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Effect of sodium chloride on the formation and stability of n-dodecane nanoemulsions by the PIT method.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
Published online 4 May 2010 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.445
Special Theme Research Article
Effect of sodium chloride on the formation and stability
of n-dodecane nanoemulsions by the PIT method
Jeffery Chin Long Liew, Q. Dzuy Nguyen* and Yung Ngothai
School of Chemical Engineering, The University of Adelaide, SA 5005, Australia
Received 30 October 2009; Revised 15 March 2010; Accepted 17 March 2010
ABSTRACT: This paper provides a fundamental study of the effect of sodium chloride on the formation and stability
of n-dodecane/nonionic surfactant (Brij30)/NaCl nanoemulsions produced by the phase inversion temperature (PIT)
method. Nanoemulsions are an emulsion system containing droplets from 20 to 200 nm and widely used in cosmetics
and pharmaceutical industries. The PIT method was chosen due to its low energy and surfactant usage to produce the
nanoemulsions by heating and quenching an emulsion system. The changes of conductivity with temperatures were
continuously monitored to determine phase inversion, and are found to be the same in low surfactant concentrations. PIT
point was found to decrease with NaCl concentration especially from 5 to 7 wt% Brij30. At the storage temperature
(20 ◦ C), the initial droplet size decreases with NaCl concentration; however, the decrement only occurs from 4 to
7 wt% Brij30 while no nanoemulsions can be produced at 8 wt%. By adding salt, the surfactant concentration needed
for the most stable nanoemulsions is reduced to 6 wt% from 7 wt%. Therefore, similar stable nanoemulsions can be
produced with less surfactant in a brine system. Furthermore, most of the ageing brine-continuous nanoemulsions could
be reproduced to their freshly prepared state by heating process but not for the most stable nanoemulsions.  2010
Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: n-dodecane; nanoemulsions; sodium chloride; phase inversion temperature (PIT); reversibility
INTRODUCTION
Nanoemulsions, also referred as miniemulsions, submicron emulsions, and unstable microemulsions,[1] which
contain at least two immiscible liquids are an emulsion system with droplets ranging from 20 to 200 nm
and appear as a transparent low-viscosity liquid. They
are classified between macroemulsions and microemulsions. Due to their ultra fine and small droplet sizes,
they are stable against sedimentation and creaming,
which are common destabilizing factors in macroemulsions. Unlike microemulsions, nanoemulsions are only
kinetically stable and so their droplet sizes tend to
grow with time leading to phase separation. However, nanoemulsions are preferable to be produced in
most industries as they have better long-term physical
stability[2] and can be produced by using less surfactant
and consuming less energy. Nanoemulsions are widely
applied in many industries, such as cosmetic, pharmaceutical, drug delivery, and food industries.[1,2]
Energy is required for the formation of nanoemulsions as they are non-equilibrium systems and cannot
*Correspondence to: A/Prof. Q. Dzuy Nguyen, School of Chemical
Engineering, The University of Adelaide, SA 5005, Australia.
E-mail: dzuy.nguyen@adelaide.edu.au
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
be formed spontaneously.[1,2] There are two methods to
supply the energy: high-energy methods make use of
mechanical devices, such as high pressure homogenizer
and high shear stirring; whereas low-energy methods
make use of the physicochemical property of an emulsion system. By low-energy methods, nanoemulsions
are produced as a result of phase transition, which can
take place either at a constant temperature or by applying the phase inversion temperature (PIT) concept.[2 – 4]
For the low-energy methods undertaken at a constant
temperature, the system needs to go through low
interfacial tension during the emulsification process.
One of the methods to achieve the low interfacial
tension is to create a transition in the curvature of
surfactant monolayer by changing the water volume
fraction, at a constant temperature (emulsion inversion
point [EIP] method). In EIP method, water droplets
are initially produced in a continuous oil phase by
adding water into oil. By increasing the water volume
fraction (stepwise addition of water into oil phase),
the curvature of surfactant monolayer will change
from concave toward water to concave toward oil
at the inversion point. A bicontinuous microemulsion
is formed at the inversion point for a short-chain
surfactant emulsion system. During the transitional
Asia-Pacific Journal of Chemical Engineering
process, minimal interfacial tensions are achieved and
promote the formation of emulsions with small droplet
sizes.[5] Low-energy methods conducted at a constant
temperature involve catastrophic phase inversion (CPI),
in which the oil-in-water (O/W), and water-in-oil (W/O)
emulsions are not interchangeable by reversing the
conditions.
The PIT method works on the basis of changes in
the affinity of a polyoxyethylene-type nonionic surfactant with temperatures. The curvature of the surfactant monolayer is more convex toward water at low
temperatures but more concave toward water at high
temperatures due to the dehydration of hydrophilic tail
in a nonionic surfactant. Therefore, O/W emulsions
are preferably formed at low temperatures and W/O
emulsions at high temperatures. PIT point is a temperature where the curvature of surfactant monolayer
is neither convex nor concave toward water, and this
creates an extreme low interfacial tension and produces
ultra small-sized emulsions.[6] However, emulsions at
that point are very unstable and so a quenching process needs to be undertaken to produce stable, fine,
and monodispersed O/W nanoemulsions, and the final
nanoemulsions need to be stored at a temperature away
from the PIT point.[7] The PIT method has received
more attention in the past few years since it has been
first introduced in 1969,[7] due to its advantages, such
as low cost over other high-energy methods. Therefore,
this method was chosen to be used in this study to
produce the nanoemulsions.
The addition of salt to stabilize emulsions is a
common practice in emulsion industries, especially the
cosmetic, pharmaceutical, and food industries. In the
formation of emulsions, the addition of salt will depress
the PIT point and phase inversion will occur at a crucial
salt concentration (at a constant temperature).[8 – 10] For
emulsions with droplets larger than 1 µm, coalescence
will be enhanced by the addition of salt in an emulsion system;[11] however, it has been reported that there
is an optimum salt concentration needed to produce
the most stable emulsions.[12] For the nanosized emulsions produced by high-energy methods, the addition of
salt has been found to have no effect on droplet sizes
but enhance the creaming stability.[13 – 15] The improved
stability is a result of the increasing interdroplet interaction and the increasing viscosity in the continuous
medium.[14] To date, few studies have been conducted
to investigate the effect of salt on the formation and
stability of nanoemulsions, especially those produced
using the PIT method; therefore, this study is important. In this research, sodium chloride is chosen as it is
a common salt to add into the production of emulsion
industries.
Nanoemulsions produced by the PIT method are very
sensitive to temperature.[16] They need to be handled
at their optimum temperature to prolong the shelving
life. However, it is very hard to store and deliver
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
N-DODECANE NANOEMULSIONS
nanoemulsions at their optimum state for practical
purposes. If an ageing or destabilized nanoemulsion
system can be reverted to its freshly prepared state in
terms of the droplet size and the size distribution, it may
be said that the nanoemulsions have the reversibility
ability. The reversibility ability plays an important role
in nanoemulsion industries as it will allow no restriction
to store and deliver nanoemulsions.
In this research, n-dodecane nanoemulsions produced by the PIT method with different surfactant and
sodium chloride concentrations will be investigated to
determine their changes in formation, stability, and
reversibility ability.
APPROACH AND METHODS
Materials
n-Dodecane (99% purity) was purchased from Merck,
Australia. Polyoxyethylene lauryl ether (Brij30), with
4 mol of ethylene oxide per surfactant molecule was
purchased from Sigma-Aldrich. The salt, sodium chloride (NaCl), was obtained from Chem-Supply with 99%
purity (analytical grade). All reagents were used without any further purification. MiliQ water was directly
used to prepare nanoemulsions and to prepare brine.
Methods
PIT point determination
PIT point has been suggested to be determined by monitoring the changes of the conductivity values with temperatures in a nonionic surfactant emulsion system.[17]
There will be a sudden drop in the conductivity curve
when a water-continuous emulsion system becomes an
oil-continuous emulsion system, when the temperature
is increased. PIT point is taken as an average temperature between the temperatures at the maximum and
minimum conductivity value.[4] The investigated emulsion systems were those with an oil/water weight ratio
at 20/80 and 0–0.1 M NaCl while the surfactant concentration was varied from 4 to 8 wt%.
Formation of nanoemulsions
Two types of nanoemulsions, O/W nanoemulsions and
O/brine nanoemulsions, were produced in this study.
The nanoemulsions produced by the PIT method were
prepared by a two-step procedure. First, the emulsions
were gradually heated to a temperature, which was 4 ◦ C
higher than the temperature at zero-conductivity. When
the system was completely formed into oil-continuous
emulsions, a quenching process was carried out for the
heated emulsions by immediately placing the system
into an ice bath and rapidly cooling to 15 ◦ C. All
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
DOI: 10.1002/apj
571
572
J. C. L. LIEW, Q. D. NGUYEN, AND Y. NGOTHAI
Asia-Pacific Journal of Chemical Engineering
◦
samples were stored at 20 ± 1 C. The emulsions were
continuously stirred during the heating and cooling
processes.
Droplet size, stability, and reversibility
determination
Mean droplet size and the polydispersity index (PdI)
of nanoemulsions were determined by dynamic light
scattering (DLS) using Malvern Zetasizer Nano, Series
3600 (Malvern, UK), with a 532 nm green laser and a
scattering angle of 173◦ . PdI ranging from 0 to 1.0 is an
indication for quality of dispersion. For the emulsions,
PdI is strictly below 0.2 to be considered as a monodispersed system and suitable for measurement. The stability of the produced nanoemulsions was assessed by
measuring droplet size and PdI as a function of time.
Nanoemulsions are considered as losing their stability
when they are phase separated and phase separation was
determined by visual observation. The reversibility testing was carried out for 3-day ageing nanoemulsions
to ensure the result be consistent with our previous
work.[18] The testing procedures were described in our
previous paper, which involved two processes, heating
and cooling.[18] All measurements were carried out in
duplicates and an average value was reported. All samples were diluted with continuous phase and the thermal
equilibrium time for each sample (around 1 ml) was
5 min prior to the start of the measurements.
RESULTS AND DISCUSSIONS
Conductivity measurements and phase
behaviors
Conductivity values of n-dodecane emulsions with
4–8 wt% Brij30 and with 0–0.1 M NaCl were continuously recorded as a function of temperature, and some
selected systems are shown in Fig. 1. For most of the
studied emulsion systems, at the beginning of a heating
process, conductivity value increases slowly with temperatures to reach a maximum. After the maximum, it
decreases to a minimum before reaching a secondary
(local) maximum. From the local maximum, the conductivity value starts to decrease again and drops to zero
(Fig. 1). These trends of conductivity curve are similar
with reported works.[4,17,19] The changes in the conductivity value with temperatures have been explained as
follows. There are water-continuous emulsions existing
at low temperatures (high conductivity), whereas oilcontinuous emulsions are at high temperatures (low/zero
conductivity). Between the water-continuous and oilcontinuous emulsions, there is a transitional zone, which
involves the formation of lamellar liquid crystal phase
and bicontinuous phase.[17,19]
The initial conductivity value is increased with surfactant concentration in a pure miliQ water system but
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 1. Conductivity values as a function of
temperature for systems with (a) pure miliQ water
and (b) 0.001 M NaCl and with 4–8 wt% Brij30.
Oil/aqueous = 20/80 w/w. This figure is available
in colour online at www.apjChemEng.com.
it is independent on surfactant concentration in brine
systems (Fig. 1). This is because the conductivity potential of sodium chloride is much stronger than that of
a nonionic surfactant. As observed, only the conductivity curves at 4–7 wt% Brij30 behave in a similar trend (decrease, local maximum, and decrease) in
both pure miliQ water and brine systems. There are
no local maximum in brine systems containing 8 wt%
Brij30 as shown in Fig. 1(b), which illustrate a different phase inversion process. The conductivity curve
at a fixed surfactant concentration appears further to
the left (lower temperature) when sodium chloride is
added. This shows that the phase transition occurs at a
lower temperature and the PIT point is lowered in brine
systems.[8,9] PIT point is decreased by 0.5 ◦ C from pure
miliQ water system to 0.001M NaCl system while it
is lowered by 2 ◦ C from pure miliQ water system to
0.01M and 0.1M NaCl systems (Fig. 2). These reductions occur due to the dehydration of nonionic surfactant (more hydrophobic), which results in the decrease
of its solubility in aqueous phase.[8,9,20] These reductions are considered small when compared to published
works due to the different investigated systems,[8,9] but
are similar to our previous studies.[18] Furthermore, the
PIT point is decreased when surfactant concentration is
increased (Fig. 2); this phenomenon has been explained
as the accumulation of short ethylene oxide chain at the
oil–water interface.[4]
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
Figure 2. PIT point as a function of different
salt and surfactant concentrations. Oil/aqueous =
20/80 w/w.
N-DODECANE NANOEMULSIONS
maximum Ttrans shows that n-dodecane is more soluble in those surfactant concentrations and affects the
stability of nanoemulsions.
The largest Ttrans is apparent in the system with
8 wt% Brij30 and 0.1M NaCl among all the studied
systems, because of not only the more stable transitional
zone (microemulsions, LLC, or bicontinuous),[9] but
also the partly dehydrated nonionic surfactant. Due to
the high concentration of sodium chloride, Brij30 may
behave between hydrophilic-liked and hydrophobicliked surfactant and form two different surfactantstructured transitional zones during the phase inversion
process.[22] Therefore, a longer Ttrans (compare to only
one type of surfactant-structured transitional zone) is
needed to completely invert the transitional zones into
W/O emulsions. In addition, internal pressure (osmotic
pressure) is increased with the addition of sodium
chloride[8] so that a larger driving force (temperature
difference in this case) is needed to achieve the phase
inversion process.
Formation and stability of nanoemulsions
Figure 3. Width of transitional zone for systems containing different salt concentrations and
4–8 wt% Brij30. Oil/aqueous = 20/80 w/w.
A temperature transition, Ttrans , is defined as a temperature difference between temperatures at highest and
lowest conductivity value. Ttrans represents the width
of the transitional zone between O/W and W/O emulsions and is linked with surfactant concentration at
water/oil interface, and a larger and more stable transitional microemulsion zone is held at a higher surfactant
concentration (higher Ttrans ).[9] Figure 3 clearly shows
that Ttrans continuously increases with surfactant concentration in 0.001M and 0.1M NaCl emulsion systems,
but there is a maximum Ttrans in pure miliQ water
and 0.01M NaCl systems. The maximum Ttrans occurs
at 7 wt% Brij30 in pure miliQ water system but at
6 wt% Brij30 in 0.01M NaCl system. Other than in
those two surfactant concentrations, Ttrans is relatively
insensitive to salt concentration. This result confirms
that Ttrans is more dependent on surfactant concentration than salt concentration.[9] As the transitional zone is
linked with the completeness of solubilization of the oil
component and the stability of nanoemulsions,[21] the
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
n-Dodecane nanoemulsions were prepared by the
method as described in previous section. The cooling
temperature (15 ◦ C) was chosen to produce nanoemulsions because PIT points for all systems are higher than
15 ◦ C, which have been shown in Fig. 2. However, all
samples were stored and analyzed at 20 ◦ C as it is a
normal storage temperature in many industries.
Some selected emulsion systems with their initial
droplet size distribution are presented in Fig. 4. The
emulsions start to be polydispersed from 8 wt% Brij30
in a pure miliQ water system but from 7 wt% Brij30 in
a brine system. Nanoemulsions with surfactant concentration lower than those concentrations (with or without
salt), are monodisperse distributed (data not shown).
Figure 4. Initial droplet size distributions at 20 ◦ C
for n-dodecane nanoemulsions with different salt
and surfactant concentrations. Oil/aqueous =
20/80 w/w.
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
DOI: 10.1002/apj
573
574
J. C. L. LIEW, Q. D. NGUYEN, AND Y. NGOTHAI
Asia-Pacific Journal of Chemical Engineering
Table 1.
Days taken for phase separation for
n-dodecane nanoemulsions (oil/aqueous = 20/80 w/w)
stored at 20 ◦ C with different sodium chloride and
surfactant concentrations.
Figure 5. Initial droplet size and polydispersity
index (PdI) at 20 ◦ C for n-dodecane nanoemulsions with different salt and surfactant concentrations. Oil/aqueous = 20/80 w/w. (solid line:
droplet diameter; dashed line: PdI).
The polydispersity may be attributed to the storage (analyzed) temperature, which is close to the corresponding
PIT points and so nanoemulsions are unstable.[7] In
addition, the PIT point for 8 wt% Brij30 is lower than
20 ◦ C (Fig. 2) and so no nanoemulsions, but only bicontinuous phase can be produced. Nanoemulsions with PdI
smaller than 0.2 and droplet sizes smaller than 120 nm
can be produced at 4–7 wt% Brij30 in a pure miliQ
water system but at 4–6 wt% Brij30 in a brine system
(Fig. 5). Therefore, the range of surfactant concentration, which is used to produce nanoemulsions, becomes
narrower when sodium chloride is added into a pure
miliQ water system.
Nanoemulsions with mean initial droplet sizes ranging from 77 to 110 nm are produced in a pure miliQ
water system, whereas 65–104 nm are produced in
brine systems (Fig. 5). The smaller droplets are formed
due to the low interfacial tension created by the addition of sodium chloride.[23] However, PdI remains
unchanged within all investigated systems. Therefore,
the addition of sodium chloride has no effect on
the fineness of nanoemulsions produced by the PIT
method. Among the brine systems, 0.1M NaCl has the
largest mean initial droplet size and PdI as the nonionic surfactant is too dehydrated to cause low interfacial tensions to produce nanoemulsions with smaller
droplet sizes. Because the use of a surfactant in an
emulsion system is to decrease the interfacial tension,
lower interfacial tension is achieved at higher surfactant concentration.[4,24] Therefore, smaller droplets
have been produced in higher surfactant concentration,
which is clearly shown in Fig. 5. The surfactant concentration with 7 wt% in a pure miliQ water system
and 6 wt% in brine systems can produce nanoemulsions with the smallest droplet size and lowest PdI
(Fig. 5), which are also the most stable nanoemulsions
(Table 1).
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
[S]
(wt%)
No
NaCl
0.001 M
NaCl
0.01 M
NaCl
0.1 M
NaCl
4
5
6
7
8
8
4
3
>30
–
9
5
>30
–
–
8
5
>30
–
–
7
2
>30
–
–
The stability of nanoemulsions was investigated by
measuring their mean droplet sizes and PdI as a function of time before the phase separation. Days taken
to phase separation for nanoemulsions are shown in
Table 1, whereas the growth of droplet size and PdI
are shown in Fig. 6. Table 1 and Fig. 6(b) clearly indicate that 7 wt% Brij30 in a pure miliQ water system
and 6 wt% Brij30 in brine systems are the most stable n-dodecane nanoemulsions. They can be stored at
20 ◦ C for more than 1 month without any phase separation, and there are no changes on their droplet sizes
and PdI during the studied periods. These highest stable nanoemulsions may be due to the complete solubilization of oil as they have the highest Ttrans in
the phase inversion process, which have been shown in
Fig. 3. The changes of droplet size and PdI for 4 wt%
Brij30 as shown in Fig. 6(a) clearly indicate its instability: the droplet size is almost doubled (110–290 nm)
while PdI increases almost six times (0.1–0.6) for a
pure miliQ water system after the sample has been
stored for 1 week. For the ageing nanoemulsions in
brine systems, 0.1M NaCl has the largest mean droplet
size and PdI because of stable O/W nanoemulsions are
less likely to be produced by the dehydrated nonionic
surfactant.[9]
In this study, it has been found that there is an
optimum surfactant concentration to produce the most
stable n-dodecane nanoemulsions at 20 ◦ C in either
a pure miliQ water or a brine system. The most
stable nanoemulsions at those surfactant concentrations
may be due to the complete coverage of surfactant
on oil–water interface and they are in transitional
microemulsions phase at 20 ◦ C (Fig. 1). For systems
containing surfactant concentrations lower than the most
stable nanoemulsions, there are no enough surfactants
to cover the interface. Furthermore, at higher surfactant
concentrations, the PIT point is close to or lower
than 20 ◦ C and so stable nanoemulsions cannot be
produced.
There is a reduction of 1 wt% of Brij30 to produce
the most stable nanoemulsions from a pure miliQ water
system to a brine system. This may be due to the compression effect from the addition of salt on the surfactant
monolayer.[8,9] Therefore, the interfacial concentration
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
N-DODECANE NANOEMULSIONS
Reversibility of nanoemulsions
Figure 6.
(a) The stability for n-dodecane
nanoemulsions containing 4 wt% Brij30 with different salt concentrations; (b) The most stable
nanoemulsions within the investigated systems
(Circle: No NaCl, 7 wt% Brij30; Square: 0.001 M
NaCl, 6 wt% Brij30; Triangle: 0.01 M.
of surfactant will be increased. In addition, the salting
out effect will decrease the solubility of surfactant in
the aqueous phase and increase the interfacial concentration of Brij30 on the oil phase. Although the 1 wt%
reduction is not significant for practical purposes, this
finding has shown the effect of sodium chloride in
the formation and stability of nanoemulsions produced
by the PIT method and will lead to additional future
works. However, the addition of salt in the production of nanoemulsions need to be done carefully, as
there is a maximum salt concentration to prevent the
nonionic surfactant to fully become hydrophobic and
produce W/O emulsions instead of to produce O/W
nanoemulsions.[10]
It has been known that O/W and W/O emulsions
can be interchanged by tailoring temperature in a
nonionic surfactant emulsion system.[4,7] However, the
comparison in physical properties, such as droplet
size of nanoemulsions between before and after the
reversibility process has not been well established.
In our research, it has been previously shown that
O/W nanoemulsions at their optimum temperatures or
nanoemulsions with sodium chloride have the ability
to revert their droplet sizes and the PdI, from ageing
to freshly prepared nanoemulsions.[16,18] In addition, it
has been found that the reversibility ability is governed
by the phase inversion process.[18] The procedures
of reversibility testing have been described in Liew
et al .,[18] in which the cooling and heating processes
of gradually cooling to 10 ◦ C from 20 ◦ C and heating
to 30 ◦ C from 20 ◦ C were discussed. The thermal
equilibrium time prior to the start of each measurement
was 5 min.
To further investigate the reversibility ability of ndodecane nanoemulsions from our previous research,[18]
a wider surfactant concentration range with only four
salt concentrations has been carried out. The reversibility of most ageing nanoemulsions in this study is similar
to our previous work (data not shown): the droplet size
and PdI are overlapped between the ageing and freshly
prepared nanoemulsions only in the heating process
(from 20 ◦ C to 30 ◦ C then back to 20 ◦ C). However,
in this study, we found the most stable nanoemulsions
(7 wt% Brij30 in a pure miliQ water system and 6 wt%
Brij30 in brine systems) behaving differently in the
reversibility testing. For these systems, the droplet sizes
of ageing nanoemulsions are close to the freshly prepared samples after the cooling and heating processes,
but their PdI are larger than the freshly prepared samples after the heating process. For example, in 0.01M
NaCl and 6 wt% Brij30 nanoemulsion system, the mean
droplet diameter of ageing nanoemulsions was 66.7 nm
(Pdl = 0.048) after the cooling process and 76.0 nm
(Pdl = 0.107) after the heating process while it was
66.1 nm (Pdl = 0.042) for the freshly prepared sample
Table 2. Reversibility testing for the most stable nanoemulsions with oil/aqueous = 20/80 w/w after 3 days
storage time. The test was done in a temperature sequence: 20 ◦ C→ 10 ◦ C→ 20 ◦ C→ 30 ◦ C→ 20 ◦ C.
No NaCl, 7 wt% Brij30
◦
Freshly prepared (20 C)
Ageing (3 days)
20 ◦ C
10 ◦ C
20 ◦ C
30 ◦ C
20 ◦ C
0.01 M NaCl, 6 wt% Brij30
Mean diameter (nm)
PdI
Mean diameter (nm)
PdI
66.8
0.091
66.1
0.042
66.4
57.9
66.0
551.2
79.8
0.080
0.038
0.065
1.000
0.191
65.9
61.2
66.7
710.6
76.0
0.038
0.023
0.048
0.815
0.107
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2010; 5: 570–576
DOI: 10.1002/apj
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J. C. L. LIEW, Q. D. NGUYEN, AND Y. NGOTHAI
(Table 2). The PdI for a freshly prepared sample is generally smaller than 0.1 but it becomes larger than 0.1
after the heating process of the most stable nanoemulsions aged over a period of 3 days.
Although the most stable nanoemulsions do not need
any practical reversibility ability due to their highest stability at 20 ◦ C, their behaviors in the reversibility testing
are very interesting. The incomplete reversibility ability
(only droplet size is reverted) of these nanoemulsions
may be due to their longer Ttrans (Fig. 3), thus they
need longer time to experience the phase inversion process, which is a crucial stage in the reversibility process.
The generalized thermal equilibrium time (5 min) may
not be enough to equilibrate those systems.
CONCLUSION
Nanoemulsions of n-dodecane/Brij30/NaCl/water with
small droplet sizes have been successfully produced by
the PIT method. The addition of sodium chloride effectively lowers the PIT temperature and the temperature at
which phase inversion occurs, especially at high surfactant concentrations. Salt helps reduce the concentration
of surfactant required (reduce 1 wt% in this case) to
form highly stable and monodispersed nanoemulsions.
This is probably caused by the salting out effect and
complete solubilization by sodium chloride. While most
of the ageing brine-continuous nanoemulsions could be
rejuvenated to their freshly prepared state by heating,
the most stable nanoemulsions show a lack of reversibility with ageing due to their high temperature transition.
Acknowledgement
The help for editing of this article by Dorothy Missingham, Chemical Engineering, University of Adelaide, is
gratefully acknowledged.
 2010 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
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DOI: 10.1002/apj
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