, food stability, crude oil emulsion, pharmaceutical emulsion and as per fluoro chemical emulsion as blood substitutes.

These emulsions are widely used in variety of industries like agro chemicals, food, pharmaceutical, paint or printing that emulsion science has taken a new level of importance and more and more research is carried out for getting the best of emulsion formation.

What are the Different Methods of Preparing Emulsions?

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Whether an emulsion is O/W (oil in water) or W/O (water in oil) type or electrolyte concentration, temperature and these characteristics have led the emulsion chemists thought that surfactants are more soluble in water than in oil and hence the have stressed more in O/W than in W/O forms. The essence of Bancroft’s rule which states that that the continuous phase of emulsion tends to be the phase in which the emulsifier is preferentially soluble. 

A non-ionic surfactant of low stabilising water on oil emulsion may form oil in water emulsion formations at low temperature and hence the prevailing conditions of temperature, electrolyte concentration, oil type and chain length along with co-surfactant concentration can modify the surfactant geometry at the interface.
  • The consistency of emulsion can be increased
  • by increasing the continuous phase viscosity,
  • by increasing the fractional volume of internal phase
  • by reducing the particle size of the internal phase
  • by increasing the proportion of emulsifying agent
  • by adding hydrophobic emulsifying agents to the oil phase of emulsion
The physical stability of emulsion may be defined by a number of expression, and they basically the movement of the droplets which either move upwards or downward depending upon the density. This is also known as creaming.

The rate of creaming may be decreased by considering the theory of creaming which relates to rate of creaming to the size of the droplets, the difference in densities and the viscosity of external phase.

The rate which is basically the utilisation of stokes law, can be either decreased or increased by decreasing the size of droplets and increasing the viscosity of external phase respectively.

Minimising the difference between densities is very challenging as there exist many practical difficulties. As these droplets gather together they act as single unit and never gel into each other. Due to this large size these droplets cream faster and create physical instability. 

A recent innovation in emulsion technology has developed multiple emulsions. The disersed phase of these types of emulsions may contain even tiny droplets that are highly miscible with the continuous phase.

The multiple emulsion may be oil – water – oil where the aqueous phase is between two oil phases or water – oil – water where the internal and external aqueous phases are separated by an oil phase.

In such systems both hydrophilic and hydrophobic emulsifiers are used and both have a role to play in the yield and stability of emulsions.
These emulsions preparation is very complicated but have prolonged action and more effective along with improved stability. These emulsions have better protection against external environment and enzyme entrapment.

These are used to separate two incompatible hydrophilic substances in the inner and outer aqueous phase by a middle oil phase. Some of the best effective drugs use this technique of emulsions.

The preparation of emulsions requires a certain amount of energy to form the interface between the two phases and addition work is carried out to stir the system to overcome resistance to flow.

Due to the variety of oils used, emulsifier agents, phase to volume ratios and the desired physical properties of the product, a wide range of equipment is available for preparation of emulsions.

The homogenisation of speed and time along with rate of cooling can influence the viscosity of the product. The emulsions can also be prepared in small quantities by pestle and mortar method in laboratories and in pharmacy units for testing purpose.

Emulsions can be prepared by agitators or by shaking vigorously and is used by pharmacists for emulsification of easily dispersed and low viscosity oils. Intermittent shaking is better than continuous shaking as continuous shaking leads to breaking of phase of dispersions but also dispersion medium and results in imperfect emulsions.

Emulsions can be prepared by using mechanical mixers where propeller and impeller type mixers are utilised for preparing convenient emulsions.
Emulsions are also prepared by small electric mixers which saves time and energy. These help in producing satisfactory emulsions where the emulsifying agents like acacia or agar are used. These instruments transfer large amounts of energy and incorporates air into the emulsions.

Examples of Emulsions

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Emulsions are dispersion of one liquid into the second immiscible liquid in form of fine droplets. Since emulsions are thermodynamically unstable or that they will finally phase out separately.

Formation of emulsions result in a large interface area between two immiscible phases and are associated with increase in free energy.
Smaller interfacial tension helps in dispersion of one phase in form of fine droplets by lowering the required interfacial energy.

 Stabilising property aggregates 
 Micelles   Bilayers   Reversed 
   Water continuous emulsions     
 Repulsive interactions
 Optimal
 Intermediate 
 Weak
 Interfacial viscosity  Weak optimal  Weak  
 Anchoring  Too water soluble  Optimal
 Acceptable
   Oil continuous emulsions    
 Repulsive interactions
 Weak  Intermediate  Optimal
 Interfacial viscosity
 Weak  Optimal
 Weak
 Anchoring
 Acceptable  Optimal
 Too oil soluble 

Marshmallow: 
Foam formed from egg, gelatine and sugar

Ice cream:
Ice crystals and fat crystals form the matrix which are aerated and refrigerated at the same time.

Dynamic foams:
The bubbles change at various stages of preparation

Foams on drying:
These are formed in distillation columns where a foam blanket at the surface act as insulating layer which causes overheating.

Metallic slags foam:
These are formed due to high viscosity and cooling finally stabilises the foam.

Types of Emulsions in Pharmacy

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Medicinal emulsions for oral administration are usually of oil in water type and require the use of similar emulsifying agents. A surface active agent non-ionic in nature like acacia are used as these have the property of reducing surface tension and interfacial tension. Radiopaque emulsions are used for X ray examinations as diagnostic agents. 

For oral type of medicines oil soluble compounds render large surface area for the oily solution and this large surface area is made available for contact at absorption site which finally results in better and faster absorption of oily liquid.

Any emulsions to be used externally or for external application may be either oil in water type or water in oil type. But the oil in water emulsions have better edge over the other as these are water washable and are non-staining to clothes.

The water in oil type emulsions which are exclusively external use purpose contain one or several emulsifying agents like poly valent soaps like calcium palmitate or synthetic non-ionic sorbitan esters.

For any kind of lotion used in pharma industries emulsification is widely used to formulate dermatological combinations and also for aerosol products, liquefied gases which help in propelling emulsion from within the container. As the emulsion is discharged from container the liquefied gases vaporise and turn into a foam.

Types of Emulsions in Food

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  • Most of these food emulsions are found to be much more complex than the existing 3 components like the oil, water and emulsifier system.
  • The aqueous phase is found to contain variety ingredients with water solubility like the surfactants of the mix, sugar, salts, along with acids, and bases.
  • The oil phase of such emulsions usually contain a very complex combination of lipid soluble component such as tri-acyl-glycerol, di-acyl-glycerol and mono-acyl-glycerol along with free fatty acids, sterols and vitamins.
  • The interfacial region might also contain a mixture of variety of surface active components like protein types, phospholipids types, and many types of surfactants, along with alcohols and various solid particles.
  • Food emulsions are complex in composition, structure as well as dynamics and these factors contribute to overall properties.

Properties of Emulsions

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The concentration of droplets in an emulsion is usually explained in terms of dispersed phase volume fraction which is equal to volume of emulsion droplets divided by total volume of emulsion. The droplet concentration influences the appearance, texture, flavor, stability and cost of emulsion based food products. It is important to be able to control, predict, measure and report the size of droplets in emulsions. 

Mono dispersion emulsions are sometimes used for fundamental studies because the interpretation of experimental measurements are understandable than a poly dispersion emulsions.
  • The emulsions are identified by either internal (water in oil) or by external (oil in water) methods
  • The droplets size and size distributions are usually greater than 1 µm
  • The concentration of dispersed phase are often quite high
  • The viscosity and conductivity of emulsions are quite different from the continuous phase
  • The rheology gives a proper set of ideas for complex combinations of viscous, elastic and viscoelastic properties
  • The electrical properties are also useful to characterise the emulsion molecule structure
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