Download Chapter 3 Phase Behavior of Triglyceride Microemulsions

Chapter 3
Phase Behavior of Triglyceride
Microemulsions
3.1
Literature Survey
Phase Diagrams involves careful mapping of a system involving surfactant/
water/ oil/ cosurfactant, for understanding aspects of microemulsion formation The phase diagram approach to study microemulsions for four component
system was introduced by Gillberg, who demonstrated the identity of w/o microemulsions [Gillberg
et al.
, 1970].
In Japan Professor Shinoda pioneered
investigations of the phase equilibria of water, nonionic surfactant and oil
[Shinoda and Ogawa, 1967; Shinoda and Saito, 1968; Shinoda and Takeda,
1970]. He emphasized the connection between nonionic surfactant properties
and temperature.
The phase behavior of nonionic surfactant/ cosurfactant/
oil/ water mixtures is inuenced by number of factors that include the type
of surfactants, surfactants mixing ratio, and the presence of additives such as
dierent alcohols, nature of oil and temperature.
Thus, the study of phase behavior of these systems is essential to determine
the extent of water and oil mutual solubilization. Also, phase behavior study
will elucidate the eects of various additives on solubilization/ microemulsication of polar or non-polar components (depending on w/o or o/w microemulsion). Among these additives, alcohols hold a special place by being
the most frequently used. The presence of alcohol inuences the extent of the
microemulsions regions and their internal structure.
The roles of alcohol in
microemulsions are to delay the occurrence of liquid crystalline phases, to increase the uidity of the interfacial layer separating oil and water, to decrease
the interfacial tension between the microemulsion phase and excess oil and water [Sanya
et al. , 1989], and to increase the disorder in these interfacial layers
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as well as their dynamic character. The miscibility of water, oil, surfactant or
mixed surfactants, and cosurfactant is a composition dependent variable [Sato
et al.
, 2006; Kunieda and Yamagata, 1993; Pes
et al.
, 1995; Sonntag, 1981].
A number of authors [Fanun and Salah Al-Diyn, 2006; Mitra and Paul, 2005;
Fanun, 2010] reported on the eect of polar oils such as triglycerides, middleor long-chain alcohols or fatty acids, fatty acid ester on the water solubilization, and properties of microemulsions for dierent technological applications
[Fanun, 2008].
Eect of chain length of alcohol/ cosurfactant on structural properties of
nonionic surfactant, water, alcohol ternary system was investigated by Tomsic
et.al, it was observed that with increasing the chain length form easily soluble
alcohols ethanol and methanol to insoluble alcohols namely n-dodecanol, the
isotropic region gradually decreased [Tomsic
et al.
, 2006]. In his work on hy-
drocarbons, Bayrak et.al observed that, the microemulsion regions are smaller
when alcohols of longer chain length are used, while shorter chain alcohols
are too soluble in aqueous phase and they are ineective as cosurfactants. In
his ndings he also mentioned the eect of surfactant/ cosurfactant ratio on
phase behavior, the optimum S:C ratio for larger microemulsion region was
found to be 2:1[Bayrak and Mehmet, 2005]. Moulik et.al, analyzed the eect
of small chain alcohols like ethanol and isopropyl alcohol on phase behavior of
coconut oil, nonionic surfactant Brij 52 (C 16 E23 ), water system. He reported
that larger single phase zones were observed with isopropyl alcohol than with
ethanol [Acharya
et al.
, 2002].
The eect of branching in cosurfactant on
phase behavior was investigated, it was observed that branched alcohols are
particularly useful to mix with linear chain surfactants because their branching
increases the average area of the surfactant in the interfacial layer. The driving force being the neutral anity for oil and water to adsorb at the interface.
Hence, these alcohols provide more spacing between surfactant molecules, and
consequently they are the best to decrease cohesion and rigidity. However, it
is was observed that they are very poor as far as the stabilization performance
is concerned [Stubenrauch, 2008].
The solubilization of oil in aqueous surfactant solutions is critically aected
by temperature, particularly in case of nonionic surfactants. Therefore, it is
necessary to study phase behavior as a function of temperature. H. Kunieda
and K. Shinoda investigated in detail the system water/ tetradecane/ C 12 E5
in the temperature interval 35 to 60
°C. At 35 °C four homogeneous phases co-
existed, a water-continuous solution phase, an oil-continuous solution phase,
a lamellar liquid crystalline phase and a microemulsion phase with a high
concentration of the surfactant. An isotropic solution phase occurred in the
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binary system water/ C 12 E5 that was in equilibrium with almost pure water
and lamellar phase. At 35
°C
this phase was able to incorporate, considerable
amounts of tetradecane but this capacity was drastically reduced at 40 °C. At
45°C the isotropic phase got still narrower, they attributed this behavior to
the decreased solubility of the surfactant in water or, essentially the increasing
attraction between the polyoxyethylene chains as temperature increases [Kunieda and Shinoda, 1982]. Similar behavior was observed by Smith et.al, in his
investigations of the phase behavior of nonylphenol ethoxylate/ cyclohexane/
water system [Smith, 1985]. The eect of temperature and cosurfactant on the
phase behavior of surfactant, n-butanol and eucalyptus oil was studied by Paul
et.al at dierent temperatures.
It was observed that the monophasic region
exhibited a decrease with increasing temperature, in the presence of nonionic
surfactant. While, the monophasic region showed a maximum at 1:1 S:C ratio,
further increase in the cosurfactant amount lead to considerable decrease in
monophasic region [Mitra and Paul, 2005].
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3.2
Materials
3.2.1 Surfactant
Non ionic surfactant of the type (C i Ej ), n-dodecyl octaoxyethylene glycol
monoether with the chemical formula C 12 H25 (OCH2 CH2 O)8 OH, abbreviated
C12 E8 , was gifted by M/S Galaxy Surfactants, India. These are generally produced by ethoxylation of a fatty chain alcohol (here dodecanol). The starting
alcohol generally has a distribution of alkyl chain lengths and the resulting
ethoxylate has a distribution of ethylene oxide chain lengths. Thus the ethylene glycol numbers mentioned refers to average numbers.
(Cp) was found to 62
°C.
The cloud point
The surfactant was used as received.
3.2.2 Cosurfactants
Medium chain length alcohols namely n-butanol (CH 3 (CH2 )3 OH), n-pentanol
(CH3 (CH2 )4 OH), n-hexanol (CH 3 (CH2 )5 OH) of A.R. grade were procured from
M/S S.D. Fine, India. Ethylene glycol monohexyl ether/ n-hexylglycol , commonly known as hexyl cellosolve (C 6 H13 OCH2 CH2 OH) was obtained from Dow
Chemicals Ltd. and Ethylene glycol mono-2-ethylhexyl ether/ 2-Ethylhexylglycol
(C4 H9 CH.C2 H5 CH2 (OCH2 CH2 OH)) was obtained from M/S Galaxy Surfactants, India.
3.2.3 Triglyceride - Coconut Oil
Triglyceride (Triacylglycerol or Triacylglyceride) is an ester derived from glycerol and three fatty acids.
There are many triglycerides, depending on the
oil source, some are highly unsaturated, some less so. Saturated compounds
are "
saturated "
with hydrogen all available places where hydrogen atoms
could be bonded to carbon atoms are occupied. Unsaturated compounds have
double bonds (C=C) between carbon atoms, reducing the number of places
where hydrogen atoms can bond to carbon atoms. Saturated compounds have
single bonds (C-C) between the carbon atoms, and the other bond is bound
to hydrogen atoms (for example =CH-CH=, -CH 2 -CH2 -, etc.). Triglycerides
are the main constituents of vegetable oil.
Coconut oil is an edible oil extracted from the kernel or meat of matured
coconuts harvested from the coconut palm.
The most abundant fatty acid
in this group is lauric acid, CH 3 (CH2 )10 COOH. More than 90% of the fatty
acids of coconut oil are saturated. The saturated character of the oil imparts
a strong resistance to oxidative rancidity.
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Rened Coconut Oil which was used for all experimental purposes was procured from by M/S Marico Industries, India. The Acid Value for the coconut
oil used was in the range of 0.0 to 0.1 mg KOH/ g of oil, Saponication Value
approximately 250 mg KOH/ g of oil and Iodine Value between 8 to 10 mg I 2
/ g of oil. The composition of the coconut oil is as shown in Table. 3.1
Table 3.1: Composition of Coconut Oil
3.3
Name of Acid
Formula
Nature of Acid
Weight %
Caproic
C5 H11 COOH
Saturated
0.3
Caprylic
C7 H15 COOH
Saturated
9.2
Capric
C9 H19 COOH
Saturated
9.7
Lauric
C11 H23 COOH
Saturated
44.3
Myristic
C13 H27 COOH
Saturated
15.9
Palmitic
C15 H31 COOH
Saturated
9.6
Stearic
C17 H35 COOH
Saturated
3.2
Oleic
C17 H33 COOH
Unsaturated
6.3
Linoleic
C17 H31 COOH
Unsaturated
1.5
Methods
3.3.1 Formulation and Stability of Triglyceride Microemulsions
The phase diagrams were generated by weighing calculated amount of surfactant and cosurfactant and mixing them in specic ratios. The Surfactant (S)
and Cosurfactant (C) were mixed till homogeneous mixture was obtained. Oil
was added to the S+C mixture and this mixture was vigorously shaken using
a vortex mixer. Weighed amount of water was carefully added to the above
mixture and the mixture was again mixed vigorously.
Water was added till
turbidity or phase separation started to appear in the clear mixture, which
conrmed the transition from monophase to bi/multi- phase.
This, estab-
lished a phase boundary distinguishing monophase and multiphase systems .
The samples near the phase boundary were allowed to equilibrate for at least
for an hour before they were examined.The resulting clear, monophasic, homogeneous, isotropic solutions were kept at constant temperature in sealed test
tubes for one week to conrm their appearance. Samples exhibiting no signs
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of turbidity, separation or presence of anisotropic phases after one week were
labeled as microemulsions.
For the temperature induced phase study, calculated amounts of components (samples which are conrmed and established as microemulsions at room
temperature) were taken in sealed test tubes, shaken vigorously in a vortex
mixture, and kept in the thermostatic water bath. The temperature was then
increased in small intervals and at the desired elevated temperature the samples were shaken vigorously and further kept for 12 hrs. The resulting changes
in the samples were observed after 12 hrs, depending on weather they still
retained the characteristics of microemulsions or had visible indications of a
binary phase/ turbidity, they were accordingly categorized into monophasic
microemulsions and multiphase systems.
3.3.2 Identication of Microemulsions
Microemulsions have become commercially valuable for various applications in
the food, pharmaceutical and cosmetic industry. Properties of microemulsions
including its transparency and isotropic nature add commercial value to many
of these applications. Microemulsions were distinguished from macroemulsions
and liquid crystals by using the following techniques.
3.3.2.1
Visual Inspection
Microemulsions can be dierentiated from coarse emulsions or other two-phase
systems by visual inspection (Microemulsions = transparent or translucent,
two-phase systems = turbid).
Visual inspection was therefore used for the
investigation of phase diagrams to establish the initial phase boundaries. The
individual systems were formulated in transparent sealed vials were visually
inspected for clarity, signs of phase separation, and birefringence, subsequently
left to equilibrate for 24 hours and then re-inspected after 1 week.
Since,
nonionic surfactants are temperature care was taken to avoid temperature
uctuations over the storage period by maintaining the samples at constant
temperature. Visually, microemulsions are clear and thus easy to dierentiate
from a coarse emulsion or other two-phase systems. Thus, transparency and
single phase (Winsor IV) compositions which does not scatter visible light as
compared to bigger aggregates like droplets in a coarse emulsion were termed
as microemulsions.
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3.3.2.2
Birefringence
Conrming and distinguishing microemulsions from liquid crystalline systems
was done using polarizing light. Microemulsion systems are isotropic and does
not exhibit birefringence and liquid crystalline systems which are anisotropic
show birefringence when viewed using polarizing light.
The optical proper-
ties of isotropic material are not direction dependent, and with only one RI
the propagation direction of light passing through this material is not inuenced. Anisotropic material like liquid crystals, on the other hand, has optical
properties that vary with the orientation of the incident light.
Liquid crys-
tals demonstrate a range of refractive indices depending on the propagation
direction of light through the substance [Fanun, 2008].
Thus, the individual samples were observed under a polarizing lter, where
white light is shone onto the polarizer through which only light propagating
into a certain direction can pass.
This leads to plane- polarized light.
The
plane-polarized light subsequently traveled through the sample specimen. In
case of samples through which light traveled undisturbed (not demonstrating
splitting of beam)were isotropic and were thus conrmed as microemulsions.
On the other hand, the samples through which plane-polarized light traveled
through and was splitted into an ordinary and extraordinary light beam exhibiting birefringence were labeled as liquid crystals.
3.3.3 Ternary Phase Diagrams
Ternary phase diagram is a practical tool for understanding the association
phenomena of importance to microemulsions, it is based on the Phase Rule
of J. Willard Gibbs. One of the pioneers in using ternary phase diagrams to
investigate the behavior of oil, water and surfactant was Per K. Ekwall. For
three-component system such as oil, water and surfactant, a simple rectangular graph cannot reveal as much information about the systems as can a
ternary diagram. This can take form of a triangular graph having three axes,
one for each of the components.
In presence of a fourth component like co-
surfactant which will signicantly modify the phase behavior of the system, a
common approach to represent these systems is by xing the mass ratio of two
components (surfactant/ cosurfactant, oil/ surfactant, oil/ cosurfactant, oil/
drug, etc.) and as such considered a single component [Ekwall
et al.
, 1970].
Such an approach is actually an oversimplication of the system, yet is acceptable for the purpose of phase behavior studies. Such systems are denoted as
pseudoternary and may comprise four (quaternary) or components yet are
represented using a Gibbs triangular phase diagram.
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Figure 3.1:
Theoretical ternary phase diagram outlining the region of exis-
tence of one- phase and two-phase systems.
Note the illustrative represen-
tation of the droplet w/o, droplet o/w, and bicontinuous microemulsions.
O, oil component; W, water component; S, amphiphile component (surfactant/cosurfactant). (Fanun 249)
The virtue of the triangular graph is that the percentage composition at
any point adds up to 100%. This enables assignment of physical attributes to
each point and so to visually trace changes in these attributes with composition. These equilibrium diagrams are widely utilized in the classication of
heterogeneous equilibria as shown in Fig. 3.1 [Fanun, 2008].
The border line joining S:C mixture and water represents a binary mixture
along all points on that line (S:C + Water), indicates increasing wt % of water
and decreasing wt % surfactant along that line (towards 100 wt % water). The
border line joining Water and Oil represents a binary mixture along all points
on that line (Water + Oil), indicates increasing wt % oil and decreasing wt
% water along that line (towards 100 wt % oil), where as oil correspondingly
increases. And, the border line joining Oil and S:C represents a binary mixture
along all points on that line ( Oil + S:C), indicates the increasing wt % of S:C
along with decreasing amount oil along that line (towards 100 wt % S:C mix),
where as S:C correspondingly increases.
It has the advantage that it based
on empirical evidence and is therefore immune to any changes in molecular or
kinetic views on a given subject. Under such circumstances it is an ideal tool
for studying micellar solutions or microemulsions [Prince, 1977a].
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3.3.4 Eect of Cosurfactant Chain Length on Phase Behavior
The eect of dierent increasing chain length of cosurfactants, was studied
on the phase behavior of nonionic surfactant C 12 E8 / cosurfactant/ Coconut
Oil/ Water system.
Straight chain alcohols namely n-butanol, n-pentanol,
n-hexanol were used for the study.
The surfactant and single cosurfactant
were weighed and mixed in predetermined weight ratio of 1:1. This mixture
was mixed with the help of a vortex mixture.
amount of coconut oil was added.
To this mixture calculated
The mixture was then titrated against
water, which involved addition of weighed amount of distilled water.
The
mixture was subjected to vortex mixing after every addition of water. After
every addition the physical appearance of the mixture was checked visually
for its transparency, and its isotropic nature was conrmed by observing the
mixture under polarizing glass sheets. Water was added till the point of phase
separation i.e; the mixture lost its monophasic/ isotropic nature and exhibited
multiphase/ turbidity.
This procedure was repeated for multiple and varying amounts surfactant/
cosurfactant/ oil mixtures where known amounts of water was added. Depending upon the S:C ratio, amount of oil, the amount of water/ oil solubilized
varied. The weight percentages of these amounts and their corresponding nature monophasic/ multi-, biphasic, were thus plotted on the pseudo ternary
phase diagram. Thus, when plotted, the phase diagram clearly distinguished
between monophasic/ isotropic and multiphase/ turbid phases, thereby indicating the impact of varying component (chain length of alcohol) on phase
behavior, hence also on formation of microemulsion. The amounts surfactant/
cosurfactant/ oil mixtures titrated against water were predetermined to enable
cover entire region within the phase diagram with corresponding addition of
water.
3.3.5 Eect of S:C ratio on Phase Behavior
The eect of varying ratio of surfactant to cosurfactant, on the phase behavior of nonionic surfactant C 12 E8 / Cosurfactant/ Coconut Oil/ Water system
was studied. Straight chain alcohols namely n-butanol, n-pentanol, n-hexanol
were used for the study. The eect of varying surfactant to cosurfactant ratio
was studied at predetermined ratios, namely 2:1 and 1:2. The procedure and
methodology for formulation and identication microemulsion/ monophasic region, along with determining and plotting the phase behavior remains same as
mentioned previously in subsections 3.3.1 and 3.3.2. The whole exercise was
49
repeated individually for all three cosurfactants.
3.3.6 Eect of Cosurfactant Branching on Phase Behavior
The eect of cosurfactant branching, on the phase behavior of nonionic surfactant C12 E8 / Cosurfactant/ Coconut Oil/ Water system was investigated.
Straight chain alcohol namely Ethylene glycol monohexyl ether and branched
alcohol, Ethylene glycol mono-2-ethylhexyl ether were used for the study. The
eect of branching of cosurfactant on the phase behavior was studied at predetermined ratios, 2:1, 1:1 and 1:2. The procedure and methodology for formulation and identication microemulsion/ monophasic region, along with determining and plotting the phase behavior remains same as mentioned previously
subsections 3.3.1 and 3.3.2. The whole exercise was repeated individually with
both cosurfactants.
3.3.7 Eect of Temperature on Phase Behavior
The eect of varying temperature, on the phase behavior of nonionic surfactant C12 E8 / n-pentanol/ Coconut Oil/ Water system was investigated.
The
eect of temperature on the phase behavior was studied with S:C ratios, 1:1,
1:1.5 and 1:2, at dierent temperatures, namely 20, 25, 30 and 35 °C. The
monophasic samples that were established as microemulsions, were sealed in
glass capped test tubes and were placed in a Thermo / HAAKE DC10-K10
constant temperature bath, for 12 hours. The change in their physical appearance was noted by visual observation.
Thus, depending upon their physical
appearance (visual observation) and nature (isotropic/ anisotropic) they were
characterized into monophasic/ isotropic and multiphase/ turbid phases. The
procedure and methodology for, determining and plotting the phase behavior
remains same as mentioned previously is sub-subsection subsections 3.3.1 and
3.3.2. The whole exercise was repeated individually with all S:C ratios at all
three temperatures.
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3.4
Results and Discussion
3.4.1 Eect of Cosurfactant Chain Length on Phase Behavior
The eect of increasing cosurfactant chain length from n-butanol to n-pentanol
and n-hexanol on the phase behavior of surfactant C 12 E8 / Coconut Oil/ Water
was investigated. As, the pseudoternary phase diagram indicates, the apex of
the ternary diagram represents 100 % surfactant cosurfactant mixture (S:C)
at 1:1 ratio, whereas the the bottom left corner represents coconut oil 100 wt
% coconut oil. The corner at the bottom right of the triangle represents 100
wt % water.
It was observed that the monophasic/ isotropic region in the phase diagram
for C12 E8 / n-butanol/ Coconut Oil/ Water extended from water in oil (w/o)
region with compositions showing isotropic nature with composition starting
from S:C 28 % + Oil 70 % and water 2 % as shown in Fig.
3.2.
As the
amount of S:C mixture increased, more amount of water got solubilized in
the system, while simultaneously the amount of oil solubilized into microemulsion decreased. When, the S:C ratio reached nearly 50 % the amount of oil
solubilized was 5 % which was very less as compared to that of water 45 %,
which indicated transition to oil in water (o/w) microemulsion. When the S:C
amount was increased to around 68 %,the the amount of oil solubilized in the
microemulsion state was 12 % and water 20 %. At further higher amounts of
S:C from 70 %, near the apex of the triangle all components showed complete
miscibility with each other. Thus, the area above the line joining the w/o and
o/w region represents isotropic, monophasic microemulsion region whereas the
region below the line correspondingly indicates anisotropic, multiphase, turbid
systems.
When n-butanol was replaced by alcohol with an additional methyl group,
n-pentanol as cosurfactant.
While the ratio of S:C was maintained 1:1, the
phase diagram showed the same inclination towards formation of w/o microemulsion.
But, with increasing amount of S:C the amount of water sol-
ubilized in in the system giving rise to o/w microemulsion region dramatically
increased. As shown in Fig. 3.3, the isotropic region is more wider than that
with n-butanol. The microemulsion region extended till 45 % S:C with amount
of oil solubilized was 10 % and water 45 %, indicating o/w microemulsion.
Thus, more coconut oil was solubilized in presence of n-pentanol as a cosurfactant than n-butanol with comparatively lesser amount of surfactants. And
when the area under the microemulsion region was computed, the microemul-
51
sion region with n-pentanol as cosurfactant was found to be 25 % where as for
n-butanol it was found to be 15 %.
Further, the chain length of cosurfactant increased by another methyl
group. And, n-pentanol was replaced by n-hexanol, the S:C ratio was maintained constant at 1:1. As shown in Fig. 3.4, it was observed that in presence
of n-hexanol the w/o isotropic region extended deep nearing the oil corner
with S:C amount as low as 10 % water 2% and oil 88 %. With increasing the
amount of S:C it was found that the region under microemulsion increased,
similar to n-pentanol. But it was observed that the minimum amount of amphiphile (S:C) required to microemulsify oil in o/w region was larger than that
of n-pentanol. The minimum amount of amphiphile required to solubilize oil
was found to be 52 %, and the amount of oil solubilized was 10 % with 36 %
water. The total region under microemulsion with n-hexanol as cosurfactant
was computed and it was found to be 24 % which is signicantly larger than
n-butanol but marginally lesser than n-pentanol.
Thus from the Fig. 3.5, it can be observed that the phase behavior drastically changed with increasing cosurfactant chain length, thereby inuencing
the spontaneous curvature of surfactant molecules at the oil-water interface.
The microemulsion region obtained with n-butanol was signicantly smaller
than both n-pentanol and n-hexanol.
It was observed that the amount of
surfactant required to obtain microemulsion was reduced drastically in presence of n-pentanol as compared to n-butanol, this corroborates the fact that
n-pentanol inuences the formation of microemulsion by interfacial and bulk
eects at a larger scale[Prince, 1977a]. This also indicates that n-pentanol is
more eective in distributing itself between the aqueous and oil phase thereby
altering the chemical composition and hence the relative hydro/ lipophilicity
[Cavalli
et al. , 1996].
Microemulsion region obtained with n-pentanol extends from w/o to a
larger o/w region, whereas with n-hexanol it extends more deeper into w/o
region than in o/w region as compared to n-pentanol.
This suggested that
n-hexanol or higher chain length alcohols reduces the hydrophilicity of the
aqueous phase, thereby rendering it more lipophilic [Broze, 1999], limiting the
formation of isotropic phase [Alany
et al.
, 2000]. Hence, it can be concluded
that with increasing the cosurfactant chain length (middle chain) from C 4 to
C5 led to better solubilization of oil with lesser amount of surfactants, thereby
increasing the eciency of the surfactant.
But when the cosurfactant chain
length was further increased from C 5 to C6 , its eect on the eciency of surfactant C12 E8 seemed to be diminished, suggesting that longer the chain, the
more eective the alcohol in making the surfactant mixture more hydrophobic
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[Strey and Jonstroemer, 1992]. Thus, adding an alcohol may increase or decrease the eciency of the amphiphile mixture depending on the chain length
of the alcohol [Aboofazeli and J., 1993].
Figure 3.2: Pseudoternary phase diagram for the system C 12 E8 / n-butanol/
Coconut Oil/ Water, with S:C ratio 1:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
53
Figure 3.3: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
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Figure 3.4: Pseudoternary phase diagram for the system C 12 E8 / n-hexanol/
Coconut Oil/ Water, with S:C ratio 1:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
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Figure 3.5:
Pseudoternary phase diagram for the system C 12 E8 / Dierent
cosurfactants/ Coconut Oil/ Water, with S:C ratio 1:1. Blue represents system
with cosurfactant n-butanol, Red represents system with n-pentanol, Magenta
demarcates system with n-hexanol .
The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
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3.4.2 Eect of S:C ratio on Phase Behavior
The eect of varying surfactant to cosurfactant (S:C) ratio on the on the
phase behavior of surfactant C 12 E8 / Cosurfactant/ Coconut Oil/ Water was
investigated. This investigation was carried out at S:C ratios 2:1 and 1:2, with
n-butanol, n-pentanol and n-hexanol as cosurfactants. In the pseudoternary
phase diagram the apex represents 100 % surfactant cosurfactant mixture (S:C)
with dierent ratios, whereas the the bottom left corner represents coconut oil
100 wt % coconut oil. The corner at the bottom right of the triangle represents
100 wt % water.
The eect of varying ratios of C 12 E8 and n-butanol on the phase behavior is
shown in Figs. 3.6 and 3.7. When the ratio of S:C is 2:1 it was observed that the
isotropic region extended steeply into w/o region with compositions containing
minimum 30 % amphiphile, 2 % water and 68 % coconut oil. Whereas, the
o/w region extended till region, where 49 % S:C could only solubilize 3 % oil
in 48 % water.
At higher amount of S:C particularly above 70 %, isotropic
region was observed. When, the S:C ratio was changed to 1:2 it was observed
that, the steep incline of the w/o region narrowed down and w/o region was
observed only till 38 % amphiphile, solubilizing 4 % water in 58 % oil. Also,
the area under o/w region showed a marked decease. The minimum amount of
surfactant required to solubilize oil rose to 68 %, solubilizing 4 % coconut oil in
28 % water. When the regions covered under isotropic region were computed
it was observed that, when the S:C ratio was 2:1 the isotropic region was 16
% and with 1:2 the region receded to 11 %.
Similarly, the eect of S:C ratio was studied with n-pentanol at varying
ratios.
The pseudoternary phase diagram showed remarkable dierence.
As
depicted in Fig. 3.8, the isotropic region considerably rose when the S:C ratio
was 1.5:1, the microemulsion region extended from w/o region (solubilizing 5 %
water in 55 % oil with 45 % amphiphile) covering large swath of middle phase
region of the ternary phase diagram. Further, the isotropic region culminated
deep in the o/w region, extending almost till wt 10 % amphiphile.
A stark
variation was registered when the S:C ratio was changed to 1:2 as shown in
Fig. 3.9, the initial w/o region remained almost constant, but the large region
composing o/w microemulsions completely ceased to exist. At this ratio, at
least 80 % surfactant would be necessary to solubilize10 % coconut oil in 20
% water. The dierence in the eciency of the S:C mixture is visible from the
fact that, the isotropic region covered when the S:C ratio was 1.5:1 fell from
28 % to 9 % when it was changed to 1:2.
Further, the eect of S:C ratio was studied with n-hexanol at varying ratios.
57
Again, the pseudoternary phase diagrams showed considerable dierence, not
only in comparison with n-butanol and n-pentanol but also with changing
amount of n-hexanol in S+C mixture. As shown in Fig. 3.10, when the S:C
ratio is 2:1 the isotropic region extends deep in the w/o region approaching
the coconut oil corner. As, the amount of S+C mixture increased, large region
extending from 50 % of S+C till the S:C apex, was found to be isotropic
in nature.
The isotropic region ceased to exist beyond 50 %.
Thus lesser
amphiphile amounts did not encourage the formation of o/w microemulsion
indicating lesser S+C eciency.
When, the S:C ratio was changed to 1:2,
the overall isotropic region under the pseudoternary phase diagram became
extremely scant as shown in Fig. 3.11. A thin zone of isotropic region extended
in w/o region, running very close to the coconut oil, S+C boundary. The large
microemulsion region encompassing the middle region of the graph ceased to
exist, the ecacy S+C mixture at this ratio was so low that it required nearly
80 % amphiphile to solubilize oil and water.
When computed the isotropic
region for S:C 2:1 was found to be 27 % and this region sharply decreased to
6 % at, S:C ratio 1:2.
The area covered under the microemulsion was found to be larger in presence of higher amount of surfactant than cosurfactant [Aboofazeli and J., 1993].
For all three cosurfactants studied S:C ratio 2:1 (1.5:1 in case of n-pentanol)
was found to be more ecient than S:C ratio 1:2 in formation of isotropic
region. The eciency of the S+C mixture decreased with decreasing amount
of surfactant, clearly evident from Figs.
3.7, 3.9 and 3.11.
This indicates
that even thought cosurfactants assists in formation of microemulsions, their
presence amounts more than necessary to inuence the interfacial parameters,
decrease the solubilization capacity of surfactant molecules [Mitra and Paul,
2005].
Also, it was observed that the eciency of S+C mixture (at varying
ratios) for microemulsion formation was highest for n-pentanol followed by
n-hexanol and n-butanol.
58
Figure 3.6: Pseudoternary phase diagram for the system C 12 E8 / n-butanol/
Coconut Oil/ Water, with S:C ratio 2:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
Figure 3.7: Pseudoternary phase diagram for the system C 12 E8 / n-butanol/
Coconut Oil/ Water, with S:C ratio 1:2. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
59
Figure 3.8: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1.5:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
Figure 3.9: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1:2. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
60
Figure 3.10: Pseudoternary phase diagram for the system C 12 E8 / n-hexanol/
Coconut Oil/ Water, with S:C ratio 2:1. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
Figure 3.11: Pseudoternary phase diagram for the system C 12 E8 / n-hexanol/
Coconut Oil/ Water, with S:C ratio 1:2. The region above the line represents
isotropic/ monophasic/ microemulsion region, whereas the region below the
line is anisotropic/ turbid in nature.
61
3.4.3 Eect of Cosurfactant Branching on Phase Behavior
The formation of microemulsion is resultant of the structural characteristics of
surfactant, cosurfactant and oil. Keeping in mind the inuence of cosurfactant
on formation of microemulsion, we investigated the eect of branched cosurfactant chain on phase behavior of the ternary system C 12 E8 / Cosurfactant/
Coconut Oil/ Water. The cosurfactants whose structural eects were studied
were Ethylene glycol monohexyl ether/ and Ethylene glycol mono-2-ethylhexyl
ether. Their eect on phase behavior was studied and compared at three S:C
ratios namely 1:1, 2:1 and 1:2.
The eect of cosurfactant branching on phase behavior when S:C ratio is
1:1 can be observed from Figs. 3.12, 3.13. In case of straight chain cosurfactant ethylene glycol monohexyl ether (Hexyl Cellosolve), the isotropic region
extends from high amphiphile concentration apex and extending in o/w region,
where oil was solubilized with 30 % S+C. Also, the isotropic region extended
slightly towards w/o region, where 38 % S+C was required to solubilize 2
% water in oil.
In case of the branched cosurfactant ethylene glycol mono-
2-ethylhexyl ether at S:C ratio 1:1, drastic variation was observed than its
straight chain counterpart. The isotropic region ceased to exist near the apex
at high S+C amounts, forming macroemulsion phase which extended towards
region with higher % of oil. The ternary system became isotropic in nature
in the region with 50 % to 70 % S+C, this broad isotropic region culminated
into a narrow band in w/o region,solubilizing water with similar proportions
to that of straight chain cosurfactant. Further the region exhibited multiphase
region till coconut oil and water corners. The dierence in the isotropic region
due to cosurfactant branching is shown in Table,
Similarly, the eect of branching of cosurfactant on phase behavior was
studied at S:C ratio 2:1. In the presence of straight chain cosurfactant ethylene glycol monohexyl ether, the isotropic region obtained at 2:1 ratio large and
was distinctive in many aspects. As shown in Fig. 3.14, the isotropic region extended from S+C rich apex, further deeply inclining in o/w region towards the
water corner. In region of 64 to 38 % S+C, solubilizing small amounts of coconut oil 1-10 %, in 30 to 52 % water, liquid crystalline structure was observed.
This region ceased to exist with decreasing amount amphiphile and increasing
amount water.
In case of branched cosurfactant as shown in Fig.
3.15, no
liquid crystals were observed with 2:1 S+C ratio. Whereas, the isotropic region decreased markedly. The isotropic region extended from the amphiphile
rich apex to w/o region, where 30 % amphiphile was required to solubilize 2
62
% water in 68 % coconut oil. A small isotropic region was also observed near
o/w region with 8-20 % amphiphile, solubilizing 2- 8 % oil in 92-72 % water.
These, regions were separated by large multiphasic region. With S:C ratio 2:1,
the isotropic region showed a marked variation only in area but also due to
presence of liquid crystals, the dierence in isotropic region due to cosurfactant
branching is shown in Table. 3.2.
Finally the eect of cosurfactant branching was studied at S:C ratio 1:2.
In case of straight chain cosurfactant it was observed that the isotropic region
was considerably lesser than that obtained in previous ratios.
Fig.
As shown in
3.16, the isotropic region extended from amphiphile rich apex toward
w/o region, but isotropic region was observed only till 45 % S+C, below which
isotropic region ceased to exist giving rise to multiphasic/ turbid region. For
the branched counterpart of the cosurfactant the phase behavior completely
changed.
Very small isotropic regions separated and surrounded by turbid
region was observed. As shown in Fig. 3.17, isotropic region was observed at
50 % S+C, solubilizing 20-30 % coconut oil and water, followed by another
small isotropic region approaching o/w region solubilizing 10 % oil in 50-60 %
water at S+C % in the range of 40-30%.
Thus, the eect of straight and branched chain alcohol on the phase behavior at dierent S:C ratios, is evident from Figs. 3.12 to 3.17. It was observed
that straight chain alcohol ethylene glycol monohexyl ether assisted in solubilizing more oil, water in o/w and w/o microemulsions respectively at all S:C ratios, as compared to branched alcohol ethylene glycol mono-2-ethylhexyl ether.
This, indicated that straight chain alcohols are more ecient in solubilization
than branched cosurfactant [Valiente and Alvarez, 2001].
Also, at S:C ratio
2:1 occurrence of liquid crystals hinted at the fact that longer chain alcohols
favors formation of liquid crystals [Prince, 1977b; Bayrak and Mehmet, 2005],
there by allowing long range ordering in the structure. The branched alcohol,
on the other hand showed very little isotropic regions, which are separated by
multiphasic region, as evident from Figs. 3.13, 3.15, 3.17. The lower eciency
of the branched cosurfactant is probably due to steric diculties [Valiente and
Alvarez, 2001; Mitra and Paul, 2005]. Also, it appears branched chain alcohols
tends to occupy more area at the interface, aecting the packing of surfactants
at the interface [Stubenrauch, 2008], resulting in less rigid structure resulting
in preventing formation of liquid crystals [Aboofazeli and J., 1993].
63
Figure 3.12: Pseudoternary phase diagram for the system C 12 E8 / Hexyl Cellosolve/ Coconut Oil/ Water, with S:C ratio 1:1.
The region above the line
represents isotropic/ monophasic/ microemulsion region, whereas the region
below the line is anisotropic/ turbid in nature.
Figure
3.13:
Pseudoternary
phase
diagram
for
the
system
C 12 E8 /
2-
Ethylhexylalcohol/ Coconut Oil/ Water, with S:C ratio 1:1. The anisotropic/
turbid region exist near the apex and also covers the base of the triangle.
Whereas, the isotropic region is present between the two anisotropic zones.
64
Figure 3.14: Pseudoternary phase diagram for the system C 12 E8 / Hexyl Cellosolve/ Coconut Oil/ Water, with S:C ratio 2:1.
The region above the line
represents isotropic/ monophasic/ microemulsion region, whereas the region
below the line is anisotropic/ turbid in nature. The area demarcated in red
represents liquid crystal region.
Figure
3.15:
Pseudoternary
phase
diagram
for
the
system
C 12 E8 /
2-
Ethylhexylalcohol/ Coconut Oil/ Water, with S:C ratio 2:1. The demarcated
regions near the apex and towards the water corner, by represents isotropic/
monophasic/ microemulsion region, whereas the region between the isotropic
regions is anisotropic/ turbid in nature.
65
Figure 3.16: Pseudoternary phase diagram for the system C 12 E8 / Hexyl Cellosolve/ Coconut Oil/ Water, with S:C ratio 1:2.
The region above the line
represents isotropic/ monophasic/ microemulsion region, whereas the region
below the line is anisotropic/ turbid in nature.
Figure
3.17:
Pseudoternary
phase
diagram
for
the
system
Ethylhexylalcohol/ Coconut Oil/ Water, with S:C ratio 1:2.
region
exists
in
form
of
two
seperated
anisotropic/ turbid region.
66
isolated
regions,
C 12 E8 /
2-
The isotropic
surrounded
by
Table 3.2: The computed gures in the table indicate the change in overall
% isotropic region due to straight chain and branched cosurfactant at various
S:C ratios.
S:C ratio
Isotropic Region %
Straight Chain
Branched Chain (2-Ethylhexyl glycol).
(n-hexylglycol/ Hexyl
Cellosolve)
1:1
29
11
2:1
25
20
1:2
21
3
67
3.4.4 Eect of Temperature on Phase Behavior
The eect of varying temperature on the phase behavior of the system C 12 E8 /
n-pentanol/ Coconut Oil/ Water was investigated.
The phase behavior was
studied at four dierent temperatures 20, 25, 30 and 35 °C, with three S:C
ratios 1:1, 1:1.5 and 1:2 as shown in Figs. 3.18,3.19 and 3.20.
The change phase behavior with temperature at S:C ratio 1:1 is shown in
Fig.
3.18.
It was observed that at lower temperatures the isotropic showed
a favored incline towards o/w region. At 20
°C
the minimum amphiphile re-
quired to solubilize 10 % coconut oil (in 50 % water) was 40 % , this region
extended till w/o region, where it took 58 % surfactant to solubilize 2 % water
in 40 % coconut oil. The isotropic region started to recede from o/w region,
and show increased tendency to form w/o microemulsion at higher temperatures. At 25°C, o/w isotropic region slightly recedes and tends to give rise to
a larger isotropic region in w/o region inclining towards oil corner. Further,
same trend is observed at 30
°C,
where the o/w region is smaller than both
previous temperatures, whereas slightly larger isotropic region was observed
near the middle phase region.
any change.
And the w/o region remained stable without
At 35 °C, the phase behavior showed a tremendous change, the
isotropic o/w region completely ceased to exist giving rise to large macrophasic/ turbid region even in the amphiphile rich apex region. But, simultaneously
it the system retained its isotropic region near the w/o region, even though
the overall isotropic region had considerably diminished as compared to lower
temperatures. Table. 3.3, shows the changes in solubilization of oil and water
i.e; on isotropic region, with increasing temperature.
Similar behavior was observed with S:C ratio 1:1.5.
At all temperature
the overall microemulsion was found be smaller than that with 1:1.
It is
clearly evident form Fig. 3.19, that the isotropic region with slight inclination
towards o/w microemulsion at 20 °C, decreases with increasing temperature.
The isotropic region shifts towards w/o region at higher temperatures.
The
isotropic region decreases at 25 °C, retaining isotropic region in the middle
phase with high amphiphile concentrations (S+C 55 % solubilizing 20 % in
25 % oil). At 30 °C, the isotropic region exists near the amphiphile - coconut
oil region, solubilizing 1-15 % water in 18-48 % coconut oil utilizing 71-31 %
S+C. The isotropic region ceased to exist at further higher temperatures.
Identical trend in phase behavior was noted with S:C ratio 1:2. As shown
in Fig.
3.20, the isotopic region which already existed at near the apex at
high S+C %, showed a decline in the isotropic region near the middle phase
when the temperature was raised form 20 to 25 °C and then to 30 °C. But, the
68
isotropic region with higher S+C amounts and isotropic w/o region remained
constant through out the temperature rise. The isotropic region ceased to exist
at further higher temperatures.
Thus, for all S:C ratios in the system C 12 E8 / n-pentanol/ Coconut Oil/
Water, it was observed that with increasing temperature the phase behavior
considerably changed. This is probably due to the fact that, increasing temperature caused structural changes in nonionic surfactant, thus greatly aecting
its solubilization capacity [Rosen
et al.
, 1982; Schick, 1963]. Increasing tem-
perature lead to preferred formation of w/o microemulsion over o/w. This is
mainly because increase in temperature leads to dehydration of oxyethylene
group which makes the nonionic surfactants more lipophilic [Mitra and Paul,
2005; Arakami
et al.
, 1997; Warisnoicharoen
et al.
, 2000]. The solubility of
surfactant in oil increases with increasing temperature because the polar end
group-water interaction becomes less favorable and as the thermal energy is
imparted to the system, many of the existing hydrogen bonds are ruptured
leading to higher lipophilicity [Broze, 1999; Sidim and Iscan, 2008]. Thus, explaining the unhindered stability and presence of w/o microemulsions at higher
temperature, whereas o/w microemulsions eventually lost their isotropic nature with increasing temperature.
69
Figure 3.18: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1:1 at dierent temperatures. The region
depicted symbol represents isotropic region at 20 °C, represents isotropic region
at 25°C, represents isotropic region at 30 °C and
at 35°C.
70
♦
represents isotropic region
Figure 3.19: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1:1.5 at dierent temperatures. The region
depicted symbol represents isotropic region at 20 °C, represents isotropic region
at 25°C and represents isotropic region at 30 °C.
71
Figure 3.20: Pseudoternary phase diagram for the system C 12 E8 / n-pentanol/
Coconut Oil/ Water, with S:C ratio 1:2 at dierent temperatures. The region
depicted symbol represents isotropic region at 20 °C, represents isotropic region
at 25°C and represents isotropic region at 30 °C.
72
Table 3.3: Eect of Temperature on solubilization of coconut oil and water.
The values in the table, mention the composition of microemulsions with minimum wt.% of S+C (1:1) required to solubilize oil and water across the isotropic
region with increasing temperature.
Temperature
20°C
25°C
30°C
35°C
°C
S+C (1:1) wt.%
Coconut Oil, wt.%
Water, wt.%
40
10
50
58
40
2
45
12
43
38
61
1
45
20
35
38
57
5
48
32
20
38
58
4
73