W8 emulsions

Question 1

What are emulsions?

Answer 1

A ‘pseudo-stable’ dispersion of at least two immiscible liquids, one of
which is dispersed throughout the other in the form of fine droplets
(generally oil and water) stabilised by the presence of an emulsifying
agent (known as emulsifier)

Question 2

Emulsions properties

Answer 2

• Cloudy or milky
• Droplet sizes are polydispersed (nm to μm)
• Low concentration of surfactant/cosurfactant
• Thermodynamically unstable but kinetically stable
  (feasible to separate, but happens very slowly)
• Prepared by input of large amount of energy, e.g. high speed homogenisation, sonication, heat

Question 3

What are the 3 emulsion types?

Answer 3

• Normal emulsion (o/w)
• Reverse emulsion (w/o)
• Multiple emulsion
  (o/w/o or w/o/w)

Question 4

What is an emulsion example?

Answer 4

o/w emulsions
- vehicles for lipophilic drugs e.g. diazemuls

w/o emulsions
- Sunscreen

w/o/w emulsions
- Vancomycin hydrochloride

Question 5

What are examples of everyday emulsions?

Answer 5

• Cosmetics
• Homogenised milk, mayonnaise, cream, butter, margarine
• Paints
• Fire extinguisher

Question 6

What are examples of pharmaceutical emulsions?

Answer 6

• Topical delivery (creams)
• Total parenteral nutrition (intravenous feeding of fat emulsions)
• Oral delivery and taste masking (cod liver oil)
• As vehicle for drug (emulsions containing propofol)

Question 7

What are creams?

Answer 7

• Semi-solid emulsions of two immiscible phases stabilised by emulsifying agent (either o/w or w/o)

Question 8

What are o/w or ‘watery’ creams?

Answer 8

Water is the continuous medium, while oil is the
dispersed phase, therefore o/w creams do not feel greasy
* Can deposit lipids so restore skin hydration, however they are non-occlusive
* Rub into the skin, leaving behind a thin film of rapidly releasing water-soluble drug
* Readily diluted and miscible with water; easily rinsed off
* Conduct electricity
* Prepared using surfactants with higher HLB (8-16)

Question 9

What are w/o or ‘oily’ creams?

Answer 9

• Are more greasy, since oil is the continuous phase
• More moisturising as they provide an oily barrier which
  reduces water loss from the outer layer of the skin
• Hydrophobic drugs are better formulated and more
  readily released
• Miscible with oil therefore not easily washable, however
  more easily spreadable
• Do not conduct electricity
• Prepared using low HLB surfactants (3.5-8)

Question 10

Creams

Answer 10

• The type of surfactant used determines the type of cream prepared
  (o/w or w/o)
• Important to consider the HLB of the surfactant mixture
• Semi-solid creams require excess surfactant/co-surfactant compared
  to that required for stabilising runny emulsions
• The excess surfactants form structures in the bulk phase producing complex
  semi-solid multiphase systems

Question 11

Ointment vs cream vs lotion

Answer 11

• Generally, the greasier an emollient, the more effective it is, as it is able to trap more moisture in the skin
• However greasy emollients are often less acceptable or tolerated
• Creams are less greasy but generally more acceptable than ointments
• Lotions are good for very mild dry skin and for the face. They can also be used in hairy places where the application of ointments or thicker creams can be quite messy!
• Ointments should not be used where an infection is present (unless it is an antibiotic ointment)
• Over-use of greasy ointments can lead to folliculitis (blockage and inflammation of hair follicles)

Question 12

Emulsion – o/w or w/o??

Answer 12

Dye solubility test
* Mix emulsion with a water-soluble dye (amaranth) and observed under
the microscope.
* Continuous phase appears red  o/w type
* Scattered globules appear red  w/o type

Question 13

Emulsion – o/w or w/o

Answer 13

• Dilution of emulsions with water or oil
• If the emulsion is o/w type
• Diluted with water  remain stable as water is the dispersion medium
• Diluted with oil  the emulsion will break

Question 14

Emulsion – o/w or w/o

Answer 14

• Conductivity
• Based on the electrical conductivity of aqueous solutions, the electric current
  is supplied through electrodes placed in the emulsion
• If the current is passed  o/w type
• Not passed  w/o type

Question 15

Emulsion – o/w or w/o

Answer 15

• Fluorescence
• Based on the fluorescence of oils under ultraviolet light, examined under the
  light in the microscope
• Whole fluid is fluorescent  w/o type
• Spotty fluorescence  o/w type

Question 16

Hydrophile-lipophile balance

Answer 16

• Hydrophile-Lipophile Balance (HLB) system
• An empirical approach to aid the choice of surfactant(s) for a particular purpose, devised by Griffin in 1949
• For emulsification, the strategy is to match the HLB of the surfactants to the HLB of the oil phase being emulsified
• A number of surfactant(s) may possess the same HLB value, therefore experiments are performed to find the best emulsifying system

The HLB of a surfactant is expressed using an arbitrary scale* For non-ionic surfactants HLB ranges from 0 to 20
* HLB = 7 + Σ(hydrophilic group no) – Σ(hydrophobic group no)
* Higher HLB (> 10)* Hydrophilic surfactants
* Act as solubilising agents, detergents and o/w emulsifiers
* Low HLB (1 – 10)* Lipophilic surfactants
* Act as w/o emulsifiers
* Mixtures of surfactants with a high and low HLB usually provide more stability

Question 17

What is the HLB of the mixture of 40% Span 60 (HLB = 4.7) and 60% Tween 80 (HLB = 14.9)?

Answer 17

HLB = 0.4 x 4.7 + 0.6 x 14.9 = 10.82