EMULSIONS

INTRODUCTION
 An emulsion is a dispersion of two immiscible liquids, one of which is
distributed uniformly in the form of fine droplets (the dispersed or
discontinuous phase) throughout the other (the external or continuous
phase).
 Emulsions are normally formed by mixing two immiscible liquids as both
liquids are immiscible into each other that’s why emulsions are
heterogenous dispersion system.
 The immiscible liquids are by convention described as `oil' and 'water',
as invariably one liquid is non-polar (e.g. an oil, wax or lipid) and the
other is polar (e.g. water or aqueous solution).
 Oil-in water (o/w) emulsions contain oil droplets dispersed in water, and
water-in-oil (w/o) emulsions contain water droplets dispersed in oil.

Fig:(a) An oil-in-water emulsion and (b) a water-in-oil emulsion. The shaded

area represents the oil.
MULTIPLE EMULSIONS
 Multiple emulsions can also be formed from oil and water by the re-
emulsification of an existing emulsion to form two dispersed phases.
 For example, multiple emulsions can be described as oil-in-water-in-oil
(o/w/o) emulsions. These are o/w emulsions which are further
dispersed in an oil continuum. Conversely water-in-oil-in-water
(w/o/w) type multiple emulsions can be prepared by further
emulsification of a w/o emulsion in water.
 Fig: A multiple w/o/w emulsion and a o/w/o emulsion. The shaded area
represents the oil.
THERMODYNAMIC STABILITY
 As emulsions are thermodynamically unstable and will attempt to
return to separate oil and water phases (i.e. crack) by coalescence of
dispersed globules unless they are kinetically stabilized by the addition
of emulsifiers.
 This happens because emulsions contain two different phases and both
phases have different physical and chemical properties. In the bulk
phase molecules are attracted to each other equally in all directions such
that no resultant forces are acting on them but upon bringing both
phases come in contact to each other they exhibit forces of attraction at
the boundary between two phases. E.g. Emulsion of water and mineral
oil.
 Thus, molecules situated the interface of both phases experience
interaction forces dissimilar to the molecules in each bulk phase. Due to
this reason, the actual contact area between dissimilar molecules will be
reduced and this is the reason emulsified droplets tend to cream and
coalesce.
 An emulsifier is operationally defined as a stabilizer of the droplet form
(globules) of the internal phase.
 On the basis of their structure, emulsifiers may be described as
molecules comprising both hydrophilic (oleophobic) and
hydrophobic (oleophilic) portions. For this reason, this group of
compounds is frequently called amphiphilic (i.e. water- and oil-
loving).

An emulsifier is a substance that helps stabilize emulsions, which are mixtures of two immiscible liquids, such as
oil and water. Here’s how an emulsifier functions as a stabilizer of the droplet form (globules) of the internal
phase:
 Mechanism of Action of Emulsifiers

1. Reduction of Surface Tension: Emulsifiers are amphiphilic molecules, meaning they have both hydrophilic
(water-attracting) and hydrophobic (water-repelling) parts. When added to an emulsion, the emulsifier
molecules position themselves at the interface between the oil and water phases. The hydrophilic part
interacts with water, while the hydrophobic part interacts with the oil. This arrangement reduces the
surface tension between the two phases, making it easier for them to mix.
2. Formation of Protective Film: As the emulsion is mixed, the emulsifier surrounds the droplets of the
internal phase (e.g., oil droplets in an oil-in-water emulsion), forming a protective layer. This stabilizing film
prevents the droplets from coming together and coalescing (merging) into larger droplets, which would
lead to phase separation.
3. Steric and Electrostatic Stabilization:
o Steric Stabilization: The physical presence of the emulsifier creates a barrier around the droplets, preventing
them from approaching each other closely enough to merge. This is particularly effective in stabilizing
emulsions with high concentrations of emulsifiers.
o Electrostatic Stabilization: Some emulsifiers can impart a charge to the droplets, causing like charges to
repel each other. This electrostatic repulsion further helps to keep the droplets separated.

4. Viscosity Increase: Emulsifiers can increase the viscosity of the continuous phase (e.g., water) in the
emulsion. A thicker continuous phase slows down the movement of droplets, reducing the rate of
coalescence and helping to stabilize the emulsion.

By reducing surface tension, forming protective films, and providing steric and electrostatic stabilization,
emulsifiers play a crucial role in maintaining the stability of emulsions. This ensures that the internal phase
droplets remain dispersed, preventing separation and maintaining the desired texture and consistency of the
product.

DECISION OF W/O EMULSION OR O/W EMULSION

 The type of emulsion that forms and the droplet size
distribution depends upon number of inter-related factors
which includes:
 • Method of preparation of emulsion
• Relative volumes of water and oil phases
• Chemical nature of emulsifying agent
 Generally, it is the dominance of the polar or non-polar characteristics of
the emulsifying agent which plays a major part in the type of emulsion
produced.
 When only oil and water are mixed vigorously in the absence of an
emulsifier, initially droplets of both liquids will be produced and as in
emulsions one liquid/phase exists in the form of droplets for the longer
time surrounded by the other liquid as continuous phase.
 So, here the liquid present in greater amount have probability to form
continuous phase because of its greater number of droplets which have
probability of rapid coalescence and subsequent formulation of
continuous phase and without emulsifier it would be temporary
emulsion. So, in order to produce stable emulsion, we need to add
emulsifier.
 When an oil, water and an emulsifying agent are shaken together, A
number of simultaneous processes have to be considered:
• Droplet formation
• Aggregation and coalescence of droplets
• Interfacial film formation
 In given Figure:
• (A) If the greater number of charged
molecules or the greater number of
hydrated polymer chains at the
interphase, there will be greater tendency
to reduce oil droplets coalescence and oil
will remain in droplets and in internal
phase while water will be the continuous
phase.
• (B) If longer hydrocarbon chain length
and greater the number of these
molecules present per unit area of the
film, the greater is tendency for water
droplets to be prevented from
coalescence and in this way water
droplets will remain in droplet form and
will be the internal
emulsion
GM Hamad

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Fig: (a) o/w emulsion, (b) w/o

emulsion
GM Hamad

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 phase of the emulsion and oil will be the continuous phase.
 The charged surface-active agents which are highly ionized and possess
strong polar groups, favor O/W emulsions such as sodium and potassium
oleates.
 The emulsifying agents which are little dissociated tend to produce W/O
emulsions such as calcium and magnesium soaps.
 Similarly, nonionic sorbitan esters favor w/o emulsions, while more
hydrophilic polyoxyethylene sorbitan esters produce o/w emulsion.

FORMULATION OF EMULSION
 The Choice of Formulation components will depend on:
• Emulsion type (o/w, w/o or multiple emulsion)
• The route of administration
• Clinical use
• Cost and Compatibility of Ingredients.
 The Processing conditions are also optimized as they control:
• Droplet size distributions and Rheological properties.
• Droplet size of internal phase of emulsion and consistency of
emulsion influence emulsion stability and its therapeutic response
because the smallest the globule size, greater will be the
absorption.
COMPONENTS OF EMULSION
 Aqueous phase  Antioxidants
 Oil phase  Flavourant for Oral Emulsion
 Emulsifying agent  Fragrance for cosmetic
 Preservatives emulsion
I. OIL PHASE
 Oil may be the medicament itself or may be used as carrier for some
lipid soluble drugs.
CONSIDERATION FOR OIL PHASE
 The desired physical properties of Emulsion
 The miscibility of the oil and aqueous phases
 The solubility of the drug in the oil
 The desired consistency of final emulsion.
FOR ORAL EMULSIONS

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  The selection of oil phase depends upon the purpose or the product. E.g.
Castor oil and mineral oil are used as laxative and fish liver oil and
arachis oil are used as nutritional supplements.
FOR EXTERNAL EMULSIONS
 Oils based on hydrocarbons are widely used. E.g. Liquid paraffin, soft or
hard paraffin. Similarly turpentine oil and various silicone oils are also
used.
FOR PARENTERAL EMULSIONS
 A range of purified vegetable oils have been used over many years in
emulsions for parenteral nutrition and as lipid soluble drug carriers. E.g.
refined fish oils, purified olive and soya oils.
II. EMUISIFYING AGENT
 The choice of emulsifier depends on many factors, these include:
• Type of emulsion to be prepared
• Emulsifier toxicity or irritancy
• Clinical use of emulsion
• Shelf life of emulsion
• Cost and availability
III. PRESERVATIVES
 An ideal preservative should:
• Exhibit a wide spectrum of activity against bacteria and fungi
• Be free from toxic and irritant activity
• Be stable to heat and storage
• Be chemically compatible (e.g. polyoxyethylene nonionic
surfactants and phenolic preservatives are incompatible)
• Have reasonable cost
• Have acceptable taste, odor and color.
 Examples: phenoxyethanol, benzoic acid and the p-hydroxybenzoates.
IV. ANTIOXIDANTS
 Antioxidants prevent oxidative deterioration of the oil, emulsifier or the
drug itself during storage.
 The antioxidants commonly used in pharmacy include butylated hydroxy
anisole and butylated hydroxytoluene at concentrations up to 0.2%, and
the alkyl gallates, which are effective at very low concentrations (0.001%
to 0.1%).
 A-Tocopherol is added to some commercial lipid emulsions to prevent
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 peroxidation of unsaturated fatty acids.
V. OTHER EXCIPIENTS
 Humectants (ability to attract and retain moisture from the environment ) are
often added to dermatological preparations to reduce evaporation of
water from the emulsion during storage and use. E.g. propylene
glycol, glycerol and sorbitol at concentrations up to 5% are added.
 Other excipients for proper formation of emulsion are flavoring agent
for oral emulsions and fragrances for topical cosmetic emulsions.

FUNCTIONS AND CLASSIFICATION OF EMULSIFYING AGENTS

 Emulsifiers are added to prevent coalescence.
 When two immiscible liquids are mechanically agitated, both phases
initially tend to form droplets. When the agitation is stopped, the
droplets quickly coalesce and the two liquids separate. Separation

of Liquids: As the droplets coalesce, they form larger droplets that

are less stable in the emulsion. Eventually, these larger droplets
may become heavy enough to break free from the dispersed phase
and rise to the top or settle at the bottom, leading to the separation
of the two liquids. This separation indicates that the emulsion has
broken down and is no longer stable.
FUNCTIONS OF EMULSIFYING AGENTS
 Emulsifiers generally impart stability by the formation of a mechanical or
electrostatic barrier at the droplet interface (an interfacial film) or in the
external phase (a rheological barrier).
 They impart thermodynamic and kinetic stability to emulsion.

RHEOLOGICAL BARRIER
 In many emulsions the external phase is thickened by the emulsifier
which means the emulsifier significantly increases the viscosity of
continuous phase.
 In this way the structured continuous phase forms a rheological barrier
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 to prevent the movement and close approach of droplets. That is why
they work as rheological barriers to prevent coalescence.
THERMODYNAMIC STABILITY
 As the surfactant emulsifier lowers the interfacial tension between the
oil and water. This facilitates the formation of droplets during

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 emulsification and reduces the thermodynamic tendency for
coalescence.
KINETIC STABILITY
 As emulsifier increases the viscosity of emulsion, that is how they
restrict the motion of globules and maintains the dispersion state of
emulsion for extended period of time.

CLASSIFICATION OF EMULSIFYING AGENTS

1. Synthetic or semisynthetic surface-active agents and polymers
 Surface-active agents and polymers
a) Ionic surfactants
 Anionic surfactants
 Cationic surfactants
b) Nonionic surfactants
c) Fatty amphiphiles
d) Polymeric surfactants
2. Natural macromolecular materials
a) Phospholipids c) Hydrophilic colloids
b) Steroidal emulsifiers d) Solid particles

1. SURFACE ACTIVE AGENTS AND POLYMERS

 Surface active agents are those compounds which
have a tendency to accumulate at the boundary
between two phases. Surface active compounds
have two distinct regions in their chemical structure,
a hydrophilic (water-liking) region and a hydrophobic
(water-hating) region.
 The existence of two such regions in a molecule is
referred to as amphipathy and the molecules are consequently often
referred to as Amphipathic molecules.
A. IONIC SURFACTANTS
I. ANIONIC SURFACTANTS
 They dissociate at high pH to form a long chain anion with surface
activity
i. ALKYL SULFATES
 Sodium lauryl sulfate (sodium dodecyl sulfate) is a commonly used
surfactant. Alone, it is a weak emulsifier of the o/w type but forms a

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 powerful o/w blend when it is used in conjunction with cetostearyl
alcohol.
ii. MONOVALENT SALTS OF FATTY ACIDS
 Emulsifiers in this group consists mainly of the alkali salts of long-chain
fatty acids and form o/w emulsions. They are generally formed in situ by
the interaction of a fatty acid with appropriate alkali. For example, in
white liniment, ammonium oleate is formed in situ from the reaction
between ammonia solution and oleic acid.
iii. DIVALENT SALTS OF FATTY ACIDS
 Calcium salts of fatty acids containing two hydrocarbon chains form w/o
emulsions because of their limited solubility in water. In zinc cream,
calcium oleate is formed in situ from the interaction between oleic acid
and calcium hydroxide.
II. CATIONIC SURFACTANTS
 They dissociate at low pH to form a long chain surface-active cation.
 Emulsions containing cationic surfactant as emulsifier are unstable at
high pH and in the presence of anionic materials including anionic
surfactants and polymers.
QUATERNARY AMMONIUM COMPOUNDS
These constitute an important group of cationic emulsifiers in

dermatological preparations because they also have antimicrobial
properties. For example, in cetrimide cream, the mixed emulsifier is
prepared by blending cetrimide (cetyltrimethyl ammonium bromide)
with cetostearyl alcohol to form cationic emulsifying wax.
B. NONIONIC SURFACTANTS
 Most nonionic surfactants are based on:
• A hydrophobic moiety with 12-18 carbon atoms. The starting
material may be a fatty acid or sorbitan.
• A hydrophilic moiety composed of an alcohol (-OH) and/or
ethylene oxide groups linked to form long polyoxyethylene
chains.
i. POLYOXYETHYLENE GLYCOL ETHERS (MACROGOLS)
 They are used as both o/w emulsifier and w/o emulsifiers. E.g.
cetomacrogol 1000.
ii. SORBITAN ESTERS (SPANS)
 These are hydrophobic and produce w/o emulsions. E.g. Sorbitan
monolaurate (Span 20), Sorbitan Monooleate (Span 80) etc.
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 iii. POLYOXYETHYLENE SORBITAN ESTERS (POLYSORBATES) (TWEENS)

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  They are more hydrophilic and used to produce o/w emulsions. E.g.
Polyethylene 20 sorbitan monolaurate (Tween20), Polyethylene 20
sorbitan monooleate (Tween 80).
C. FATTY AMPHIPHILES
 Fatty alcohols and fatty acids E.g. Cetyl alcohol and stearic acid used as
auxiliary emulsifiers. Glycerol monoesters E.g. Glyceryl monostearate
and glyceryl monooleate are the most common monoesters used in
dermatological preparations.
D. POLYMERIC SURFACTANTS
 The poloxamers are a series of neutral synthetic polyoxyethylene-
polyoxypropylene block copolymers which are used either alone or as
auxiliary emulsifiers with lecithin in small-volume parenteral injections.
2. NATURAL MACROMOLECULAR MATERIALS
A. PHOSPHOLIPIDS
 E.g. Purified lecithin derived from egg yolk or soya bean oil. They are
used extensively as o/w emulsifiers in parenteral and oral emulsions.
B. HYDROPHILLIC COLLOIDS;POLYSACCHARIDES
 Polysaccharides, including gums, such as acacia and tragacanth
and alginate and cellulose derivatives are hydrophilic colloids used
as emulsifying agents in oral preparations.
C. STEROIDAL EMULSIFIERS
 E.g. Wool fat (lanolin), wool alcohols (lanolin alcohols), beeswax and
cholesterol, used for their emollient properties and as w/o emulsifiers.
D. SOLID PARTICLES
 E.g. Bentonite, aluminum hydroxide, magnesium hydroxide,
aluminum magnesium silicate.

INSTABILITIES OF EMULSION
1. CREAMING
 Creaming is a process which occurs when the dispersed droplets
separate under the influence of gravity to form a layer of more
concentrated emulsion, the cream.
 There is aggregation of globules of the dispersed phase at the top
or bottom of the emulsion, similar to cream on milk.
 Creaming occurs inevitably in any dilute emulsion containing relatively
large droplets if there is a density difference between the oil and water
phases.

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  Creaming is a reversible process.

Original emulsion Creaming

REASONS OF CREAMING
 If emulsion contains large droplets of dispersed phase.
 If emulsion is dilute.
 If there is a density difference between the oil and water phases.
TO REDUCE CREAMING
 Small droplet size.
 Increase viscosity of external phase.
Q: When the globules of dispersed phase will rise to surface and when they
sediment to bottom?
Ans: It depends upon density of dispersed phase. If density of dispersed phase
is less than that of continuous phase the droplets of dispersed phase will rise
to the surface of emulsion. For example: More oils are less dense than water.
So, in o/w emulsion where the dispersed phase is oil. So, in this emulsion oil
droplets will rise to the surface to form an upper layer of cream and opposite
occurs in w/o emulsion.
2. FLOCCULATION
 Flocculation is a weak, reversible aggregation of droplets of the internal
phase in the form of clusters.

Original emulsion Flocculation

 Tendency for flocculation can be reduced by the use of a suitable

emulsifier, viscosity enhancer and internal phase volume.
 Flocculation is undesirable because floccules cream more rapidly under

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 the influence of gravity than individual droplets.
3. COALESCENCE AND CRACKING
 It is irreversible process in which dispersed phase droplets merge to
form larger droplets. The process will continue until the emulsion breaks
(cracks) and there is complete separation of the oil and water phases.

Original emulsion Coalescence Phase separation/

Cracking
 Absence of coalescence can be achieved by the formation of a thick
interfacial film from macromolecules or from particulate solids.
Q: How the droplets retain the individuality?
Ans: This is because of the charges on the surface of emulsified globule or
presence of mechanical protective barrier at the interface of globules which
prevents their coalescence. So, if the amount of emulsifier is insufficient to
work as proper mechanical or electrical barrier at these individual droplet
interfaces. These droplets can not remain intact and their coalescence will
occur rapidly.
4. OSTWALD RIPENING
 It is an irreversible process which involves the growth of large droplets
at the expense of smaller ones.

Original emulsion Ostwald ripening

 Ostwald ripening can be inhibited by the addition of the surfactant,

which is strongly adsorbed at the o/w interface and does not form
micelles in the continuous phase.
 It can also be inhibited by increasing the viscosity of the
emulsion external phase.

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 5. PHASE INVERSION
 It is an irreversible process in which an emulsion changes from one type
to another, for example o/w to w/o.

Original emulsion Phase inversion

REASONS FOR PHASE INVERSION
 If the amount of disperse phase approaches or exceeds a theoretical
maximum of 74% of the total volume.
 If the emulsifier solubility is changed.

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