Emulsion

Emulsion
Md. Saiful Islam
BPharm, MSc
North South University
Fb Group: Pharmacy Universe

Emulsions - Properties and Production
An emulsion is a thermodynamically
unstable dispersion of two mutually
“insoluble” liquids, such as water and
oil. One of these two components is
present in the form of finely
distributed spherical droplets in the
second, continuous phase. If oil is
dispersed in water, the emulsion is
referred to as an oil-in-water (o/w)
emulsion; the reverse case is a water-
in-oil (w/o) emulsion. Figure shows
the size ranges in which these liquid-
liquid dispersions occur.

THEORIES OF EMULSIFICATION
The natural instability of dispersions is due to a large “A” and a large “γ ” which cause a large “G,” and a large “G”
and a large “γ ” cause emulsified droplets and suspended particles, or the internal phase, to aggregate to reduce
“A” to reduce “G.”
Stable emulsions and suspensions must have a large “A” and a small “G” concurrently for consistent and uniform
dosing. This is done by decreasing “γ,” which will decrease “G,” which will decrease self attraction of dispersed
phase particles.
Gibbs Free Energy in an Emulsion
In emulsions, creaming (a reversible weak association of
internal phase droplets) and cracking (an irreversible
coalescence of internal phase droplets) may occur. The latter
may result to minimize Gibbs free energy by minimizing the
surface area of the internal phase.
Gibbs free energy states
Δ G = γ Δ A

Droplets are said to coalesce when they unite irreversibly. The individual stages of
the coalescence of two oil droplets in water are sketched in Figure. The individual
character of the droplets disappears on their coalescence. Coalescence can also
occur in a sediment or in creamed emulsion.
Coalescence of insufficiently stabilized droplets.
The stability of an emulsion with respect to
coalescence has greater practical significance than
does sedimentation, since droplets may exist collected
together for a long time without actually coalescing.
Such aggregates are maintained by a stable barrier
consisting of a thin layer of the outer phase separating
the droplets. Only when this barrier has been
destroyed may the droplets coalesce.
The cause of coalescence is the tendency to minimize
surface area while maximizing volume. A drop formed
by coalescence has a smaller surface area than that of
its two parent droplets together.

Sketch of the processes occurring in an unstable olw emulsion.
Only when coalescence occurs is
the emulsion irreversibly
destroyed: the droplets of an
emulsion can collect together by
diffusion or convection if the
forces holding them apart are
small. In the multiples thus
formed, the droplets are separated
by thin, liquid films.
The shapes of droplets in the collection depend on the interfacial tensions. Subsequent
processes involving stability, coalescence, and inversion are related to the properties of
the barrier films. In addition to the properties of these layers, the densities of the
substances involved are important; they control creaming and sedimentation.

Addition of only 2% of a soap to an emulsion of mineral oil in water reduces the interfacial
tension from 57 to only 2 mN/m; in the example above, this corresponds to a decrease in
the interface energy from 8 cal to 0.3 cal. The formation of the emulsion therefore requires
little energy in this case, and the emulsion is protected against coalescence. It is still
thermodynamically unstable, but is kinetically stabilized by the emulsifier.
Bancroft’s Rule
The type of emulsifier used is a decisive factor in the type of emulsion that results, olw or
wlo. Low molecular weight, hydrophilic emulsifiers induce formation of olw emulsions,
lipophilic emulsifiers favor wlo emulsions. Use of water-soluble macromolecular
emulsifiers also results in formation of olw emulsions.
In order to produce a long-lived technical emulsion, a third component is required, namely,
an emulsifier. The emulsifier must accumulate at the interface and form a protective layer
in the form of a tough, elastic film that is not broken when droplets collide. To optimize
these properties in practice, mixtures of emulsifiers are often used.

Bancroft’s Rule, can be expressed as follows:
The phase in which the emulsifier is more soluble is the outer phase. As the boundary film
has two surface tensions, we may also say: the film curves in towards the side with the
higher γ value, that is, the disperse phase is the one with the higher γ.
oriented wedge theory Furthermore, the type of emulsion may be
explained by the oriented wedge theory.
The part of the tenside molecule that has
the greater cross-sectional area is oriented
towards the dispersion medium. This
maximizes the density of the interfacial
film of emulsifier. Monovalent soaps form
ofw emulsions, polyvalent soaps form wio
emulsions. However, this theory does not
hold for monovalent silver salts, which give
wlo emulsions.

Emulsifiers
An emulsifier must have the following properties:
1. It must show good surface activity and create a low surface tension. If this is not the
case, the emulsifier can be combined with a suitable tenside. It must tend to migrate to
the surface rather than remaining in solution in the bulk phase. Therefore, it must have
both hydrophilic and hydrophobic groups. Too great a solubility in either one of the two
phases impairs the efficacy.
2. It must form a film at the interface, either on its own or in conjunction with other
molecules also adsorbed there. The film must be a condensed film; that is, in the case of
o/w emulsions, the hydrophobic groups in the interfacial film should interact strongly in a
lateral direction, i.e. with their neighbors in the film; in the case of w/o emulsions, the
hydrophilic groups should interact thus.
3. It must migrate to the interface fast enough to ensure that the interfacial tension is
lowered sufficiently while the emulsion is being manufactured.

4. Emulsifiers that are preferentially soluble in oil give wlo emulsions; low molecular
weight, hydrophilic emulsifiers such as soaps induce olw emulsions, as do water-
soluble, macromolecular emulsifiers.
5. A mixture of a preferentially oil-soluble tenside with a water-soluble tenside
creates more stable emulsions than a single tenside.
6. The more polar the oil phase, the more hydrophilic the emulsifier should be; the
less polar the oil to be emulsified, the more lipophilic the emulsifier.

Emulsifier Types
A. Low molecular weight emulsifiers of principally hydrophilic nature. Preferred
type as o/w emulsifiers
1. Anionic
soaps (Na, K , NH4, and morpholinium salts of fatty acids), Na lauryl sulfate, Na
cetyl sulfate, Na mersolate, Na 2-ethylhexyl sulfate, Na xylenesulfonate, Na
naphthalenesulfonate, Na sulfosuccinate, R-COOC2H4S03Na, R-CONHC2H4S03Na (R
= C17H33), Na oleyl lysalbinate, Na oleyl protalbinate, Turkey-red oil, natural
sulfonated oils, Na salts of dialkyl sulfosuccinate esters, bile salts, resin soaps.
2. Cationic
laurylpyridinium chloride, lauryltrimethylammonium chloride, laurylcolamine
formylmethylpyridinium chloride.
3. Nonionic
polyoxyethylene fatty alcohol ethers, polyoxyethylene fatty acid esters.

B. High molecular weight emulsifiers of principally lipophilic nature. Preferred type as
w/o emulsifiers
Mg stearate, Mg oleate, Al stearate, Ca oleate, Ca stearate, Li stearate, di-, tri- etc. esters of
fatty acids with polyols, cholesterol, lanolin, oxidized fats and oils.
C. High molecular weight emulsifiers with less pronounced properties
fatty acid esters of polyols and polyoxyethylene, polyoxypropylene fatty alcohol ethers,
polyoxypropylene fatty acid esters, lecithin, monoesters of fatty acids and polyols,
triethylcetylammonium cetyl sulfate, laurylpyridinium laurate, chloronitroparaffins.
D. High molecular weight emulsifiers
albumin, casein, gelatin, products of protein degradation (glue), gum arabic, tragacanth,
carrageenan, saponin, cellulose ethers and esters, polyvinyl alcohol, polyvinyl acetate,
polyvinylpyrrolidone.

Examples of emulsifiers.
Sodium oleate
Calcium stearate
Sodium lauryl sulphate
hexadecyltrimethylammonium
bromide (cetrimide)
Generic structure of the sorbitan fatty acid
esters.

Example: How much Span 80 (HLB = 4.3) and how
much Tween 80 (HLB = 15) are required to give an
HLB value of 12?
Example: How much Span 80 (HLB = 4.3) and how
much Tween 80 (HLB = 15) are required to give an
HLB value of 12?
Stabilization by Solid Particles
Very finely divided particles that are smaller
than the droplets in an emulsion and are well
wetted by the water or oil phase can stabilize an
emulsion. They accumulate at the water/oil
phase boundary in the form of a solid layer
which prevents the inner phase from coalescing.

Accumulation of solid particles at the o/w interface.
If the solid is preferentially wetted by one of the two phases, it
can gather at the interface if this curves away from the wetting
phase. Bentonites, which are preferentially wetted by water,
therefore form olw emulsions, in contrast to gas soot, which is
preferentially wetted by oil and yields wlo emulsions.

The fine particles at the interface prevent
both coalescence of the droplets and
also, if the solid particles repel one
another, aggregation. Solid particles with
a contact angle of 90˚ form the most
stable emulsions
In fact, the particles are wetted
principally by the outer phase. This
results in the minimization of the
interface between the two phases and
thus in further stabilization of the
droplets in the emulsion.

If only inorganic solids are used to stabilize an emulsifier, that is, if no actual
emulsifieris employed, the energy necessary for the formation of the droplets must
come from vigorous mixing, for example with a high-speed impeller, and the
particles must be added gradually. Alternatively they can be added together with the
inner and outer phases.
Benzene/water emulsions are stabilized by CaC03, toluene/water emulsions by
pyrite, water/benzene with powdered charcoal.
The accumulation of the solid at the oil/water phase boundary creates an interfacial
“film” of high stability and firmness, which increases the stability of the emulsion.
Indeed, an emulsion stabilized with tenside does become still more stable if the
interfacial properties of the disperse phase can be made to resemble those of a
solid. It is also possible that high zeta potentials build up due to the presence of the
powder in the interface, and consequently the emulsion is stabilized.

Identification: emulsion type
The emulsion type, olw or wlo, can be identified by various observations and
experimental methods:
1. olw emulsions have a creamy consistency, whereas w/o emulsions are oily or greasy
(the viscosity of an olw emulsion is often little different from that of a true aqueous
solution; in contrast, wlo emulsions often have an unctuous or buttery consistency,
which is generally a result of a liquid crystalline gel structure);
2. an emulsion mixes immediately with any liquid that is miscible with its dispersion
medium;
3. an emulsion can be colored with dyes that are soluble in the dispersion medium (for
example, methylene blue for o/w emulsions, Sudan blue for w/o emulsions);
4. o/w emulsions are usually fair electrical conductors.

Stability of Emulsions
Since emulsions are thermodynamically unstable, the word “stable” is used with
reference to the emulsion lifetime. In this context, three important concepts should
be mentioned :
I . Creaming and sedimentation. These phenomena occur as a result of disparities in
density. The rising or settling of the dispersed droplets is not necessarily associated
with aggregation and is generally not considered as instability. The droplets can be
redispersed.
2. Flocculation. Flocculation or coagulation of the dispersed liquid particles is a type
of emulsion instability. However, as long as the individual droplets exist, the
emulsion has not been destroyed, as the droplets can be redispersed.
3. Breaking of the emulsion; coalescence. The emulsion is only disrupted when the
droplets coalesce, and thus the phases separate and the emulsified system is
destroyed. Therefore, the rate of coalescence of the droplets was chosen as the only
quantitative measure for the stability of an emulsion.

Creaming
This phenomenon occurs primarily as a result of the density difference between the oil
and water phases and involves either the sedimentation or elevation of the droplets of
the internal phase, producing a layer of concentrated emulsion either at the top or
bottom of the container. Creaming is predominantly an aesthetic problem as the
resulting emulsion is rather unsightly; however, upon shaking the emulsion is rendered
homogeneous.
Patients often believe that an emulsion that shows evidence of creaming has exceeded
its shelf-life.
It is therefore important to understand the physicochemical basis of creaming in
emulsions and, in so doing, reduce the rate of or inhibit this phenomenon. The rate of
creaming ( δv /δt ) in an emulsion (in a similar fashion to suspensions) may be described
by Stokes’ equation: where: r refers to the average radius of the droplets
of the internal phase; (ρo – ρw) refers to the density
difference between the oil phase and the water phase; g
refers to gravity (which is negative if upward creaming
occurs); and η refers to the viscosity of the Emulsion .

Flocculation
The ability of emulsion droplets to flocculate is well known. In the flocculated state the
secondary interactions (van der Waals forces) maintain the droplets at a defined
distance of separation (within the secondary minimum).
Application of a shearing stress to the formulation (e.g. shaking) will redisperse these
droplets to form a homogeneous formulation. Although flocculation may stabilise the
formulation, there is also the possibility that the close location of the droplets (at the
secondary minimum) would enable droplet coalescence to occur if the mechanical
properties of the interfacial film are compromised.

Rate-Determining Factors in Coalescence
1. Nature of the interfacial film
2. Electrical and steric barriers
3. Viscosity of the dispersion medium
4. Volume ratio of the disperse phase and the
dispersion medium
5 . Droplet size distribution
6 . Temperature
Examples of a dense film (a), a film with loose packing (b), and a film with
insufficient packing (c), and the corresponding emulsion stabilities.

A frequently used mixture of emulsifiers
consists of oil-soluble sorbitan esters (Span)and
water-soluble POE-sorbitan esters (Tween). In
this Figure, a possible structure is depicted for
the stabilizing film built up at the phase
boundary out of these molecules.
The stronger interaction of the POE-sorbitan
with the aqueous phase causes the hydrophilic
group of this molecule to extend further into
the water than does the ester without
oxyethylene groups, and this permits the
hydrophobic groups of the two different types
of molecules to get closer to each other in the
interfacial layer and to interact better than
would be the case if only one of the tensides
were present.

Electrostatic and Steric Barriers
In olw emulsions, the charged, hydrophilic part of
the tenside faces the water, and the electrical
charge on the droplet acts as a barrier to prevent
coalescence. For ionic tensides, the sign of the
charge on the droplet is the same as that on the
tenside.
In emulsions stabilized with nonionic tensides, the
disperse phase is charged either by adsorption of
ions from the aqueous phase, or by motion and
friction of the droplets in the dispersion medium
separating electrical double layers.
High molecular weight emulsifiers stabilize
emulsions principally by steric repulsion.
Volume Ratio of the Disperse Phase
and the Dispersion Medium
An increase in the volume of the disperse
phase in relation to the volume of the
continuous phase leads to the enlargement of
the interfacial film area and thus to a decline
in stability. If the volume of the disperse phase
exceeds that of the continuous phase, the
emulsion becomes unstable with respect to
the inverted emulsion. The tenside layer
around the disperse phase is now larger than
the one that would be necessary to surround
the continuous phase; therefore, it is unstable
with respect to the smaller emulsifier film
(which has a lower free surface energy). If
both types of emulsion are possible with the
emulsifier used, then a phase inversion can
occur.

Size distribution of droplets in a stabilized
emulsion at different times (in days)
Temperature
The rate of coalescence of an emulsion depends
heavily on the temperature. A change in temperature
alters the interfacial tension between the phases. For
most liquids, y decreases linearly with increasing
temperature, as in the empirical formula of Ramsay
and Shields or Eotvos. In addition, the viscosity of the
interfacial film and the homogeneous phases, the
solubility of the emulsifier in both phases, and the
thermal motion of the particles all change.
Size Distribution of the Droplets
Larger droplets are thermodynamically more stable than smaller ones, since the ratio of interface (surface
area) to volume is lower. As a result the larger droplets grow at the expense of the smaller ones until the
emulsion breaks. The narrower the size distribution of the droplets, therefore, the more stable the
emulsion. The change in size distribution of droplets in an emulsion over time is represented in Figure.

Inversion of Emulsions
Phase inversion makes it possible for us to change an o/w emulsion into a wlo
emulsion or vice versa.
The type of emulsion depends, for example, on the order in which the phases
were added, the sort of tenside used, the ratio of phases, the temperature,
and the presence of electrolyte or other additives.
If water is added to an apolar tenside solution, a wio emulsion usually results,
whilst addition of oil to an aqueous solution of tenside yields an oiw emulsion.
Temperature-related change in the type of emulsion is caused by the change in
tenside hydrophobicity with temperature. For nonionic tensides, elevated
temperatures encourage conversion from olw to wio emulsion type; for ionic
tensides a change from wlo into olw tends rather to occur on cooling. The
behavior of the interfacial tension during the phase change is interesting. In a
narrow temperature range yoIw tends to 0, and the emulsion droplets cease to
be stable.

The inversion of an olw emulsion stabilized by an interfacial film of
sodium cetyl sulfate and cholesterol is depicted in Figure. Addition
of strong electrolytes, that is, polyvalent cations (Ba2+ or Ca2+ ),
neutralizes the charge on the droplets. Small quantities of water
are trapped inside the aggregating oil droplets. The molecules in
the interfacial film align themselves such that irregularly shaped
water droplets are formed, which are stabilized by a rigid,
uncharged film, and dispersed in the oil. The coalescence of the oil
droplets into a continuous phase completes the process of
inversion. The phenomenon of coalescence is important as the
start of both the creaming process and the inversion process.
Addition of electrolyte can also change the
hydrophobicity of the interfacial film and
thus cause phase inversion. Strong
electrolytes lower the electrochemical
potential of the particles, and the
interactions between tenside ions and
counterions are amplified. This can reduce
the stability of olw emulsions (salting out).

Manufacture of emulsions
Generically the manufacture of emulsions involves the following steps:
1. dissolution of the oil-soluble components in the oil vehicle and the (separate)
dissolution of the water-soluble components in the aqueous phase
2. mixing of the two phases under turbulent mixing conditions to ensure the
dispersion of the two phases into droplets.
At the laboratory the manufacture of emulsions usually involves the use of a
mechanical stirrer whereas the manufacture of creams involves mixing the two
(heated) phases using a mortar and pestle. The emulsification of production-scale
batches is normally performed using mechanical stirrers, homogenisers, ultrasonifiers
or colloid mills. The use of colloid mills is usually reserved for formulations of higher
viscosity, e.g. creams, due to the high running cost and slow production rate of this
apparatus.

Emulsion

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Emulsion