SCS Summer School 2015
Past Exam
Question
Russell Cox
2014 Emulsion Exam Question
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Read the question fully before starting to
write your answers
Look at the allocation of marks for particular
sections of the question
Don’t panic!
a;
• Define what an emulsion is, and describe the two
main types that are found in cosmetic formulations
(3)
a;
• Define what is an emulsion, and describe the two
main types that are found in cosmetic formulations
(3)
• An emulsion is a dispersion of one phase in another
non-miscible phase.
• Cosmetic formulations tend to be made from oil and
water, the two most common types are oil in water
(O/W) and water in oil (W/O).
b;
• The type of emulsion cannot simply be determined by
considering the relative percentages of the water and
oil phases. How does one theoretically describe the
emulsion formed?
(1)
b;
• The type of emulsion cannot simply be determined by
considering the relative percentages of the water and
oil phases. How does one theoretically describe the
emulsion formed?
(1)
• Bancroft rule states “The phase in which an
emulsifier is more soluble constitutes the continuous
phase”, therefore an O/W emulsion requires an
emulsifier that is preferentially soluble in the water
phase
c;
• Are emulsions stable?
(1)
• Thermodynamically emulsions are metastable –in the
emulsified state, they are stable for a period of time
however, the two immiscible phases will eventually
separate out. The exception to the rule is
microemulsions
d;
• How can one visually tell if an emulsion is stable or
not?
(6)
• Creaming, Where the less dense phase migrates to
form a layer at the top of the emulsion
• Sedimenting, occurs when the denser dispersed
phase migrates to form a layer at the bottom of the
emulsion
d;
• Flocculation, where the dispersed droplets cluster
together, but do not lose their droplet identity.
Flocculation gives an appearance of a fluffy cloud like
suspension in the emulsion, which can be redispersed by mixing the emulsion again.
• Coalescence, similar to flocculation, except the
droplets merge with each other to form a larger
droplet. Coalescence is a non-reversible process,
and can only be prevented by re-formulating the
emulsion.
d;
• Phase inversion. Where the phase of the emulsion
inverts from one type to another, for example an O/W
inverts to a W/O emulsion. This can occur when the
emulsifier becomes more soluble in the dispersed
phase
• Ostwald ripening. In solid solutions or liquid sols, the
small crystals or sol particles dissolve and redeposit
themselves as larger particles, as thermodynamically
larger particulates or droplets are more stable. In the
case of emulsions, molecules diffuse from small
droplets to form large ones within the continuous
phase.
e;
• ∆G=(γA)-(T∆S)
• Define the terms in the equation and describe how
they relate to the stability of an emulsion.
(5)
e;
• ∆G=(γA)-(T∆S)
• Define the terms in the equation and describe how
they relate to the stability of an emulsion.
(5)
• ∆G = gibbs free energy, an indicator of the
thermodynamic stability of an emulsion. If the value
of ΔG is positive then spontaneous emulsification is
unlikely, if ΔG is negative then spontaneous
emulsification will occur. Additionally the closer the
value of ΔG is to zero the easier the emulsification
process will be.
e;
• γ = interfacial tension. Reducing the surface energy
and thus reduce the interfacial tension between the
dispersed droplet and the continuous phase.
Therefore using an emulsifier will reduce the γA value
and lower the value of ΔG.
• A = interfacial area. Reducing the surface area
reduces the contribution of the γA value, thus
reducing the value of ΔG
e;
• T = temperature. Increasing the temperature will
increase the magnitude of the TΔS value, and thus
reduce the overall value of the Gibbs free energy for
the system.
• ΔS = entropy of mixing. Increasing the entropy (or
level of disorder within the system) will also increase
the TΔS value, and thus reduce the magnitude of ΔG.
f;
• What is an emulsifier and how does their use
influence the stability of an emulsion?
(2)
f;
• What is an emulsifier and how does their use
influence the stability of an emulsion?
(2)
• An emulsifier can be visualised as containing a
hydrophilic (water loving) head, and a hydrophobic
(oil loving) tail. Due to its structure the emulsifier will
sit at the interface between the dispersed droplet and
the continuous phase. The inclusion of an emulsifier
into an emulsion system will have the effect of
reducing the surface energy of the dispersed droplet
g;
• Explain what is HLB? Discus the advantages and
disadvantages of the HLB theory
(4)
g;
• Explain what is HLB? Discus the advantages and
disadvantages of the HLB theory
(4)
• HLB is the hydrophilic lipophilic balance, proposed by
Griffin to account for the behaviour of emulsifiers
• The theory assigns the emulsifier a value to
represent its affinity towards either the oil or water
phase. The number is in the scale of 1-20 where the
lower the number the more oil miscible the emulsifier
and the higher the number the more water miscible
g;
• The obvious advantages of the HLB theory is
emulsifiers can be categorised by their potential
behaviour and emulsifier blends can be tailored to
specific oil systems
• There are a few disadvantages; it does not consider
the ratio of oil phase to water phase, no indication as
to the level of emulsifier required, it doesn’t work well
for emulsifying waxes or account properly for the use
of fatty alcohols, no consideration of the effect of
temperature or electrolytes on the HLB of ethoxylates
h;
• How may HLB be used to select emulsifiers for a
particular emulsion system, and how does it relate to
defining the type of emulsion formed?
(3)
• The HLB requirement of a blended oil phase can be
either calculated or determined experimentally.
• Once the HLB requirement is known, the percentage
of emulsifiers required to achieve the target HLB can
be calculated.
h;
• To determine the type of emulsion O/W or W/O, one
would review the relative percentages of each
emulsifier and the respective individual HLB values
• The emulsifier present in the greater proportion would
be, according to Bancrofts rule, considered for its
solubility in its preferred phase
• The higher the HLB value (>10) for the larger quantity
emulsifier then the more likely a O/W emulsion will be
formed –and vice versa, if its HLB value was lower
(<10) then the emulsion will tend to be W/O