Disperse systems 3

Question 1

Types of emulsifier

Answer 1

Surface active agents
Hydrophilic polymers
Finely divided solids

Question 2

Choosing an emulsifier

Answer 2

Desired type of emulsion o/w or w/o
Route of administration- potential toxicity/ irritancy
emulsifying ability- mechanisms f action, strength of film
Compatibility with other excipients

Question 3

Oil in water or water in oil

Answer 3

Type of emulsifier- the phase in which the emulsifier is more soluble usually becomes the continuous phase
Phase volume ratio (PVR)- the phase present in the greater amount usually becomes the continuous phase

Question 4

Hydrophile-Lipophile balance (HLB)

Answer 4

Most emulsifier actions rely on them accumulating at interface of two phases
Different emulsifiers have different affinities for polar/ non-polar liquids: number and nature of polar and non-polar groups in the emulsifier dictates whether it is more hydrophilic, lipophilic or a balance of both

Question 5

HLB values

Answer 5

3-6 w/o emulsifier
7-9 wetting agent
8-16 o/w emulsifier
12-15 detergent
15-20 solubiliser

Question 6

Using HLB to choose emulsifiers

Answer 6

The HLB of an emulsifier dictates its suitability for a given oil, and the type of emulsion that will be formed
Tables of required HLBs for oils and emulsifiers are available
For mixed oil or mixed emulsifier systems RHLB/HLB can be calculated
It is also possible to calculate the minimum amount of emulsifier for a system, but the actual amount is determined experimentally

Question 7

Emulsifiers- mixtures

Answer 7

A combination of emulsifiers is often used to construct a strong complex film at an interface
A good mixture will produce a closely packed and/or complex film by using two agents with different hydrophilicities
The chemical structure of the chosen emulsifiers will affect the film produced

Question 8

Assessing emulsion stability

Answer 8

Visual examination- non-invasive, large sample size, only detects gross instability
Microscopy- particles visualised, quantitative, small sample size, subjective
Particle size distribution- many particles analysed, rapid, sample requires dilution
pH measurement- simple, rapid, invasive

Question 9

Factors affecting stability

Answer 9

Interfacial film properties
Globule size
Electrostatic repulsion
Phase volume ratio
Viscosity of continuous phase
Density difference between phases
Temperature