Emulsions Flashcards
What is an emulsion?
A dispersed system containing 2 immiscible liquids
Consists on an internal phase distributed throughout the external phase
What are droplet sizes in emulsions
0.1 - 100 micro metre
What are the 2 liquids in an emulsion
one is aqueous
other is oleaginous (oil based)
What are the 2 types of emulsions
O/W and W/O
What is a W/O/W emulsion
small water droplets enclosed in a large oil droplet which are dispersed in water
what are microemulsions
droplets are colloidal dimensions (1 nm - 1 microm)
What are the 3 tests to identify emulsion type
Dilution test, conductivity test, dye-solubility test
What is the dilution test
Dilution can only occur with the external phase (ex. O/W can be diluted with water)
What is the conductivity test
An emulsion will conduct electricity if water is the continuous/external phase
What is the dye-solubility test
Dyes can be used to determine the emulsion (ex. a water soluble dye would result in uniform colouring in a water based emulsion)
What type of emulsion is used in oral administration
o/w (better taste)
What type of emulsion is used in intravenous administration
o/w (or else embolization can occur)
What type of emulsion is used in intramuscular administration
w/o (sustained/slow release)
What emulsion is used for external administration?
can be either o/w or w/o depending on use
o/w - not greasy, water washable
w/o - greasy, occlusive, form water repellent film
Which oils can be used in oral administration
liquid paraffin
castor oil
cod liver oil
peanut oil
Which oils can be used in IV admin
cottonseed oil
soya bean oil
safflower oil
Which oils can be used for external admin
turpentine oil
benzyl-benzoate
various oils
Can ionic emulsifying agents be taken orally
No, irritant to GIT (both cationic and anionic)
Which emulsifying agents can be used parenterally
certain non-ionic (lecithin, polysorbate 80, poloxamers
What are the types of emulsifying agents
synthetic and semi-synthetic (anionic, cationic, non-ionic, amphoteric)
naturally occurring
finely divided solids
Structure of anionic surfactants
Hydrophobic tail
Hydrophilic head with negative charge
Examples of anionic surfactants
sodium stearate, calcium oleate, triethanolamine stearate, SLS
Structure of cationic surfactants
hydrophobic tail
hydrophilic head with positive charge
Structure and solubility of non ionic surfactants
hydrophobic tail (carbon chain) hydrophilic head (alcohol, etc)
if hydrophobic portion > hydrophilic - oil soluble
hydrophilic > hydrophobic - water soluble
Examples of non-ionic surfactants
glycol/glycerol
Sorbitan/Spans (hydrophobic)
Polysorbates/Tweens (hydrophilic)
poloxalcohols (poloxamers - can be used for IV)
higher fatty alcohols (cetyl/stearyl alcohol - used with other surfactants)
What are disadvantages of naturally occurring emulsifying agents
batch to batch variation
susceptible to bacterial/mold growth (not good for products needed long shelf life)
Examples of naturally occurring materials
polysaccharides (acacia - o/w)
methylcellulose - o/w
lanolin - w/o
What are finely divided solids
accumulate on surface of droplet and stabilize them
Examples of finely divided solids
colloidal clays
bentonite
veegum
What are 3 emulsion additives
antioxidants, preservatives, humectants
what are antioxidants
prevent auto-oxidation of oil/lipid in emulsion
examples of antioxidants
BHA and BHT - true antioxidants (synergistic with chelating agents)
ascorbic acid
what are preservatives
prevent growth of microorganisms
- usually lipid and water preservatives combined (methylparabens (water) and propylparabens (oil))
- some emulsions inactivate preservatives, more required (ex. Tween)
what are humectants
absorb water from air to reduce evaporation of water phase
examples of humectants
propylene glycol, glycerol, sorbitol
what is interfacial tension
exists at the surface between two immiscible liquids
= force/length
Graph of micellization - what happens to interfacial tension as concentration increases
increase in surfactant = decrease in interfacial temsion
- interfacial tension decreases until CMC (when the surface layer is fully saturated) at which it becomes constant
- when additional surfactant is added, it forms micelles
- each surfactant has its own CMC
What is aggregation number
the number of surfactant molecules that form a micelle
what happens if an emulsion has a low CMC
the surfactant will form micelles before the oil/water droplets can be covered (therefore it is less efficient than a surfactant with higher CMC)
- more oil droplets covered = more stable emulsion (decreases interfacial tension)
What is Gibbs Adsorption Equation
the relationship between interfacial tension and adsorption of molecules
(review - page 42)
What is the HLB scale
hydrophile - lipophile balance
What is HLB 10-20 indicative of
hydrophilic
form o/w
What does HLB 0-10 represent
lipophilic
w/o
What are the 3 types of interfacial films
monomolecular film (ionic, non-ionic surfactants) multimolecular film (hydrocolloids - acacia, gelatin) solid particle film (finely divided solids - bentonite)
what is a stable emulsion
droplets remain uniformly distributed throughout the continuous phase
what are the 3 phenomena of unstable emulsions
- creaming or sedimentation
- flocculation
- coalescence
what is creaming/sedimentation
- creaming = droplets move upwards
- sedimentation = droplets move downwards
- movement depends on density of droplets compared to external phase
- reversible
- increases probability of coalescence
What factors decrease the rate of creaming/sedimentation (Stokes Law)
- decreasing droplet size
- decreasing density difference btwn phases
- increasing viscosity of external phase
- smaller concentration of dispersed phase
what is flocculation
aggregation of droplets into loose clusters
- increases rate of creaming (increased particle unit size)
what is coalescence (breaking, cracking)
- complete fusion of droplets
- separation of immiscible phases
- irreversible
How to formulate an emulsion
- using HLB calculations
- In situ soap formulation
- Wet gum and dry gum method
Calculations for a single surfactant for a multicomponent oil phase
- calculate the required HLB (review - page 43)
2. choose the surfactant with the closest HLB
Calculation of surfactant if a mixture of surfactants are used
Have a desired HLB but using 2 different surfactants
- use alligation to determine quantity of each surfactant to use
In Situ soap formation emulsions
- oleic acid (olive oil) + Ca(OH)2 (lime water) = ca-oleate (soap)
- stearic acid + NaOH or KOH = sodium or potassium stearate (soap)
- stearic acid + triethanolamine = triethanolamine stearate (soap)
What is the dry gum method
4: 2:1 (oil:water:gum)
1. acacia is added to oil
2. water is added all at once (hard fast trituration)
3. should hear a crackling sound
what is the wet gum method
4: 2:1 (oil:water:gum)
1. water is added to acacia in small portions
2. oil is slowly added
How does the surfactant determine the type of emulsion formed
more water soluble surfactant - O/W emulsion
more lipid soluble - W/O
What is phase inversion
used to achieve a more stable emulsion
ex. Wet gum method
2 parts water + 1 part gum -> + 4 parts oil = w/o emulsion 1st
add remaining water = o/w emulsion
What types of equipment are used for emulsification
- mechanical stirrers (large scale, low viscosity)
- homogenizers
- ultrasonifiers (makes a uniform emulsion)
- colloid mills (most used, reduces particle size)
How is shelf life of emulsions tested
employing stress conditions:
- aging and temp
- centrifugation
- agitation
