Discussion
1. What are the HLB values that will produce a
stable emulsion? Discuss.
Eight simple emulsions were prepared in eight test tubes each with
the same amount of oil and distilled water which is 4mL, surfactant but with a
different HLB values. The test is conducted with different types of oil which
are Palm Oil, Arachis Oil, Olive Oil, and Mineral Oil. The emulsion appears to
be the most stable were observed. Stable emulsion meaning here is the one least
separated or the one that separates last. The HLB value of the surfactant used
in this emulsion is the HLB requirement for that particular oil phase. The HLB
of an emulsifier is an expression of its Hydrophile-Lipophile Balance, which
are the balance of the size and strength of the hydrophilic (water-loving or
polar) and the lipophilic (oil loving or non-polar) groups of the emulsifier.
All emulsifiers consist of a molecule that combines both hydrophilic and
lipophilic groups. An emulsifier that is lipophilic in character is assigned a
low HLB number (below 9.0), and one that is hydrophilic is assigned a high HLB
number (above 11.0). Those in the range of 9-11 are intermediate. When two or
more emulsifiers are blended, the resulting HLB of the blend is easily
calculated by using the formula:
HLB value =
(quantity of surfactant 1)(HLB of surfactant 1)+(quantity
of surfactant 2)(HLB of surfactant 2)
quantity of
surfactant 1 + quantity of surfactant 2
The HLB of an emulsifier is related to its
solubility. Thus, an emulsifier having a low HLB will tend to be oil-soluble,
and one having a high HLB will tend to be water-soluble, although two
emulsifiers may have the same HLB and yet exhibit quite different solubility
characteristics. The SPAN emulsifiers are usually lipophilic and the TWEEN
products are usually hydrophilic.
2. Compare
the physical appearance of the mineral oil emulsions produced and give your
comments. What is Sudan III test? Compare the colour dispersion in the
emulsions produced and give your comments.
In this experiment, Emulsion I
contains 20ml of mineral oil, Emulsion II contains 25 ml of mineral oil,
Emulsion III contains 30ml of mineral oil while Emulsion IV contains 35ml of
mineral oil.
Before homogenization, generally
Emulsions Ι, ΙΙ, ΙΙΙ and ΙV are less viscous, more oily and not consistent.
Phase inversion is likely to occur. After homogenization, generally Emulsions
Ι, ΙΙ, ΙΙΙ and ΙV are more viscous, less oily and more consistent. Emulsions
produced after homogenization is more stable.
Sudan III stain is a biological
stain used to stain sudanophilic substances, usually lipids and it is usually
provided as a saturated alcoholic solution. Sudan dyes have high affinity to
fats; therefore they are used to demonstrate triglycerides, lipids, and
lipoproteins. Thus, Sudan III test can be used to differentiate between
oil-in-water emulsion and water-in-oil emulsion. If Sudan III solution is added
to an emulsion and the disperse phase appears red, then it is oil-in-water
emulsion. However this test is only applicable if the non-ionic emulsifier is
used in the preparation (e.g. Tween 80 and Span 20).
The red stain of Emulsions Ι, ΙΙ,
ΙΙΙ and ΙV is uneven before homogenization. So, we can say that these emulsions
formed are water-in-oil emulsion. However, after homogenization, the red stain
is even. Red globules have been seen in uniform dispersion on a colourless
background. The size of the globules is small. Hence, oil in water emulsion is
formed after homogenization. Here, phase inversion occurred.
3. Plot and give comments on:
a) Graph of the sample’s viscosity before and after the temperature
cycle against the different amount of oil content.
Amount of Oil (ml)
|
Average Viscosity (cP) (x ± SD)
|
Viscosity difference (%)
(x ± SD)
|
|
Before temperature cycle
|
After temperature cycle
|
||
20 mL of Palm oil
|
78.33 ± 57.57
|
114.17 ± 103.86
|
45.76% ± 80.41%
|
25 mL of Arachis oil
|
60.00 ± 28.28
|
105.00 ± 11.18
|
75% ± 60.47%
|
30 mL of Olive oil
|
490.00 ± 46.19
|
2626.67 ± 219.97
|
436.06% ± 376.23%
|
35 mL of Mineral oil
|
7363.33 ± 1563.83
|
11325.00 ± 4129.14
|
53.80% ± 164.04%
|
In the experiment, we use different oils to prepare 4 types of emulsion
but the same emulsifying agent is used for both emulsions. The emulsifying
agent used is acacia. Acacia is a hydrophilic colloid that forms multimolecular
adsorption at the oil/ water interface. It has low effect on the surface
tension. Its main function as emulsion stabilizers is by making coherent
multi-molecular film. This film is strong and resists the coalescence. They
have, also, an auxiliary effect by increasing the viscosity of dispersion
medium. The HLB value of acacia is 8 and form oil-in-water emulsions. Acacia
contains Arabic acid and proteins (COOH and NH3) and provides
electrostatic repulsion.
Different formulations
of emulsion have different physical properties and also stability. Viscosity is
one of the physical properties that used to characterize emulsion. Temperature
cycle is one of the accelerated stability testing method. In the experiment,
different amount of oils (20mL of palm oil, 25mL of arachis oil, 20mL of olive
oil and 35mL of mineral oil) are used in the formulation of emulsion. Oil has
higher viscosity than water. Thus, emulsion with higher amount of oil will have
higher viscosity. This can be shown on the graph above. From the graph, the
viscosity of emulsion increases as the proportion of oil increases. This trend
is same for both before and after temperature cycle graphs.
Increase
in temperature of emulsion will cause decrease in viscosity of continuous phase
and will increase the kinetic motion of dispersed phase. The rate of creaming
increases as the viscosity of continuous phase decreases. As the kinetic energy
of dispersed phase increases, the globules gain energy and collide with each
other more frequently, thus results in breakdown of emulsifying agent and cause
coalescence. Partial coalescence could
also have occurred during the cooling of the emulsions. At low temperature,
cold denaturation of proteins may occurred which could lead to a loss of
protein functionality when the emulsions were thawed. Besides, the
crystallization of some of the water causes the globules to be forced closer
together, thereby promoting globule-globule interactions. When the water phase
froze, insufficient free liquid water present to fully hydrate the emulsifying
agents present at the globule surfaces, thereby promoting globule-globule
interactions. The temperature cycle will disrupt the adsorbed layer of
emulsifying agent at the oil/water interphase.
Furthermore, an oil-in-water emulsion
stabilized by non-ionic emulsifying agent such as acacia will cause phase
inversion to water-in-oil emulsion on heating. This is because as the
temperature increases, the HLB value of a non-ionic emulsifying agent will
decrease as it becomes more hydrophobic. At the phase inversion temperature (temperature
at which the emulsifier has equal hydrophilic and hydrophobic tendencies), the
emulsion will be inverted. From the graph, the viscosity of all the emulsions
after the temperature cycle increases because the oil-in-water emulsion has
become water-in-oil emulsion where oil is the continuous phase, contribute to
the higher viscosity of emulsion.
b) Graph of the difference
of viscosity (%) against the different amount of oil content.
From
the graph above, the viscosity differences before and after the temperature
cycle are almost the same which are 45.76%, 75% and 53.8% for emulsions
prepared from 20 mL of palm oil, 25 mL of arachis oil and 35 mL of mineral oil
respectively. However, there is a very large viscosity difference for emulsion
prepared from 30 mL of olive oil which is 436.06%. By theory, larger viscosity
difference indicates a weaker and less stable emulsion. As the volume of
dispersed phase increases, the stability of the emulsion decreases and phase
inversion may occur. The small viscosity differences for emulsions composed of
20 mL of palm oil and 25 mL of arachis may be caused by coalescence of
dispersed phase after the temperature cycle, not due to phase inversion. On the
other side, the emulsion prepared from 30 mL of olive oil may undergo phase
inversion after the temperature cycle. This causes oil phase become the
continuous phase which has a much higher viscosity than water, so contribute to
much higher viscosity difference. The
emulsion prepared from 35 mL of mineral oil may already undergo phase inversion
before the temperature cycle, so there
is only a slight increase in viscosity (small viscosity difference cycle) after
the temperature cycle. To make the result more accurate, there are several
precaution steps that can be carried out in this experiment. Firstly, stir the
emulsion thoroughly before getting the viscosity readings from the viscometer.
Secondly, only take readings after the readings in the viscometer become stable
and constant. Finally, clean the rotor in the viscometer by dipping it in the
distilled water before taking the second and third readings.
4. Plot the graph of
the separated phase ratio after centrifugation process against the different
contents of mineral oil. Give your comments.
Ratio of phase separation for different amount of different types of oil
Types of oil (mL)
|
Ratio of Phase Separation (x ± SD)
|
Coconut Oil
(20mL)
|
0.715±0.085
|
Arachis Oil
(25mL)
|
0.670 ± 0.03
|
Olive Oil
(30mL)
|
0.250 ± 0.03
|
Mineral Oil
(35mL)
|
0.245 ± 0.025
|
Types
of oil
|
Ratio
of Phase Separation
|
Coconut
oil
|
0.715
|
Arachis
oil
|
0.670
|
Olive
oil
|
0.250
|
Mineral
oil
|
0.245
|
From the graph, it could
be seen that the separation ratio decreases from coconut oil to arachis oil to
olive oil and to mineral oil. So, as a conclusion, mineral oil has the lowest
ratio of phase separation. This may be caused by the structure of mineral oil
which is made up of only carbon and hydrogen. This causes the structure to
become non polar. When comparing mineral oil and the other oils such as coconut
oil, arachis oil and olive oil, they contain carboxylic group which renders
them more polar than mineral oil. Hence, if the polarity of an oil is low, it
will cause the ratio of phase separation to be low. This will also cause the
emulsion formed to be more stable. As such, the separated phase ratio must be kept at
minimum to produce a stable and homogenous emulsion to ensure that the drug can
be dispersed uniformly in the emulsion and an accurate dose of drug can be taken
for each volume of emulsion administered.
5. What are the functions of every substance used in this emulsion
preparation? How the different contents of substances can affect the physical
characteristics and stability in the formulation of an emulsion?
There are some
ingredients that has to be used to prepare an emulsion. They are oil phase (mineral
oil for example turpentin oil), aqueous phase (syrup, alcohol and distilled
water), emulsifying agent (acacia) and other substances such as vanillin.
First and foremost, the oil phase of the emulsion,
given by the mineral oil, functions as the oil phase in this oil in water (o/w)
emulsion. Mineral oil is the dispersed phase in this emulsion. The oil content
is set in range of 40% to 60% in order to produce a stable o/w emulsion.
Besides that, the oil phase of an emulsion can provide emollient effects.
Syrup is responsible as a sweetening agent. This is
because emulsion normally will give a taste of oil. To mask the taste of the
oil phase, sweetening agent is required for this purpose. Moreover, syrup
increases the viscosity of the emulsion. However, the amount of syrup used has
to be controlled as this will determine the flow of the emulsion. Sufficient
amount of syrup will produce an emulsion which has good flow and can be easily
poured out, Besides that, emulsion will not form a viscous layer on the side of
the container.
Alcohol functions as a preservative to prevent the
growth of this microorganism in the emulsion. Basically, the amount of alcohol
used as a preservative depends on the amount of water phase in the emulsion as
water is the best medium for microorganisms to grow. Distilled water on the
other hand, act as the aqueous phase (external phase or continue phase) in the
oil in water (o/w) emulsion.
The next ingredient which is also very important is
Vanillin. Vanillin acts as flavouring agent to improve the taste of the
emulsion so that it is more acceptable to the patients.
On top of that, another ingredient that is
important too is the emulsifying agent. It can increase the viscosity of the
surface of the oil and aqueous phase. Besides, it also provides a protective
sheath for the oil droplets in emulsion. Acacia is also a natural product and
it is a suitable medium for the growth of microorganisms. Hence, antimicrobial
agent needs to be added to the emulsion to stabilize it. Acacia will not
decrease the surface tension.
The ingredients used in preparing emulsion will
affect the physical and chemical characteristics of the emulsion. Different
types of mineral oil used in the preparation of emulsion will give rise to a
different physical characteristics and chemical stability in an emulsion formulation. Besides that, the different
compositions of mineral oil and distilled water used in the preparation of
emulsion will affect the types of emulsion produced, either oil in water (o/w)
emulsion or water in oil (w/o) emulsion. If the amount of distilled water used
is in excess compared to the oil used, then a oil in water emulsion will be
produced. While water in oil emulsion is produced if the amount of oil used is
in excess. Hence, any change in the content of water or oil will results in
phase inversion.
Furthermore, the content of syrup can affect the
flow and physical characteristics of the emulsion formed. The content of the
syrup used must be controlled to prevent
any problems related to the emulsion's rheology properties. Physical
characteristics can also be affected in this case. Some of the oils used have
different colours and this will produce emulsion with different colours. In
addition, palm oil has antioxidant property and this can improve the stability
of the emulsion formed. The differences discussed above will produce an
emulsion with different physical characteristics and chemical stability.
Conclusion
An emulsifier
having a low HLB will tend to be oil-soluble emulsion, while a high HLB
will tend to be water-soluble emulsion. The less polar of the emulsifier been used, the more stable the
emulsion to be formed.
References
Appendix
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