Emulsions Flashcards
Definition of emulsion
Emulsion is a system consisting of 2 immiscible LIQUID phases, one of which is dispersed as fine globules throughout the other.
Stabilised by addition of EMULSIFYING AGENT
What is the phase that is subdivided into globules
Disperse phase or internal phase
What types of emulsions are there
simple emulsions (o/w or w/o) multiple emulsions (wow or owo) micellar emulsions (oil-in-water micellar emulsion and reverse micellar emulsion) microemulsions (small globules 10-75nm)
Which type of emulsion’s globules cannot be seen under microscope
microemulsion
micellar emulsion
Oil-in-water micellar emulsion consists of what
oil found inside the non-polar interior of micelles, present in aqueous continuous phase
reverse micellar emulsion consists of what
water found inside the polar interior of the micelles, present in an oily continuous phase
What methods can be used for determination of emulsion type
Colour feel on skin filter paper wetting filter paper impregnated with cobalt chloride fluorescence conductivity dye test with sudan III dilution
Sudan III dye is soluble in which phase
oil, will dye the oily part.
Which simple emulsion is miscible in water
o/w emulsion
What are the basic components of an emulsion?
oily phase
aqueous phase
emulsifying agent
Properties of oil phase which affect the performance of the emulsion
consistency (viscous)
Feel or tactile characteristic (tacky or sticky feeling)
stability
drug solubility
What are the factors affecting the selection of emulsifying agents
Type of emulsion (o/w or w/o)
compatibility with other components (i.e with preservatives)
toxicity of emulsifying agent
cost of emulsifying agent
What are surfactants and what can they do
Compounds with hydrophilic and lipophilic group in their molecular structure.
Cause molecules to become attached to interfaces, thereby lowering interfacial tension
What are the 4 major groups of surfactants
anionic
cationic
amphoteric
non-ionic
What are anionic surfactants usually used for and what are the different types of anionic surfactants
Generally used for external preparations. Incompatible with cationic compounds, low pH, and high concentration of electrolytes.
Soaps of monovalent bases, polyvalent bases, amine soaps, sulphated and sulphonated fatty acids and alcohols, and quaillaia saponins.
What emulsion is suitable to use soaps of monovalent bases. Name one e.g. of this base
oil in water,
Sodium, potassium or ammonium stearate
what emulsion is suitable to use soaps of polyvalent bases. Name one e.g. of this base
water in oil
calcium or zinc oleate
What is the difference between amine soaps vs soaps of monovalent bases.
amine soaps are suitable for o/w emulsions where a high pH cannot be tolerated (won’t cause pH to be very high) whereas monovalent soap bases can make the pH high.
Name examples of sulphated and sulphonated fatty acids and alcohols. What emulsion can they be used for and are they strong surfactants?
Sodium lauryl sulphate and sodium cetyl sulphate.
used in o/w emulsion
strong
Properties of cationic surfactants?
Emulsifying and antiseptic, incompatible with anionic compounds
promote formation of o/w emulsions
examples of cationic surfactants?
quaternary ammonium compounds like cetrimide, cetyl pyridinium chloride and benzalkonium chloride.
Example of amphoteric surfactant?
lecithin, used for IV fat emulsions
cationic at low pH, anionic at high pH
Properties of nonionic surfactants?
low toxicity and irritancy
less sensitive to pH changes and addition of electrolytes
Used for external and internal preparations
types of Nonionic surfactants
sorbitan esters and polyoxyethylene sorbitan esters (POE)
glycol and glycerol esters
fatty acid polyglycol esters
faty alcohol polyglycol ethers (macrogol ethers)
higher fatty alcohols
what are the trade names for sorbitan esters and polyoxyethylene sorbitan esters respectively?
Span and tween respectively
What preservative is incompatible with all nonionic surfactants with POE or polyglycol group and why?
phenols due to hydrogen bond.
Which nonionic surfactant is used as auxiliary emulsifying agents?
higher fatty alcohols like cetostearyl alcohol
Which nonionic surfactants promote formation of o/w emulsions
glycol and glycerol esters (glyceryl monostearate)
Fatty acid polyglycol esters (POE fatty acid esters)
Which nonionic surfactants require blending of hydrophilic and lipophilic members?
fatty alcohol polyglycol ethers (POE fatty ethers or macrogol ethers)
Spans and Tweens also
Types of hydrophilic colloids
natural and synthetic clays (bentonite)
natural and synthetic gums (acacia, tragacanth, sodium alginate, carrageenan locust bean, guar, xanthan, sodium CMC, methyl cellulose)
proteins (gelatin and soluble casein)
Properties of hydrophilic colloids
used as emulsifying agents and as thickening agents. generally favour the formation of o/w emulsion
What is one reason for finely divided solids to be sterilised before use?
those of mineral origin may contain tetanus spores
What are the two types of finely divided solids
polar inorganic i.e heavy metal hydroxides, non-swelling clays. These favour the formation of o/w emulsions
Non-polar solids like carbon, glyceryl tristearate. they favour the formation of w/o emulsions.
Based on the theory of emulsification, what can we do to make the system of 2 immiscible liquids more thermodynamically stable?
Formation of rigid interfacial film with a surfactant
Formation of electric double layer (applies to ionic surfactant - prevents coalescence of droplets)
Increasing the viscosity of continuous phase using gums and waxes (for water and oil respectively)
What is the Bancroft theory
adsorption of surfactants at oil/water interface give rise to interfacial film.
the film experiences 2 interfacial tensions, one between film and the aqueous phase and the other between film and the oil phase.
the film will curve in the direction of the greater interfacial tension.
thus the disperse phase is on the side of the film with higher interfacial tension.
According to bancroft theory, if the interfacial tension between film and oil is greater than that of film and water, which emulsion will it form?
oil in water emulsion. because the disperse phase will be the one with higher interfacial tension with the film.
Oriented wedge theory by Harkin tried to explain about what?
Type of emulsion formed by soaps of different valencies and how the surfactant molecules will orientate at the oil/water interface such that a tightly packed film can be formed.
How are the wedges shaped differently for monovalent base vs di/polyvalent bases
Monovalent base the fat part of the wedge is the hydrophilic head, whereas the narrow part is the lipophilic tail. (o/w)
di/polyvalent bases the fat part of the wedge is the lipophilic tail, whereas the narrow part is the hydrophilic head (w/o)
the narrow part of the wedge will always face the disperse phase.
Theory based on angle of contact applies to what kind of emulsifying agents?
Finely divided solids which are:
insoluble in both aqueous and oily phases
preferentially wetted by one of the phases
able to form a thin interfacial film
and of colloidal size (less than 1 micron)
If the finely divided solid is preferentially wetted by the oil, it means that the contact angle will be higher (>90 degrees), what does this tell you about the emulsion.
It is an water in oil emulsion
How to calculate HLB
(weight of hydrophilic group/ molecular weight) * 20
higher the HLB, the more hydrophilic the surfactant is.
what is the HLB range for solubilising agents?
10 to 18
Does varying different blends of surfactants with the same HLB value lead to varying emulsifying efficiency?
yes
After finding the most effective HLB of a given pair of surfactants, what else can we do to find the optimal blend of surfactants?
Vary the surfactants at that specific HLB number to see which emulsion is most stable.
what are the techniques of emulsification?
agent-in-water method
agent-in-oil method
nascent soap method
alternate addition method
(hydrophobic agent in oil and hydrophilic agent in water method) - Like in experiment 1.
What is the agent-in-water method
emulsifying agent incorporated in water, then oil is added with vigorous agitation.
produces o/w emulsion directly.
further addition of oil will lead to phase inversion for w/o emulsion
what is the agent in oil method
emulsifying agent incorporated in oil, which is then added into water with vigorous agitation
produces o/w emulsion
continue adding oil to result in phase inversion to form w/o emulsion.
describe nascent soap method
suitable for emulsions with soap as the emulsifying agent.
Fatty acid is dissolved in the oil and the base in the water.. the soap is formed in situ as the oil and aqueous phases are brought into contact during mixing and an emulsion is subsequently produced.
depending on the soap (mono or divalent base), o/w or w/o emulsions are produced.
describe alternate addition method
particularly suitable for vegetable oils.
water and oil are added alternatively in small amounts to the emulsifying agent.
can produce either o/w or w/o emulsions.
Types of emulsifying machines
simple stirring via propeller mixer, turbine mixer and paddle mixer
colloid milling
vibration and ultrasonification
How can emulsion instability be shown
creaming, flocculation, coalescence and/or cracking
What is the limitation of stokes law on creaming?
stokes law refer to 1 isolated sphere only whereas in real life there are alot of droplets and they mutually interfere.
what can stokes law tell us about creaming
can tell us which factors to vary to slow down the creaming rate and improve stability of emulsion
what are the factors that can be used to reduce the rate of creaming based on stokes law
reduce radius of droplet (r)
increase viscosity of continuous phase (n)
having lower difference (p1-p2) in densities of disperse phase and continuous phase
whats the meaning of flocculation
reversible aggregation of droplets of disperse phase in the form of 3D clusters.
how can flocculating increase the rate of creaming
flocculated particles behave as single particles i.e their radius of particle increases = higher rate of creaming by stokes law.
what does coalescence mean
droplets joining to form larger drops. this process is irreversible, leading to a decrease in the number of droplets and finally cracking.
What does cracking mean
complete breakdown of the emulsion, with coalescence of the droplets and a separation of the two phases into two layers. this process is irreversible.
What are some factors that may cause cracking
addition of substances incompatible with emulsifying agent
pH changes
temperature changes
bacterial and fungal action
Examples of substances that are incompatible
with the emulsifying agent and cause
cracking of the emulsion
Strong acids + Alkali-metal soaps
Anionic compounds + Cationic surfactants
Cationic compounds + Anionic surfactants
High concentration of electrolytes + Soaps
Alcohol + Gums
What are the various ways of stress testing?
centrifugation (3750rpm for 5h in 10cm radius centrifuge, this test is similar to 1 year of shelf standing)
agitation
freeze-thaw cycles from -25 deg C to 25 deg C
heating-cooling cycles from 45 deg C to 5 deg C
Types of tests for stability testing?
determination of degree of separation (greater degree of separation = less stable)
size analysis of globules (greater increase in mean globule size with time = less stable)
determination of electrophoretic property (reduction in conductivity in o/w emulsion = instability; conductivity of w/o emulsion = instability)
determination of viscosity changes (when viscosity has an abrupt change i.e sudden increase = cracking)
