Malaysian Journal of Analytical Sciences, Vol. 7, No. 2 (2001) 439-444
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Physicochemical Properties Of Isotropic Region In Tween 80/Brij
30/Ester/Water Systems
Wan Rusmawati W.M1, Faujan B.H.A2, Hamdan S3, Anuar K2, Wen-Huei Lim4 and Dzulkefly K2
1
Faculty of Science and Technology, Universiti Pendidikan Sultan Idris, 35900 Tanjung Malim, Perak
2
Chemisty Department, Universiti Putra Malaysia, 43400 Seri Serdang, Selangor, Malaysia
Faculty of Science and Technology, Universiti College Terengganu, 21030 Kuala Terengganu, Terengganu
4
Advanced Oleochemical Technology Centre (AOTC), Malaysia Palm Oil Board (MPOB), 43650 Bandar Baru Bangi
Selangor
3
(Received 6 September 2000)
Abstract: Phase behaviour of Tween 80/Brij 30/H2O system was constructed at 25.0±0.5oC. Isotropic and liquid crystalline
(hexagonal and lamellar) regions were observed in this system. The isotropic solution was extended to the rich water corner
from the Tween 80 and Brij 30 axis. Quaternary phase behavior was constructed by incorporate methyl acetate (MA) into
Tween 80:Brij 30(80:20)/H2O system. Association structure of the mixed surfactants in the isotropic region of MA/Tween
80:Brij 30 (80:20)/H2O system was carried out via electrical conductivity, light scattering and solubility measurements. The
results obtained show the formation of water-in-oil (w/o) microemulsion, bicontinuous and oil-in-water (o/w) microemulsion
structures occurred between 20 – 40 wt%, 40 – 60 wt% and > 60 wt% H2O, respectively.
Abstrak: Gambarajah fasa sistem Tween 80/Brij 30/H2O dibina pada 25.0±0.5oC. Kawasan-kawasan isotropik and cecair
hablur (heksagonal dan lamela) kelihatan terbentuk dalam sistem tersebut. Larutan isotropik didapati tersebar dari kedua-dua
sudut Tween 80 and Brij 30 ke sudut kawasan air. Lanjutan gambarajah fasa dibina dengan penambahan metil acetat (MA)
kepada sistem Tween 80/Brij 30/H2O Penyatuan struktur campuran surfaktan dalam kawasan isotropik sistem MA/Tween
80:Brij 30 (80:20)/H2O dikaji dengan menggunakan elektrik conduktiviti, sebaran cahaya dan kelarutan. Data menunjukkan
pembentukan struktur air-dalam-minyak (w/o) mikroemulsi, "bicontinuous" dan minyak-dalam-air mikroemulsi pada julat 2040 wt%, 40-60 wt% dan > 60 wt% H2O.
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Keywords: phase behavior, isotropic region, percolation, conductivity, solubility
Introduction
It has long been recognised that
microemulsions are thermodynamically stable
isotropic mixtures of oil, surfactant, and water [13,6]. There are homogeneous on a macroscopic
scale but heterogeneous on a molecular scale. They
consist of oil and water domains, which are
separated by surfactant monolayer. With the
progress of studies on microemulsions and their
increased technical applications and instrumentation
- electrical conductivity and light scattering knowledge of their microstructure has become more
and more important.
In the food and pharmaceutical industries,
mixed nonionic surfactants are used, as they are
generally less toxic, low irritant and exhibit better
solubility properties in microemulsions systems [5].
Moreover, nonionic surfactants have the capability
to form microemulsion without the assistant of cosurfactant. The presence of ester compounds (polar
substances) as an important raw material in the
cosmetics and pharmaceutical industries, were
shows to exhibit significant different solubility
region [4].
This paper reports the phase behaviour of
Tween 80/Brij 30/H2O system. The presence of
methyl acetate in the system was also been
conducted. Association structures of mixed
surfactants in the isotropic region of Tween 80/Brij
30/Methyl Acetate/H2O system were determined by
using electrical conductivity meter, light scattering
instrument and solubilisation method.
Materials And Methods
Materials
Polyoxyethylene
sorbitan
monoester
(Tween 80) with an average of 20 ethylene oxides
per surfactant molecule was provided by
ICICorp.Sodium
bis(2-ethylhexyl)sulfosuccinate
(Brij 30), >99% purity, supplied by Fluka. Methyl
acetate (MA) >99% purity, supplied by Fluka. All
the chemicals were used without further
purification. Deionised water of 18.0 S/cm was
used in all the experiments.
Phase Diagram
Phase behaviour of MA/(Tween 80:Brij
30)/H2O system was constructed at 25.0 ± 0.5oC in
two experimental steps. The best weight ratio of
Tween 80:Brij 30, which exhibit maximum water
WAN RUSMAWATI W.M . et al.: PHYSICOCHEMICAL PROPERTIES OF ISOTROPIC REGION
solubility, was chosen in Tween 80/Brij 30/H2O
system. The phase behaviour of this ratio was
further constructed by incorporating MA into the
system. A preliminary study on various phase
regions was conducted by mixing different
compositions of MA, Tween 80, Brij 30 and H2O in
the test tubes with screw caps. The samples were
sealed with parafilm to avoid evaporation, vortexed
thoroughly and allowed to stand for at least 30
minutes in a water bath at 25.0 ± 0.5oC to
equilibrate. Phase boundaries of the systems were
precisely located by titration method with H2O or
MA [1,2]. All the samples showed clear phases,
separated by sharp interfaces when reached
equilibrium. The types of phases in each sample
were recorded to construct the phase diagram.
Conductivity measurement
Several microemulsion samples in the
MA/Tween 80:Brij 30(80:20)/H2O system were
prepared at 60:40, 50:50, 35:65 and 20:80 weight
ratios of MA:(Tween 80:Brij 30). Conductivity
property of the samples was measured by using a
conductivity meter (Jenway Model 4020 and
conductivity cell Model PCM 141) with a cell
constant of 1.01 at 25.0 ± 0.5oC.
Light-scattering measurement
The association structures of MA/Tween
80:Brij 30(80:20)/H2O system was also been
investigated by using a dynamic light scattering
instrument (DLS) at 25.0 ± 0.5oC. The instrument
(Malvern 4700C) is able to determine the
aggregation size of surfactant. The scattering
intensity and auto-correlation functions at 90o were
recorded with a PCS5 photomultiplier in tandem
with a real-time correlation of 128 channels. The
results were scaled to the intensity of dust-free
toluene, which was taken as the cells were cleaned
in a chromix solution, washed with twice-distilled
water and dried overnight at 80oC. Dust and other
extraneous matter were excluded from the solvent
through a fine porosity glass filter (0.022 µm).
Another fine porosity glass filter was used to
remove dust from the deionised water. The
microemulsion samples were then prepared and
carefully transferred to the cells after centrifugation
at ~104 g for 2 hours to precipitate fine dust. The
cells were tightly sealed and placed in a water bath
(25.0 ± 0.5oC) for 6 hours before measurement.
440
Results And Discussion
In order to understand synergistic
interaction between Tween 80 and Brij 30 in an
aqueous system, the phase behaviour of Tween
80/Brij 30/H2O system was constructed (Figure 1).
An isotropic region was observed to form along the
apex line of Tween 80 and Brij 30 axis. The
isotropic region extended into the rich water corner
at 80:20 weight ratio of Tween 80:Brij 30. Whereas,
a liquid crystalline region was formed between 20
wt% to 55 wt% of Tween 80 axis, extended until 73
wt% Tween 80. In this system, a transition of liquid
crystalline structures from lamellar, D, to hexagonal,
E, phases were observed (under a polarised light
microscope) as the weight ratio of Tween 80:Brij 30
increased. At low percentage of water (<15 wt%),
all Tween 80:Brij 30 mixtures were dissolved in
water (L). The maximum water solubility of 100
wt% occurred at 80:20 weight ratio of Tween
80:Brij 30. This ratio was then selected for further
studies of phase behaviour by incorporating ester
compound, methyl acetate (MA).
Figure 2 showed the phase behaviour of
MA/Tween 80:Brij 30(80:20)/water system. The
isotropic region was observed to cover almost 75%
of the system. However, two-phase region was
observed in the range of 10 to 75 wt% as weight
ratio of MA:MS increased above 65:35. The
association structures of this isotropic region were
studied via electrical conductivity and light
scattering measurements. All the results were
presented as a function of percentage H2O, wt%, at
constant weight ratio of MA/MS.
Figure 3 shows the conductivity (K) of
microemulsion at different weight percentage of
H2O, wt%. Four tie lines of different MA/MS
weight ratios (20:80, 35:65, 50:50 and 60:40) in the
MA/Tween 80:Brij 30 (80:20)/H2O system was
conducted. At the binary mixtures of MA/MS
(Tween 80:Brij 30), a small increased of electrical
conductivity with the increase of MA/MS was
observed. The electrical conductivity (K) of
different MA:MS weight ratios in MA/Tween
80:Brij 30 (80:20)/H2O system slowly increased
with the increase of H2O content up to 20 wt%
(Figure 3). The low K value could be due to the
hydrated surfactant species in solution. Above the
critical weight percentage of H2O, the electrical
conductivity increased linearly with H2O up to about
40 wt%. A steep increase in K value indicated a
percolative threshold, which decreased with a lower
weight ratio of MA/MS from 60:40 to 20:80.
WAN RUSMAWATI W.M . et al.: PHYSICOCHEMICAL PROPERTIES OF ISOTROPIC REGION
Tween 80
0
100
10
90
20
80
30
70
L
40
60
60
E
50
50
T
40
70
30
80
20
D
T
90
10
100
0
0
10
20
30
40
50
60
70
80
90
100
Brij 30
W ater
Figure 1 : Phase diagrams of Tween 80/Brij 30/water system at 25.0 ± 0.5oC.
L = isotropic region; D = lamellar liquid crystal region; E = hexagonal liquid crystal region and T = two-phase region.
MA
0
100
10
90
20
80
30
70
40
60
50
50
T
60
40
70
L
30
80
20
90
10
100
0
Water
0
10
20
30
40
50
60
70
80
90
100
Tween 80:Brij 30 (80:20)
Figure 2: Phase diagrams of MA/Tween 80:Brij 30(80:20)/water system at 25.0 ± 0.5oC.L =
isotropic region; and T = two-phase region.
441
WAN RUSMAWATI W.M . et al.: PHYSICOCHEMICAL PROPERTIES OF ISOTROPIC REGION
140
Conductivity, K ( S/cm)
120
100
80
60
60:40 (MA/MS)
50:50 (MA/MS)
40
35:65 (MA/MS)
20
20:80 (MA/MS)
0
0
10
20
30
40
50
60
70
80
H2O (wt%)
Figure 3: Conductivity of the MA/Tween 80:Brij 30(80:20)/water system as a function
of its weight percentage of water at constant MA/MS weight ratio at 25.0 ± 0.5oC.
The percolative threshold corresponds to
the formation of the first infinite cluster of droplets.
The number of cluster increases rapidly above the
percolation threshold, increasing the conductivity as
the H2O content increases. The phenomenon may be
either due to “hoping” of the surfactant ions from
droplet to droplet within the droplet clusters, or a
transfer of counterions from one droplet to another
through the water channels opening between
droplets during “sticky” collisions or transient
merging.
At this point, an increase in MA/MS
(Tween 80:Brij 30) weight ratio will increase the
conductivity (Figure 4a). The results showed that
high MA content (MA is a polar solvent) will
contribute to the high charge carrier and mobility
around the inverse micelle which resulting in a high
conductivity. The conductivity thereafter continued
to grow as H2O molecules dissolved in it. The
second critical weight percentage of H2O occurred
as the H2O content increased further.
The translational mobility of H2O
molecules only started to increase when the oil
droplets began to collapse to form a bicontinuous
structure (i.e. both oil and water in a continuous
medium). In the bicontinuous structure, movement
of the charge carriers was less restricted then in the
w/o microemulsion structure. However, the
electrical conductivity decreased with an increase in
442
the MA/MS (Tween 80:Brij 30) weight ratio (Figure
4b). In addition, the conductivity increased with
Tween 80:Brij 30 weight ratio. Hence, Tween 80
greatly affected the conductivity of the surfactant
mixtures in the systems. An increase in conductivity
values with the formation of the bicontinuous
structure was due to the high Tween 80 content.
The conductivity value was then decreased
again with further increase of H2O, which possibly
due to a transition from bicontinuous structure to
o/w microemulsion droplets. The microemulsion
droplet decreases with the addition of H2O content
thus decrease the conductivity, concentration and
mobility of isotropic system. The effect of different
MA/MS weight ratios in the system with the
conductivity value in o/w microemulsion formation
was plotted in Figure 4c. The conductivity value
was observed to decrease with the increase of
MA/MS weight ratio.
In addition to the solubility properties of
MS in MA/Tween 80:Brij 30 (80:20)/H2O system
(Figure 3), the thermodynamics of the system were
also been investigated. The size of surfactant
aggregates was studied by using light scattering
instrument. The implicit assumption of this
approach was that the micelle aggregates and
microemulsion droplets behave like macromolecular
solutions, and therefore pressure,
temperature
and the attachment of labels do not substantially
influence their size.
WAN RUSMAWATI W.M . et al.: PHYSICOCHEMICAL PROPERTIES OF ISOTROPIC REGION
Figure 5 shows the average particle size
(Zave) of MA/Tween 80:Brij 30(80:20)/ H2O system
as a function of the H2O wt% at various weight
ratios of MA/MS. With the addition of water, a
similar trend was observed in all weight ratios
investigated. In binary mixtures, 20:80 weight ratio
had the highest Zave. Thus, MS increased with Zave.
Addition of small amount of H2O decreases Zave to
130.8 nm, 34.2 nm and 10 .0 nm at 20:80, 60:40 and
50:50 weight ratios, respectively. The mixed
surfactants are believe to start interacting with each
other but still did not aggregate to form micelles or
inverse micelle structures.
Further increase of H2O content increase
the Zave to a maximum of 125 nm and 230 nm for
the weight ratios of 60:40 and 20:80. The
40
15
20
10
35:65
50:50
0
60:40
140
55
120
50
H2O (wt%)
100
45
80
40
60
40
35
30
20:80
20
35:65
50:50
80
0
60:40
140
120
H2O (wt%)
75
100
80
70
60
40
65
20
60
20:80
Conductivity, K ( S/cm)
10
20:80
35:65
50:50
Conductivity, K ( S/cm)
H2O (wt%)
30
Conductivity, K ( S/cm)
20
0
60:40
Figure 4: Variation of H2O, wt% and conductivity, K, with the weight ratio of MA:Mixed Surfactants
(Ma/MS) in the MA/Tween 80:Brij 30(80:20)/water system at 25.0 ± 0.5oC : (a) percolation threshold;
(b) bicontinuous structure; and (c) micelle.
443
WAN RUSMAWATI W.M . et al.: PHYSICOCHEMICAL PROPERTIES OF ISOTROPIC REGION
percolation threshold started to form as the H2O
content increased above 10 wt%. This phenomenon
increased the conductivity value as discussed
previously. Theoretically, the particle size of
microemulsion is about 5 ~ 100 nm. Thus, w/o
microemulsion only formed in MA/MS system at
weight ratio of 60:40. At 20:80 weight ratio of
MA/MS system, the Zave increases between 125 nm
and 225 nm.
At this point, the H2O content was still too low for
the formation of a bicontinuous structure (>40 wt%
H2O is needed to form a branched structure). The
small Zave value may be due to the decomposition of
inverse micellar structure. The trend was unchanged
by further increase of H2O content above 70 wt%
and no micelle structure was formed above this H2O
content.
The higher the MS content in the system,
the higher the droplet size, Zave, with the highest
value obtained at 20:80 weight ratio of MA/MS.
Therefore, MS (Tween 80:Brij 30) played an
important role in changing the droplet size.
Particle Size (Z
Ave ,
nm)
The presence of large water droplets in the
w/o microemulsion increases Zave to the maximum
at 20 wt% water. However, the increase of H2O
content to above 20 wt%, decreased the droplet size.
450
400
350
300
250
200
150
100
50
0
60:40 (MA/MS)
50:50 (MA/MS)
20:80 (MA/MS)
0
20
40
60
80
100
H2O (wt%)
Figure 5: Plots of Zave against water weight percentage (wt%) at different MA/MS weight ratios of
the MA/Tween 80:Brij 30(80:20)/water system at 25.0 ± 0.5oC.
Conclusions
MA/Tween 80:Brij 30(80:20)/H2O system
form large isotropic region (almost 75% of the
system). This was mainly contributed by the
presence of Tween 80 in the system, which has
higher HLB value compared to Brij 30. Moreover,
mixed surfactants system improved solubilisation
property thus an isotropic region was more likely to
form. Electrical conductivity measurements showed
that transition from w/o microemulsion to
bicontinuous structure and lastly o/w microemulsion
was formed between 20 – 40 wt%, 40 – 60 wt% and
>60 wt% H2O content, respectively. This transition
was also supported by light scattering measurement.
2.
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Elworthy, P.H., Florence, A.T., and Macfarlane,
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4.
Fiberg, S., and L. G. Zhuo (1983) “
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Huibers, P.D.T., and Shah, D.O. (1996).
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Solubilisation
in
water-in-oil
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