• Surfactant (Monomolecular adsorption): To reduce the interfacial tension Oil droplets are surrounded by a coherent monolayer of the surfactant which prevents coalescence. If the emulsifier is ionized, the presence of strong charge may lead to repulsion in droplets and hence increasing stability.
• Hydrophillic Colloids (Multimolecular adsorption)
• Finely divided solid particles (solid particles adsorption):They are adsorbed at the interface between two immiscible liquid phases to form Particulate film.
• Monomolecular Adsorption Amphiphiles (surfactants) reduce interfacial tension (to 1 dyne/cm) because of adsorption at interface o/w
• Droplets are surrounded by coherent monolayer that help prevent coalesence (merging) between two droplets
• Surface Charge cause repulsion between globules.

Monomolecular Adsorption (Combination of surfactants) • Combination of surfactants is generally used as it is more effective

• Combination of Sodium cetyl sulphate and cholestrol leads to complex film that produce excellent emulsion
• Hydrophilic Tween can be combined with Lipophilic Span, varying proportions produce desired emulsion… w/o or o/w.

• Multimolecular Adsorption (Hydrocolloids) Hydrophilic colloids (mucilage of gum acacia) are different in action from surfactants
• They do not cause lowering of interfacial tension
• They form multimolecular layer at o/w interface, action of hydrocolloids is because of this reason
• They increase viscosity of dispersion medium

        Solid Particles Adsorption • Solid particles that can be wetted by oil as well as water can         act as   emulsifying agent

• Their concentration is higher at interface
• They form particulate film around dispersed droplets to prevent coalescence
•  Example of agents: Bentonite (Al2O3.4SiO2.H2O), Veegum (Magnesium Aluminum Silicate).

Multiple choice questions (MCQs)

1.In the stability of emulsion, which important instability step should be prevented?

a)Breaking

b)Coalescence

c)Flocculation

d)Creaming

2.W/O emulsions is called as

a)Aqueous emulsion

b)Oily emulsion

c)Alcoholic emulsion

d)Electrophoresis

3.The substances which are added to stabilize the emulsions are called

a)Dialyser

b)Aggregates

c)Emulsifying agent

d)Peptizing agent

4.Any two immiscible liquids when mixed form an

a)Aerosol

b)Emulsion

c)Gel

d)Sol

5.The dispersion of one liquid in another liquid is called as

a)Solid sol

b)Sol

c)Emulsion

d)Gel

6.Milk is an emulsion in which

a)Acid particles are dispersed in water

b)Base particles are dispersed in water

c)Water particles are dispersed in liquid fat

d)Liquid fat particles are dispersed in water

7.Why is the emulsion stable with the egg yolk?

a)Due to presence of fat in it

b)Due to presence of lecithin in it

c)Due to presence of albumin in it

d)Due to presence of proteins in it

8.Vanishing cream is an example of

a)Aerosol

b)Gel

c)o/w emulsion

d)w/o emulsion

9.What is the process of separating emulsion into its constituents called?

a)Emulsifying agent

b)Centrifugation

c)Demulsification

d)Emulsification

10.Which of the following is an example of an emulsifier?

a)NaCl

b)CaCO₃

c)C₁₅H₃₁COONa

d)CH₃COOH

11.Which of the following is not an example of a water-in-oil emulsion?

a)Cod liver oil

b)Butter

c)Cold cream

d)Milk

12.What is the difference between vanishing cream and cold cream?

a)Both are examples of oil-in-water emulsions

b)Vanishing cream is an oil-in-water emulsion whereas cold cream is a water-in-oil emulsion

c)Vanishing cream is a water-in-oil emulsion whereas cold cream is anoil-in-water emulsion

d)Both are examples of water-in-oil emulsions

13.Which of the following is not a method to test the type of emulsion?

a)Microscopic method

b)Conductance method

c)Coagulation method

d)Dye method

14.Which of the following statements regarding emulsions is false?

a)Emulsions cannot be separated into their constituent liquids

b)Emulsions show Brownian motion

c)Emulsions show Tyndall effect

d)Emulsions exhibit properties like Electrophoresis and Coagulation

15.Which of the following statement about emulsions is true?

a)Oily drugs cannot be prepared in the form of emulsions

b)Digestion does not involve the process of emulsification

c)Disinfectants like Dettol and Lysol give emulsions of water-in-oil type on mixing with water

d)The cleansing action of soap is based upon the formation of water-in-oil emulsion

Solution:

1. b)Coalescence
2. b)Oily emulsion
3. c)Emulsifying agent
4. b)Emulsion
5. c)Emulsion
6. d)Liquid fat particles are dispersed in water
7. b)Due to presence of lecithin in it
8. c)o/w emulsion
9. c)De-emulsification
10. c)C₁₅H₃₁COONa
11. d)Milk
12. b)Vanishing cream is an oil-in-water emulsion whereas cold cream is a water-in-oil emulsion
13. c)Coagulation method
14. a)Emulsions cannot be separated into their constituent liquids
15. b)Digestion does not involve the process of emulsification

References: 1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY

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Some common emulsions are milk (where the dispersion of fat molecules or droplets in the aqueous solution) and also butter (which is the dispersion of droplets of particles of an aqueous solution in the fat).

Types of Emulsion:

Emulsions can be classified on the basis of the properties of the dispersed phase and the dispersion medium.

• Oil in water (O/W): In this type of emulsion, the oil will be the dispersed phase and water will be the dispersion medium. The best example for o/w emulsion is milk. In milk, the fat globules (which act as the dispersed phase) are suspended in water (which acts as the dispersion medium).
• Water in oil (w/o): In this type, water will be the dispersed phase and oil will be the dispersion medium. Margarine (a spread used for flavouring, baking and working) is an example of water in oil emulsion.

Microemulsions- In spite of their similarity, the terms microemulsion and emulsion characterize two very different systems both by their physical and thermodynamic properties and by their structure. In both cases, the systems consist of an aqueous phase, a lipophilic phase and a surfactant agent. A co-surfactant is also required for microemulsions. Microemulsions actually exist when the percentage of oil or water in the internal phase is low (<10%). These dispersions of oil or water nanodroplets in an external phase are stabilized by an interfacial film of surfactant and co-surfactant. The addition of co-surfactant results in a homogeneously dispersed system, which can diffuse the light, appear clear and homogeneous to the naked eye and, as opposed to emulsions, is thermodynamically stable. The co-surfactants have three functions: (1) they provide very low interfacial tensions required for the formation of microemulsions and their thermodynamic stability, (2) they can modify the curvature of the interface based on the relative importance of their apolar groups and (3) they act on the fluidity of the interfacial film.

The main characteristic of microemulsions is their transparent appearance due to the high level of dispersion of the internal phase, the size of which ranges from 100 to 1000 Å.

The microemulsions are Newtonian liquid and are not very viscous. These dispersed systems are isotropic and in terms of the manufacture, their formation is spontaneous, do not require much energy and are thermodynamically stable.

O/w micellar solution- Blending of a small amount of oil with water results in a two-phase system because ‘water and oil do not mix’. If the same small amount of oil is added to an aqueous solution of a suitable surfactant in the micellar state, the oil may preferentially dissolve in the interior of the micelle because of its hydrophobic character. This type of micellar microemulsion is called an o/w micellar solution.

W/o micellar solution- In these systems, sometimes called reverse micellar solutions, water molecules are found in the polar central portion of a surfactant micelle, the nonpolar portion of which is in contact with the continuous lipid phase.

A microemulsion in which a water-insoluble oil or drug is ‘dissolved’ in an aqueous surfactant system plays an important role in drug administration.

Table 1 – Methods for the determination of type of emulsion

Multiple choice question (MCQs)

1.Dye test which is used to identify type of emulsion is that which dye is used?

a)Scarlet red

b)Scarlet green

c)Crystal violet

d)Methylene blue

2.In conductivity test, if bulb glows on passing electric current so what would be the type of emulsion?

a)w/o emulsion

b)o/w emulsion

c)micro emulsion

d)multiple emulsion

3.In micro emulsion, the size of globule are

a)1µm

b)0.1 µm

c)0.01 µm

d)10 µm

4.Multiple emulsion can be designed as

a)o/w

b)w/o/w

c)o/w/o

d)both b and c

5.Which emulsion is mainly used for taste masking purpose?

a)Multiple emulsion

b)w/o

c)micro emulsion

d)o/w

6.Dilute emulsion follows

a)Non Newtonian flow

b)Newtonian flow

c)Thixotrophy

d)Rheopexy

7.If viscosity of continous phase increase so creaming

a)No change

b)Increase

c)Decrease

d)None of the above

8.In case of coalescence

a)Dispersed droplet does not fuse

b)Globule size decrease

c)No. of globules increase

d)Dispersed droplets tend to fuse

9.If the density of two phase is more so

a)Stability of emulsion is less

b)Stability of emulsion is more

c)Does not affect the stability

d)None of the above

10.In case of o/w emulsion, creaming takes place at

a)Down side

b)Up side

c)At interface between two phase

d)None of the above

11.Ideal phase volume ratio per stable emulsion

a)25:75

b)50:50

c)75:25

d)33:67

12.Thermodynamically emulsion is

a)Stable

b)Unstable

c)Both

d)None

13.Creaming is

a)Reversible

b)Irreversible

c)Both

d)Difficult to predict

14.Upward creaming is observed in

a)w/o

b)o/w

c)both

d)micro

15.HLB value of w/o emulsifying agent is

a)2-3

b)3-6

c)7-9

d)8-16

Solutions:

1. a)Scarlet red
2. b)o/w emulsion
3. c)0.01 µm
4. d)both b and c
5. a)Multiple emulsion
6. b)Newtonian flow
7. c)Decrease
8. d) Dispersed droplets tend to fuse
9. a)Stability of emulsion is less
10. b)Up side
11. a)25:75
12. b)Unstable
13. a)Reversible
14. b)o/w
15. d)8-16

References:

1. GAURAV KUMAR JAIN – THEORY & PRACTICE OF PHYSICAL PHARMACY, 1st edition 2012 Elsevier, page no. 237-240.

2. Martins Physical Pharmacy, 6th edition 2011, page no. 763.

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