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International Journal of Drug Delivery 3 (2011) 133-142
http://www.arjournals.org/ijdd.html
Research Article
Effect of Ultrasonication on Stability of Oil in Water Emulsions
Kiran A. Ramisetty1 and R. Shyamsunder1*
*Corresponding author:
R. Shyamsunder
1University College of
Technology, Dept of
Pharmaceuticals and Fine
chemicals,
Osmania University,
Hyderabad, India.
E-mail:
ramskiranict@gmail.com
Abstract
Effect of ultrasonic waves on stability of oil in water system of light
liquid paraffin oil (HLB = 12) as internal phase and tween20 (HLB =
16.7), span20 (HLB = 8.6) as emulsifying agents was studied. A
comparison was made to determine the stability of emulsions
prepared by mechanical agitation method and ultrasonication
technique. Droplet size measurement method was used to determine
the stability of emulsions. Physico-chemical parameters like
concentration of emulsifying agent, volume fraction of dispersed
phase, viscosity of continuous phase by adding glycerin to water
were compared apart from the effect of emulsification time on
stability of emulsions prepared with mechanical stirring and
ultrasound. Ocular micrometer was used to determine the droplet
size of the dispersed phase.
Emulsions prepared by ultrasonic technique were found to be more
stable for longer duration of time when compared to emulsions
prepared by mechanical agitation which can be attributed to the
small droplet size which is thermodynamically stabilized.
Ultrasonic technique gave more stable emulsions than with
mechanical agitation method. Emulsification time, volume fraction
of dispersed phase, viscosity of continuous phase and concentration
of emulsifying agents played a major role in the stability of
emulsions.
Keywords: Liquid paraffin, Tween 20, Span 20, emulsification,
ultrasound technique, volume fraction of dispersed phase..
Introduction
In an emulsion system, the finely divided droplets
are referred to as the dispersed phase,
discontinuous or internal phase; the liquid
surrounding the droplets is called the non-
dispersed phase, continuous phase or external
phase. The addition of a third component acting
at the interface to retard phase separation is called
emulsifier or emulsifying agent.
Since emulsions, in most instances, are two-phase
systems, it is customary to define the type of
emulsion by considering whether the oil is in the
internal or external phase. If the oil is in the
internal phase, the emulsion type will be an oil-
in-water, o/w; or, conversely, if the water is in the
internal phase, water-in-oil, w/o; type is achieved.
Secondary emulsion (multiple-emulsion): it
contains two internal phase, for instance, o/w/o or
w/o/w. It can be used to delay release or to
increase the stability of the active compounds.
ISSN: 0975-0215
doi:10.5138/ ijdd.2010.0975.0215.03063
©arjournals.org, All rights reserved.
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
134
The first objective to be attained in emulsification
is to reduce the internal phase (oil or water) into
small globules [3-6]. This can be accomplished
only if an external source of energy in the form of
work is supplied. The energy may be in the form
of human or mechanical work. Theoretically, it is
possible to calculate the energy required to
produce a quantity of an emulsion having a
definite particle size by the use of the equation
[7].
Where W is the free surface energy in ergs is, is
the surface tension in dynes/cm, and , the
surface area in cm2. The work necessary to
produce an emulsion of a specific volume and
particle size may be reduced if the internal
tension ( ) is lowered [8-9]. This may be
accomplished by the addition of an emulsifying
agent having surface-active properties. The
selection of an emulsifying agent, which lowers
the interfacial tension considerably, will be an
important factor to consider when emulsification
is desired [10-12]. However, it is not necessarily
true that those agents who are not markedly
reducing the interfacial tension are poor
emulsifying agents [13].
Surface orientation
At the interface of oil and water, the molecule
must posses a polar group and a nonpolar group,
both of about equal magnitude. In this particular
case, the molecule would orient itself in such a
way that the polar group (hydrophilic head) will
face the water, while the nonpolar group
(hydrophilic tail) faces the oil [14-19]. Consider
now the case of a molecule having a very large
polar group in comparison to the non-polar
group. This type of molecule will be more soluble
in the water and, thus, will move away from the
oil and enter the main portion of the water. The
molecule with a large nonpolar group will
migrate into the main portion of the oil phase
[20].
Hydrophilic
head
Hydrophobic
tail
Water
Oil
Experience has shown that emulsifying agent
having a greater degree of hydrophilic property
than hydrophobic will usually produce an oil in
water emulsion, while a more hydrophilic
surface-active agent will usually give water in oil
emulsion [20-23]. Bancroft noted this tendency a
number of years ago and concluded that the phase
in which the emulsifying agent was more soluble
would be the continuous or the external phase
[24-25].
Materials and methods
Light liquid paraffin oil (Sd Fine-chem. ltd),
Tween 20 (polyoxyethylene sorbitan mono
oleate), Span 20 (Sorbitan monooleate), Glycerin,
distilled water, ocular microscope with stage
micrometer.
Preparation of Emulsions
By mechanical agitation method
An emulsion of 60ml was prepared by taking,
20% of light liquid paraffin oil and span20 in a
beaker and tween20 was added to 77% of
distilled water in another beaker as tween20 is
miscible in water and span20 is oil miscible
followed by pouring dispersed phase to
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
135
continuous phase. Here percentage of
emulsifying agent was kept to 3%. This
composite solution was then subjected to
mechanical agitation by placing the agitator at the
middle of interface between the dispersed phase
and continuous phase, at 1000 RPM. Proper
mixing of the phases gives the good emulsion
which is white in color.
By Ultrasonication method
Emulsion with above concentrations was
prepared by applying the ultrasound using
sonicator with adjustable height hadle with
operating frequency of 20 KHz at 3mm of depth
from the surface of the emulsion solution. Time
of insonation is variable, which was measured by
using stopwatch. Temperature of the sample was
measured with thermometer, as time of
insonation increases the temperature of the
emulsion will increases. Emulsion prepared from
ultrasonicator is milky white in colour.
Evaluation of emulsions
In this experiment, a study was made to compare
the stability of emulsion prepared by two
different methods i.e. mechanical stirring and
ultrasonic horn tip method by examining the
following physicochemical parameters affecting
the emulsion stability.
1. Effect of stirring time and irradiation
time.
2. Effect of volume fraction emulsifying
agent.
3. Effect of volume fraction of oil phase or
dispersed phase.
4. Effect of viscosity of continuous phase.
Effect of stirring time and irradiation time
Effect of time was studied on emulsions stability
at fixed amount of emulsifying agent and fixed
volume fraction of dispersed phase. Here 20% of
dispersed phase volume and 3% of the
emulsifying agent is used in total amount of 60ml
oil. Volume fraction of dispersed phase φ = 0.2
and Volume of the dispersed phase is 12ml. Total
amount emulsifying agent used is 3% means
1.8ml. in this volume fraction tween20 is 42%
means 0.756ml and volume fraction span20 is
58% means 1.048ml. And continuous phase
volume is 46.2ml.
Eight samples were taken and subjected to
mechanical stirring at 1000rpm by varying the
time from 1, 2, 3, 4, 5, 6, 7 and 8minutes.
Similarly another eight samples of same
compositional biphasic mixture were subjected to
ultrasoincation by ultrasound horn tip, at 20 KHz
frequency with increasing the time of insonation
in the range of 1, 2, 3, 4, 5, 6, 7 and 8 minutes.
Immediately after completion of emulsification
1ml of emulsion sample was taken and diluted
with 10ml of water in a test tube and was
observed under microscopic stage micrometer to
observe the number of droplets in the
microscopic premises. By counting this number
of droplets the average droplet size was measured
by sauter diameter.
d
32 = Σ ni di3/ Σ ni di2
Effect of volume fraction of emulsifying agent
Effect of volume fraction of the emulsifying
agent was studied by keeping the time of
emulsification (5min) and volume of the
dispersed phase (φ = 0.2) as constant. The
volume fraction of the emulsifying agent was
varied from 3% to 15% and were subjected to
emulsification by mechanical and ultrasonic
method. Finally the droplet size of the emulsion
was measured by stage micrometer as mentioned
earlier.
Effect of volume fraction of oil phase or
dispersed phase:
In this experiment emulsification time (5minutes)
and volume fraction of emulsifying agent (9%)
were kept constant with varying concentrations
volume fraction of dispersed phase. In order to
prevent the phase inversion, the total percentage
of oil was not exceeded more than 40%. The
prepared emulsions were subjected to
emulsification by mechanical and ultrasonic
method. Finally the droplet size of the emulsion
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
136
was measured by stage micrometer as mentioned
above.
Effect of continuous phase viscosity
Viscosity of the solution was measured by using
broke field viscometer. From the experiments
considering the emulsification time, amount of
emulsifying agent and volume fraction of the
dispersed phase were kept constant, now by
adding the glycerin to the water increased the
viscosity of the continuous phase. In this process
the volume fractional volume of emulsifying
agent was 0.9 means 5.4ml, volume fraction of
the dispersed phase is φ = 0.2 means 12ml, time
of emulsification was 5minutes. To the remaining
amount of 42.6ml of water, glycerin was added in
the volume fraction 0.5, 1.0, 1.5 and 2.0 volume
continuous phase. After the emulsions were
prepared by mechanical agitation and ultrasonic
method, the viscosity of the prepared emulsions
was determined by viscometer followed by
droplet size measurement using stage micrometer
method as discussed earlier.
From the above experiments, a graph was plotted
taking volume % of droplets and droplet size in
microns with time of mechanical agitation or time
of sonication applied to the sample.
Results and discussion
Effect of stirring time and irradiation time
By comparing the above figures (1 and 2)
obtained for droplet size distribution of the
droplets of emulsions, prepared from mechanical
stirring, and ultrasonic irradiation, it can be
observed that, as the emulsification time is
increased the droplets distribution curve become
narrow in shape, which indicates that number of
droplets in uniform size in this narrow range are
more and hence the stability of emulsion will be
more. At minimum time of stirring or irradiation
the droplet size distribution curve, become wider
in range indicating droplets in this region are less
in number, with wide spread of non uniform
particles size distribution. As the time of
emulsification was increased from 1min to 8min
droplet distribution is in narrow range, increasing
the stability of emulsions. As the time of
sonication was increased temperature of the
emulsion was found to increase which can be
attributed to physical effect of ultrasonic
irradiation.
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
137
Figure 1: Volume of drop size distribution with emulsification time by mechanical stirring at constant emulsifying agent (3%)
and volume fraction (φ = 0.2) of the oil.
Figure 2: Volume of drop size distribution with emulsification time by ultrasonic irradiation at constant emulsifying agent (3%)
and volume fraction (φ = 0.2) of the oil.
With an increase in the temperature, the
interfacial tension as well as the viscosity is
expected to decrease considerably. The decrease
in the interfacial tension is observed to set in the
interfacial instability, which increases the number
of dispersed phase droplets. With an increase in
temperature, the number of nuclei giving rise to
cavitation may increase due to an increase in the
vapour pressure of the cavitation medium. With
an increase in the cavitational events and
intensity, the breakage of large droplets to form
small droplets is observed to be increasing. When
the power is kept constant at 30 W, the droplets
formed are initially small. They show a slight
increase in size at a time of 1 min and then go on
reducing if irradiated further until 8 min.
Effect of volume fraction of emulsifying agent
The surfactant plays a critical role in both droplet
break-up and coalescence. The surfactant aids
droplet break-up by lowering the interfacial
tension, which reduces the resistance to droplet
deformation. The most important role of the
surfactant is to prevent the immediate re-
coalescence of newly formed droplets by rapid
adsorption to, and stabilization of, the newly
formed interface.
Invariably the requirements of both droplet
break-up and coalescence dictate that small
molecule surfactants are the most suited to the
formation of nano-emulsions because of their
greater ability to rapidly adsorb to interface and
their much lower dynamic interfacial tensions. As
observed from the Figures (3 and 4), with the
increase in the surfactant concentration from 3%
to 9%, the particle size of the droplets was found
to decrease up to 15% giving narrow particle size
distribution. As the emulsifying agent amount
increases it surrounds the oil droplets uniformly
decreasing the interfacial tension between the oil
droplets. With the increase in the emulsifying
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
138
agent concentration resulted in the formation of
more stable emulsions with ultrasonicator than in
the mechanical stirring.
Figure 3: Effect of emulsifying agent on droplet size distribution of emulsions prepared from mechanical stirring.
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
139
Figure 4: Effect of emulsifying agent on droplet size distribution of emulsions prepared from ultrasonication
Effect of volume fraction of oil phase or
dispersed phase:
From Figures (5 and 6) it can be observed that
with the increase in the volume fraction of the
dispersed phase the droplet size of the oil phase
was found to increase in both mechanical
agitation and ultrasonication method. The droplet
size of the oil phase in mechanical agitation
varied from 13.9 to 25.1µm while in
ultrasonication method it varied in the range of
1.29 to 4.21 µm. As the volume fraction of
dispersed phase increases viscosity of dispersed
phase increases then it is hard to break the oil
droplets, and it is observed that droplet size is
decrease in ultrasound than in the mechanical
stirring because it requires more power to break
the oil droplets and penetrate the droplets into the
continuous phase. Mechanical stirring could not
give required energy to break oil droplets. Using
ultrasonic irradiation fragmentation of the
droplets becomes more. However, at the starting
of insonation it is hard to form a cavity bubble.
This cavity bubble increases rapidly and collapse
then the rupturing of the oil droplets becomes
more. As the dispersed phase concentration
increases, it requires more time to rupture the
droplets. So size of the droplets increases by
further addition of oil content. From the graphs, it
can be observed that as the dispersed phase
fraction increases droplet size distribution
becomes wider with increase in mean number of
droplets having non-uniform size. As the non
uniformity of the droplets becomes more the
stability of emulsion was found to decrease.
Emulsions prepared from ultrasound were found
to have narrow range of particles in the low
fraction of dispersed phase. However, it becomes
wider as the dispersion phase content increases.
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
140
Figure 5: Droplet size distribution of the oil particles with varying concentrations of dispersed phase at constant emulsification
time of 5minutes and emulsifying agent 9% in Mechanical agitation.
Ramisetty et al. International Journal of Drug Delivery 3 (2011) 133-142
141
Figure 6: Droplet size distribution of the oil particles with varying concentrations of dispersed phase at constant emulsification
time of 5minutes and emulsifying agent 9% in Ultrasonication
Effect of continuous phase viscosity
Here glycerin is act as secondary stabilizing
agent, which was added to increases the viscosity
of continuous phase thus reducing the mobility of
droplets in order to prevent them from
coalescing. As viscosity of the continuous phase
increased stability of emulsion was found to
increase which can be attributed to the continuous
medium surrounding the droplets and it resist the
coalescence of the droplets mean while there was
increase in droplet size (Figure 7). Agglomeration
of the droplets was found to decrease, but with
the increase in the continuous phase viscosity it
require more time of insonation or stirring to
incorporate the oil phase into continuous phase.
Droplet size also more as continuous phase
viscosity increases.
Figure 7; Droplet size of the oil with varying viscosity of continuous phase in mechanical agitation and ultrasonication
technique.
Conclusion
With our simple, three-component, model
system, comparison of two types of
emulsification processes was made, first one
using mechanical agitation method and the
second one involving power ultrasound at low
frequency (20 kHz), affords several interesting
results in favor of the ultrasound technique.
Smaller average drop sizes d32 (down to
1.125µm) can be obtained with ultrasound. Time
of emulsification plays a major role in decreasing
the droplet size of the emulsions. Ultrasound
technique gives more stable emulsions than the
mechanical stirring technique. Emulsifying agent
resist the coalescence of droplets to agglomerate
and brought the stable emulsions in both the
mechanisms but in the ultrasound it effects more
on stability as increased in volume fraction.
Droplet size distribution of low content
emulsifying agent emulsions is wider in range
than with that are having more content of
emulsifying agent. There is an increment in
droplet size has been observed with increment in
dispersed phase volume fraction but it was less in
ultrasound technique when compared with
mechanical stirring. Here droplet size increases as
the viscosity of continuous phase increases but
the stability increases with time of storage. An
extension of this work would require the study on
ultrasonic parameters like irradiation power
irradiation frequency.
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