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Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
Vesicles in fatty acid salt–fatty acid stabilized o/wemulsion—emulsion structure and rheology
Boris B. Niraula1, Tiong Ngiik Seng, Misni Misran∗
Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
Received 13 May 2003; accepted 12 January 2004
Abstract
Texture and rheology properties of fatty acid salt-fatty acid mixture stabilized oil–water (o/w) emulsions are being explored. While NaOH
solution was introduced as aqueous phase, oil phase contained in it dissolved lauric acid. When these oil and water phases are brought in
contact, a portion of lauric acid transforms into sodium laurate in situ via acid–base reaction, the combination of which at a certain threshold
concentration and above is believed to act as emulsifier in stabilizing the given o/w emulsion system. Light microscopy under cross polarizing
conditions suggested that in addition to the formation of surfactant stabilized emulsion droplets, formation of molecular aggregates of vesicle
type is imminent in these conditions, both the size and number density of which is found to be a function of emulsifier concentration ratio.
The concentration of emulsifier (sodium laurate–lauric acid) is being assessed in this work indirectly as a function of NaOH concentration,
due to the problem associated with its quantification. Non or a very small number of vesicles was observed at NaOH concentration below
0.4 M, whilst in the presence of 0.4–0.8 M NaOH the vesicle population increased substantially. While vesicle population density decreased
past 0.8 M NaOH, their size increased with NaOH concentration from 0.8 to 1 M. These observations suggested that sodium laurate–lauric
acid concentration ratio that corresponds to 0.6–0.8 M NaOH is the optimum NaOH concentration at which maximum vesicle number density
is observed. As far as rheology properties are concerned, at any given γ̇ , in the absence of NaOH, the η of the oil solubilized lauric acid was
found to be much lower than the η of corresponding sodium laurate–lauric acid stabilized o/w emulsion. Though η(γ̇) profiles of sodium
laurate–lauric acid stabilized o/w emulsion was not a linear function under the given sets of experimental conditions, at all conditions ηstrongly depended on sodium laurate–lauric acid concentrations (concentration of NaOH in this case). What more is that at any given γ̇ ,
the η was observed to be maximum at sodium laurate–lauric acid concentration ratio that corresponded to 0.8 M NaOH, a situation which
corresponds to the presence of the maximum vesicle number density. In addition to η, in the presence of 0.8 M NaOH, higher values of σ Y was
observed, implying that at this particular NaOH concentration these emulsions possessed greater degree of emulsion structuring compared to
other NaOH concentrations. These observations lead to the conclusion that sodium laurate concentration plays a great role in the formation
of fatty acid salt vesicles, that under the given set of conditions sodium laurate–lauric acid concentration corresponding to 0.8 M NaOH is the
optimum concentration at which maximum numberof vesiclesare produced, andthat rheology property of these emulsions depend not only on
emulsion droplets but also on the vesicle number density and size. Test on oscillatory shear mode also suggested that much like o/w emulsions
stabilized with other type surfactant, the viscoelastic properties of these sodium laurate–lauric acid stabilized o/w emulsions depended highly
on surfactant concentration. The fact that G response was dominant over G response at all measured frequency and sodium laurate–lauric
acid concentration, it is evident that viscous property dominated over elastic property in these emulsion. The good news is that improved
elastic property is observed at sodium laurate–lauric acid concentration that corresponded to 0.6–0.8 M NaOH. And not only, in the presence
of 0.8 M NaOH, while G response of these samples was dominant over G response at low domain, the dynamic moduli crossed-over at
around 2 Hz, past which the G response became dominant over G . This implies that, at this particular surfactant concentration, emulsions
stabilized with sodium laurate–laurate mixture displays an excellent viscoelastic property pertaining to emulsions used in pharmaceutical
and cosmetic products. This further suggests that in the presence of 0.8 M NaOH these o/w emulsion both spread easily and stabilized better
as well as possess long storage stability and shelf life. With these dynamic moduli responses it is not surprising that the δ of these sodium
laurate–lauric acid stabilized emulsions decreased with NaOH concentration from 0.4 to 0.8 M. The fact that these emulsion showed smallest
δ in the presence of 0.8 M NaOH as opposed to other NaOH concentrations, it is evident that solid-like elastic property of these emulsions
∗ Corresponding author. Tel.: +60-3-7967-4079; fax: +60-3-7967-4079.
E-mail addresses: [email protected] (M. Misran), [email protected] (B.B. Niraula).1 Co-corresponding author.
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.colsurfa.2004.01.002
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8 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
is enhanced and improved at this NaOH concentration. As far as oil type is concerned mineral oil based systems showed better rheology
properties compared to corresponding paraffin oil based systems.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Sodium laurate; o/w Emulsion; Vesicles in emulsion; Shear viscosity; Shear thinning; Yield stress; Viscoelastic property; Storage modulus; Loss
modulus; Cole–Cole plot; Phase angle
1. Introduction
Lipids including fatty acids are important group of am-
phiphilic molecules that are used in stabilizing oil–water
(o/w) interfaces. As amphiphiles lipid as well as fatty acid
molecules may self-assemble into a variety of aggregates
ranging from micelles to bilayers to multilayered stacked of
lamellar type to other form of liquid crystalline structures
[1]. Among these structures both the unilamellar vesicles and
the array of closely-packed multilamellar vesicles referred
to as the onion phase finds scientific and industrial interest
for several reasons including, encapsulation of drug parti-cles inside their inner compartments, compartmentalization
of fluids and in transporting macromolecules through the
blood stream and skin [2]. In any case vesicles in emulsion
with these appealing feature finds potential applications in
pharmaceutical and cosmetic industries due to their ability
in releasing drugs and other biologically active substances
encapsulated in them in a slow and controlled manner
[3].
Low molecular weight amphiphiles such as phospholipids
are capable of forming vesicles from both their aqueous so-
lutions and o/w emulsions. Strong evidence of the presence
of fatty acid vesicles in o/w emulsions was provided byFerezou et al. [4], Hajri et al. [5] and Westsen and Wehler
[6], who after exploring TG-water and PC-water systems a
decade ago reported that in most fatty acid stabilized com-
mercial o/w formulations lipid vesicles coexist as dispersed
phase with emulsified droplets.
Vesicles in emulsions are produced in many ways, but in
most cases their production require input of external energy.
For instance, vesicles are often produced from solutions con-
taining lamellar bilayers through dispersion technique. Dis-
persion may involve either dilution of the solution or its me-
chanical agitation. In a simplest way vesicles are produced
from solutions containing lamellar bilayers through dilution
or through the input of external mechanical energy. Alterna-
tively, while sonication of aqueous lipid dispersion is a clas-
sical way of forming phospholipid vesicles, in some cases
vigorous shaking and vortexing are required [7]. In any case
both the vesicle type and their particle size distribution are
strongly affected by method of their preparation [8].
Thin film hydration is another method with which vesi-
cles are produced. It involves evaporation of solvent from
amphiphilic solutions with the result that a thin surfactant
film is produced. Such thin films get hydrated when they
come into contact with aqueous environment, leading to the
formation of vesicles [9].
On the other hand, reports often suggest that vesicles
are usually produced spontaneously from surfactant solu-
tion without the application of any external stimuli. Aqueous
fatty acid solutions are one of many systems identified as be-
ing capable of forming vesicles spontaneously, where vesi-
cles are formed as a function of pH via deprotonation of fatty
acid molecules [10]. In these systems carboxylate anion are
regarded as surfactant, whilst the corresponding fatty acid as
a cosurfactant. Therefore, the formation of vesicles in these
systems depends strongly on carboxylate anion–carboxylic
acid mixing ratio, with the result that higher fractions of fatty
acid generally lead to the reduction of the head group area of the amphiphilic molecules, and thereby reducing curvature
of the surfactant aggregates.
Pautot et al. [11] report that, provided right initial com-
position is chosen, ternary systems consisting of fatty acid,
oil and water are capable of generating both the emulsified
droplets and multilamellar aggregates of vesicle type spon-
taneously under most experimental conditions. Olson et al.
[12] on the other hand argue that, though production of both
the emulsified droplets and multilamellar aggregates of vesi-
cles type usually require input of external energy under most
conditions, large MLV are often produced spontaneously
in the presence of excess water. Unilamellar aggregates of vesicle type with high degree of monodispersity can be pro-
duced when these systems are extruded through filter paper
of narrow pore size.
The problems is that despite these simple procedures, in
most circumstances vesicles are only produced from both the
surfactant solutions and o/w emulsions upon the application
of external mechanical stimuli, where shearing forces are
being the most commonly employed form of external stimuli
[13].
As far as surfactant type is concerned, among other AOT
[14] and C12E6 [15], both anionic in nature, tend to pro-
mote spontaneous emulsifications provided a co-solvent that
is soluble in both the oil and water phases is added to the
system. By contrast, giant unilamellar vesicles with broad
particle size distribution are produced by rehydration and
swelling of thin film [16] In any case although production
of both the emulsions along and a combination of vesicles
in emulsion system is a well-documented subject, the mech-
anism on how these systems are generated is still not under-
stood fully and it largely remains a subject of speculation.
In this regard while Saito et al. [17] and Handa et al. [18]
treat the presence vesicles in an o/w emulsions on the basis of
monolayer/bilayer equilibrium transition, Israelachvili et al.
[19] suggest that lamellar liquid crystalline phase is formed
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 9
when the packing parameter, R, of the surfactant is in the
range of 0.5–1. The packing parameter is defined as:
r =V H
lHaPH,
where, V H is the volume of the hydrogen carbon chain, lH the
length of the hydrocarbon chain and aPH the cross-sectionarea of the polar head group. With this general formula
in mind it is not difficult to visualize that many surfactant
molecules, including lecithin, monoglycerides, combination
of long chain alcohol/long chain soap and long chain fatty
acid/long chain fatty acid soap, form lamellar structure of
vesicle type [20].
The good news is that emulsions stabilized with liquid
crystals show high viscosity and excellent stability. The other
advantage of the presence of liquid crystalline aggregates in
o/w emulsions is that they help dissolve substances that oth-
erwise show only a limited solubility. A good example is hy-
drocortisone, whose solubility in isotropic solvents in rather
small: in the order of 1.5% in ethylene glycol. On the con-trary, hydrocortisone can be dissolved above 4% in the pres-
ence of lamellar liquid crystalline phase of lecithin in water.
Texture and flow characteristics of emulsions used in
cosmeceutical and pharmaceutical products are generally ex-
plored with the help of rheology [21]. The Rheology proper-
ties of surfactant stabilized emulsions can be directly linked
to emulsion structural networks, which is generally formed
as a consequence of self-association of surfactant molecules
at the o/w interface, and the inter-droplet interactions
that result as a consequence of formation of such droplet
networks.
As to emulsion rheology, most emulsions used in pharma-ceutical and cosmetic products show a combination of shear
thinning property and pseudo-plastic behavior characterized
by yield stress (σ Y ). It is widely known that emulsions dis-
playing higher degree of shear thinning at low shear rate
spread easily, whereas emulsions showing higher values of
σ Y offer larger degree of resistance to an external force be-
fore they start flowing. These implies that the higher the σ Y the greater the degree of emulsion structuring [22] and the
greater the degree of its stability.
Overall, all creams, semisolids and emulsions used in
pharmaceutical and cosmetic products not only exhibit vis-
coelastic properties [23] usually described by a combination
of high degree of shear thinning and high value of yield
stress, but they are better described by frequency dependence
dynamic moduli profiles.
For example, in oscillatory shear mode, domination of
storage modulus (G) over loss modulus (G) at a high ω
domain indicates good emulsion stability, whereas domina-
tion of G over G at a low frequency domain indicates that
emulsions are easily spread over human skin [24]. As far
as viscoelastic model is concerned, fluids showing Maxwell
[25] model type flow behavior usually display a semi-circled
shaped Cole–Cole plot, whilst the G responses of such flu-
ids scales with ω2 whereas their G responses scales with
ω. Ideal viscoelastic material usually show phase angle, δ,
of around 45◦ at a wide range of frequency. Material show-
ing a δ of <45◦ are usually more elastic in nature, whilst
those showing δ of >45◦ are more viscous in nature. The δ
is described as a ratio of loss modulus to storage modulus
and it is given by tan−1 [G / G], where δ can range from 0◦
for ideal elastic material to 90◦
for ideal viscous material[26]. δ(ω) profiles thus indirectly tells us about the degree of
viscoelasticity of the emulsion. Higher value of δ indicates
that emulsions are more liquid-like in nature, whereas lower
values of δ indicate that emulsions are more solid-like in
nature.
While Rheology property of surfactant solution [27] and
surfactant stabilized emulsions [28,29] is a widely studied
topic, literature search suggested that Rheology properties
of surfactant stabilized complex emulsions with vesicles and
emulsified droplets as dispersed phase is a virgin area of
study. Considering potential applications of fatty acid based
vesicles in encapsulation and control drug release this work
explores rheology properties of fatty acid salt/fatty acid sta-bilized o/w emulsions. The beauty is that in the presence of
both the vesicles and emulsion droplets as dispersed phase,
a surfactant stabilized o/w emulsions display fascinating
emulsion texture and rheology property.
Fatty acid is oil soluble and a solution of fatty acid usually
shows shear thinning Rheology properties. Depending upon
the concentration, in the presence of NaOH fatty acid solu-
tion transforms into fatty acid salt, which in turn is found
to acting as emulsion stabilizer. The beauty is that, in addi-
tion to stabilizing emulsion droplets, depending upon con-
centration ratio a system consisting of fatty acid salt/fatty
acid mixture as stabilizer produces surfactant aggregates of vesicle type in an o/w emulsion. This means that fatty acid
salt/fatty acid mixture stabilized o/w emulsions contain in
it both sterically stabilized droplets and vesicles of multil-
amellar type as dispersed phase.
2. Experimental
2.1. Chemicals
All solutions and emulsions studied in this work were
prepared in deionized water with an ionic conductivity of
18S/cm. All chemicals used in this work were of TLC
grade unless other wise stated. They were used without
any purification. Both lauric acid of GC grade and NaOH
were purchased from Fluka, both of which were of 98%
purity. To make it moisture free, before its use lauric acid
was placed in vacuum oven at an ambient temperature of
25 ◦C and for a period of 24 h. A 15 mmHg vacuum was
employed for this purpose.
Both the mineral oil and paraffin oil used in this work
were of 0.84–0.86 mg/ml density. While the mineral oil used
was a complex mixture of both the straight and branched
chain naphthenic and aromatic hydrocarbons with 15 or
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10 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
more carbon atoms, the paraffin oil contained in it a mixture
of straight-chain hydrocarbons with an average chain length
of 20–30 carbon atoms. Both the mineral and paraffin oil
were purchased from Fluka and these were used as soon as
received to avoid rancidity.
2.2. Sample preparation
All samples examined in this work were prepared by mix-
ing 0.7 fractions of oil phase in 0.3 fractions of aqueous
phase, where a known amount of lauric acid (0.5 M in this
case) was dissolved in oil phase prior to mixing it with wa-
ter phase. Conversely, a series of NaOH solutions of known
concentration (ranging from 0.2 to 1 M) was freshly pre-
pared by dissolving NaOH pellets into deionized water prior
to mixing water phase in oil phase. This means that the oil
phase being studied in this work consisted of lauric acid
dissolved in oil, whereas the aqueous phase consisted of al-
kaline solution of NaOH.
As oil droplets are formed the dispersed surfactantmolecules in oil phase start accumulating at the oil–water
interface forming surfactant monolayer at the interface and
by doing so stabilize oil droplets.
After mixing the oil phase and NaOH solutions at various
mixing ratios, the mixtures that resulted were homogenized
for at least 5 min to give corresponding o/w emulsions. A
homogenizer, with a rotational speed of 13,000 rpm, was
employed for this purpose. All emulsified samples were then
stored for a period of 3–4 days before running any rheology
on them. This would allow equilibration and stabilization of
dispersed phase in continuous phase.
The dynamic moduli, in the oscillatory shear mode, weremeasured within the linear viscoelastic regime, where the
material function was a function of angular frequency alone.
Please note that in order to erase shear history each sample
was subjected to pre-shear sweep before commencing any
oscillatory shear test on them. A shear sweep from 0.001 to
0.1s−1 was performed for this purpose. This followed a rest
period of 120 s. This was necessary to ensure that shear his-
tory of samples is erased and that the samples established
their equilibrium structure before under-going any shear de-
formation under oscillation mode.
2.3. Instrumentation
2.3.1. Optical microscopy
A LEICA DMRXP Germany made light polarizing mi-
croscope was used to scan optical micrographs of emulsions
under investigation. Images were focused with the help of
both the dark field and bright field mode between cross po-
larizers with long working distance objectives with 20×,
50× (oil immersion) and 100× magnifications. This system
consists of a high voltage beam source, a polarizing unit and
a detector unit. The detector unit is interfaced with a per-
sonal computer equipped with image analysis Leach Qwin
Standard version 2.6 software. This software helps captur-
ing as well as importing images from microscope stage into
a personal computer with a further possibility of saving and
processing them as electronic documents.
2.3.2. Rheometer
All rheology data presented in this work were scanned
with the help of a Bohlin VOR strain controlled Rheome-ter. It was equipped with a cone plate measuring geometry
(CP 2.5/30, with a cone of 30 mm diameter and cone angle
of 2.5◦). This geometry consists of a rotating lower plate
and a fixed upper cone, with a measurement gap of 70 m.
This instrument operates on the principle that a controlled
shear/strain rate is applied to the sample and the resulting
stress response is monitored, while doing so the shear rate
can be maintained within a range of 0.3%.
The advantage of using this instrument is that it can be
configured to measure a variety of characteristics within a
wide range of shear rate and oscillation frequencies. It covers
five orders of magnitude in shear rate range from 10−3 to
1.5 × 103 s−1, which allows measurement of shear stressvalues from 10 to 104 Pa. The accessible range of viscosities
is being 10−2 to 104 Pa s. Oscillation experiments can be run
from 10−3 to 20 Hz, covering shear strain values from 0.01 to
20 s−1. In this work, both the shear rate dependent viscosity
and shear stress were obtained through steady-shear sweep
measurements, whereas the frequency sweep measurements
were performed at a constant shear rate in the frequency
region from 10−3 to 20Hz.Shear rates from10 to 5×102 s−1
was applied in this work.
As viscosity versus shear rate profiles did not show an
apparent Newtonian plateau at low shear rates, zero shear
viscosity (η0) could not be identified. Hence, instead of η0low shear viscosity (ηlow) was monitored at a shear rate
of 0.01 s−1. On the contrary, before commencing any test
in oscillatory shear mode, a strain sweep test was carried
out for the determination of the proper strain to be applied;
where the lower limit is usually automatically set by built-in
transducer, while the upper limit was set within the limit
of sample’s linear viscoelastic regime. Because these emul-
sions did not show an apparent plateau within the measured
frequency range, GoN , the liquid-like entanglement storage
modulus, could not be estimated. In such situations GoN is
usually estimated from the on-set of plateau region at high
frequency G (ω = 100 Hz). The yield Stress was obtained
from the stress versus shear rate profiles through the extrap-
olation of the stress towards the zero-strain.
All measurements presented in this work were obtained
with the help of a torsion bar of 10 g cm. It is also worth
noting here that all results shown in this work are within the
sensitivity range of the Rheometer, whereas data collected
below rheometer sensitivity of 20 mPa were discarded. Like-
wise all measurements presented in this work were carried
out at 25 ◦C. The temperature was controlled with the help
of oil bath. IKA Labortechnik T25 homogenizer was used
for the homogenization of the samples. While a Thermo
Orion made Ag/AgCl (3 M KCl) pH electrode was em-
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 11
ployed to sense pH of these emulsions, a CyberScan PC 510
pH/conductivity meter was used as a pH meter.
3. Results and discussions
3.1. Acid–base profile and ternary phase diagram
Acid–base profile of oil solubilized 0.5 M lauric acid as
a function of NaOH concentration is given in Fig. 1. This
suggests that no sodium laurate is produced at NaOH con-
centration below 0.2 M, whilst past 0.2 M NaOH it is evi-
dent from this profile that sodium laurate is produced in situ.
It means that, under the present set of experimental condi-
tions, lauric acid starts dissociating only past 0.2 M NaOH,
producing sodium laurate. This implies that the concentra-
tion of sodium laurate increases with the increase in NaOH
concentration past 0.2 M NaOH, whilst the concentration of
lauric acid decreases. This acid–base profile thus gives a
ternary oil–water and surfactant mixtures with known oil to
water mixing ratios and surfactant concentration. The prob-
lem here is that under these set of experimental conditions
sodium laurate–lauric acid concentration ratios can only be
estimated indirectly in terms of NaOH concentration. Thus
in the discussion that follows surfactant concentration is ex-
pressed in terms of NaOH concentration instead of sodium
laurate–lauric acid concentration ratio.
As can be seen from this acid–base profile, given oil to
water mixing ratio constant, this acid–base profile produces
a series of sodium laurate–lauric acid stabilized o/w emul-
sions, the texture profiles of whose largely depend on NaOH
concentration. The texture profiles (Fig. 2) of these ternarysystems suggest that in the presence of water-soluble sodium
laurate as a surfactant and lauric acid as cosurfactant both
the mineral and paraffin oil produces stable o/w emulsions,
whose dispersed phase contain not only emulsified droplets
but also surfactant aggregates of vesicle type. This means
0.0 0.2 0.4 0.6 0.8 1.0
7.4
7.6
7.8
8.0
8.2
8.4
8.6
8.8
9.0
Paraffin Oil
Mineral Oil
p H
[NaOH], mol dm-3
Fig. 1. Acid–base profile of lauric acid solution as a function of NaOH.
that in situ generation of sodium laurate as a surfactant under
the given sets of experimental conditions not only leads to
the generation of emulsified droplets but also to the genera-
tion of multi-lamellar liquid crystalline aggregates of vesicle
type at the same time.
Optical microscopy under cross polarizing light suggests
that at oil to water mixing ratio of 7:3 and in the presenceof a certain sodium laurate–lauric acid concentration ratio
that corresponds to 0.2 M NaOH and higher, in addition
to emulsified droplets a lamellar liquid crystalline phase of
vesicle type is generated instead of isotropic surfactant so-
lution (Fig. 2). In other words, these images further suggest
that emulsion systems under investigation contain emulsified
droplets both with and without liquid crystalline character.
The presence of lamellar and hexagonal liquid crystals
phases is evident in these emulsions as they exhibited optical
anisotropy characteristic to emulsified droplets covered with
layers of surfactant. Also, as shown in these images droplets
with liquid crystalline character not only exhibit a strong
halo at the center under cross polarizing condition, but theyusually appear radiant under cross polarizer with spherical
patterns characteristic of Malthesian crosses. Both of these
characteristics are being inherent properties of layered liquid
crystalline aggregates of vesicle type.
As depicted in Fig. 2, typical indicator of the occurrence
of vesicle like structures in both the concentrated surfactant
solution and surfactant stabilized o/w emulsions is the pres-
ence of optical birefringence under cross polarizing condi-
tion. Characteristic to optical birefringence is the presence
of a variety of color inside individual vesicle unit (Fig. 2b
and e), which usually arises due to optical interferences. Op-
tical interference in samples with vesicle droplets arises dueto refractive index differences inside the droplet. Refractive
index difference in vesicle droplets usually arises due to the
presence of layers of different densities.
As depicted in Fig. 2c, under dark field, the image of these
vesicles resemble to fan like structures, white shades filled
with black patches. As it turned out, these images also sug-
gest that, along with vesicles, oil droplets of a variety of sizes
existed as a dispersed phase in a continuous matrix of aque-
ous surfactant solution. As far as emulsion packing is con-
cerned, as can be seen in these figures, under the given set of
experimental conditions and optical magnification it is hard
to figure out as to how droplets are packed in these emul-
sions. However, much like vesicle number density and their
size, it is not hard to figure out that both the droplet size and
population density depended highly on NaOH concentration.
These microscopy images indirectly suggest that in the
presence of given fatty acid salt/fatty acid concentration ra-
tios the mutual solubility of the given oil and water system is
largest. This further suggests that at 7:3 oil to water mixing
ratios and in the presence of these surfactant–cosurfactant
concentration ratios a higher degree of emulsion stability is
achieved.
Images under cross polarizer also suggest that sodium
laurate–lauric acid stabilized o/w emulsions displayed both
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Table 1
Changes in vesicle size and vesicle number density of sodium laurate–lauric acid stabilized mineral oil–water emulsion as a function of NaOH concentration
NaOH concentration (M) pH HLB values Vesicles sizes (m) Number density
0.4 7.91 9.7 No vesicles –
0.6 8.19 12.6 Small 9–11.5 Low
0.8 8.62 15.5 Small 3–7, large 50 High
1 8.85 18.4 Small 3–10, large 10–50 Medium
while their size increased (Fig. 2d and e). These images fur-
ther suggest that depending upon NaOH concentration the
size of the vesicles varied from 1 to 50 m, whilst the size
of the dispersed oil droplets varied from 3 to 10 m. Shapes
of both the vesicles and oil droplets were being of predom-
inantly spherical.
Table 1 summarizes the change in vesicle number den-
sity and sizes as a function of NaOH concentration. Vesicle
number density increased with NaOH concentration from
0.4 to 0.8 M NaOH. On the contrary, past 0.8 M NaOH the
vesicle number density slightly decreased, whilst their sizeincreased with the increase in NaOH concentration. From
these considerations it is evident that 0.8 M is the optimal
NaOH concentration at which population of smallest sized
vesicles was highest.
Table 1 also suggests that the HLB values of the emul-
sion system increased with NaOH concentration, while pH
of these emulsion systems, though increased with NaOH
concentration, did not increase significantly. This indirectly
tells us that water insoluble lauric acid gets neutralized to
give a more water-soluble sodium laurate as NaOH concen-
tration increases, and that the acid–base reaction was not
fully realized to completion under these conditions.
Depending upon surfactant–oil–water mixing ratios theternary phase diagram (Fig. 3) of sodium laurate–lauric
acid–water–oil system displayed a variety of complex
phases. For example, except at surfactant concentration level
below 5% at the far left side of the diagram most volume
0.0
0.2
0.4
0.6
1.0 0.0
0.2
0.4
0.6
1.0
VG
LQ-E
L2
UE
L1
Na-Laurate/
Lauric Acid
(wt%)
Oil (wt%)
Water
(wt%)
80
60
40
20 80
60
40
20
20 40 60 80
Fig. 3. Ternary phase diagram of oil, water and sodium laurate–lauric acid system at 25 ◦C.
fractions are two phases. Firstly, a rather unstable emulsion
(UE) was identified at all oil to water mixing ratios at this
low surfactant concentration domain. Secondly, common to
most surfactant–oil–water systems two isotropic solubilized
regions were identified at surfactant concentration level
above 5%. Of which two regions show isotropic solution
properties. L1 region is characterized by the presence of
excess of water with surfactant solubilized in it.
Considering water as a solvent, the first L1 phase, ap-
peared at high water phase concentration where the aqueous
surfactant solution usually incorporates the oil phase in itsmicellar structure. The second isotropic region, L2, appeared
at high oil phase concentrations, where the solubilizate-
solvent roles are reversed. Here water is solubilized by oil
phase and the micelles within this phase are reversed.
The area under LQ-E in this diagram represents homo-
geneous liquid crystalline region. This suggests that in the
presence of 10–30 wt.% of surfactant and oil to water mix-
ing ratio ranging from 15/85 to 35/65 a stable emulsion was
observed which showed an anisotropic and optically bire-
fringent liquid crystalline phase in addition to emulsified
droplets. This means that in situ generation of sodium lau-
rate from lauric acid not only lead to the formation of emul-
sified oil droplets but also to the formation of multilamellarliquid crystalline aggregates of vesicle type. On the other
hand, with the increase in surfactant concentration this sys-
tem increasingly became first gel like and than it turned into
solid even at these rather low surfactant concentrations (VG).
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14 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
0.6 0.7 0.8 0.9 1.0
0
2
4
6
8
10
12
14
16
18
20
A v e r a g e s i z e ( µ m )
NaOH concentration (M)
Fig. 4. Plot of vesicle sizes as a function of NaOH concentration.
Note a viscous isotropic gel like region (VG) was identified
as soon as the surfactant concentration level exceeded 20%.
As else where, this ternary phase diagram demonstrates
that under the given sets of experimental conditions stable
emulsions are produced only in a certain oil to water mix-
ing ratios, which in this case ranged from 1.5:8.5 to 7:3, and
these could only be accomplished in the presence of at least
0.2 M NaOH. Interestingly, stability of the resulting emul-
sions increased with NaOH concentration. This further im-
plies that the stability of these emulsions can be enhanced
and improved by increasing sodium laurate concentration.
As far as vesicles are concerned with the increase insodium laurate concentration their population density in-
creased from 0.2 M equivalent of NaOH to 0.8 M equiva-
lent of NaOH, past which NaOH concentration the vesicle
0.0 0.2 0.4 0.6 0.8 1.0
0
200
400
600
800
1000
mineral oil
paraffin oil
, P
a s
[NaOH], mol dm-3
Fig. 5. ηlow As a function of NaOH concentration (emulsion samples consisted of 0.7vol. fractions of oil phase in 0.3 vol. fractions of water phase,
where NaOH was dissolved in water phase prior to mixing, whereas lauric acid was dissolved in oil phase).
number density decreased (Fig. 4). As far as vesicles size
is concerned it reduced with NaOH concentration from
0.4 to 0.8 M, past which NaOH concentration the vesicle
size increased again. These latter observations lead to the
conclusion that a certain threshold sodium laurate to lauric
acid concentration ratio plays an important role in forming
multilamellar aggregates of vesicle type, that for the given
surfactant system this threshold concentration ratio can be
attained around 0.2 M NaOH, and that in the neighborhood
of 0.8 M NaOH a optimal surfactant–cosurfactant ratio is
attained so as to enable the formation of maximum vesicle
population density.Also noteworthy is the fact that the presence of liquid
crystalline phase enhances emulsion stability [30] because
they not only form a covering skin like layer around droplets
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 15
but also a kind of rigid three-dimensional interconnected
network throughout the continuous phase. The importance
of the skin like covering layer is that it prevents droplet co-
alescence because of its high viscosity and the fact that a
layered structure changes inter particle distance dependence
Van der Waals pair potentials [31]. In addition, the presence
of heavily charged systems in the liquid crystalline phaseenhances emulsion stability due to the influence of the elec-
tric double layer [32], which obviously is the case with in-
creasing NaOH concentration.
3.2. Emulsion rheology
3.2.1. ηlow Profiles
Fig. 5 shows low shear viscosity (ηlow) profile of sodium
laurate–lauric acid stabilized o/w emulsions as a function
of NaOH concentration. This was measured at a shear rate
0 100 200 300 400 500
0.018
0.019
0.020
0.021
0.022
0.023
0.024
0.025
0.026 mineral oil with lauric acid
paraffin oil with lauric acid
η ,
P a s
γ , s-1
0 100 200 300 400 500
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.4M NaOH
0.6M NaOH
0.8M NaOH
1.0M NaOH
η , P a s
γ , s-1
(a)
(b)
Fig. 6. η(γ̇) Profiles of sodium laurate–lauric acid stabilized o/w emulsions as a function of NaOH concentration. (a) η(γ̇) Profile in the absence of
NaOH; (b) η(γ̇) profiles in the presence of NaOH.
of 0.1s−1. This figure demonstrates that, irrespective of oil
type, the ηlow of sodium laurate–lauric acid stabilized o/w
emulsion is a function of sodium laurate–lauric acid concen-
tration ratios (in this case function of NaOH concentration).
It is low at lower NaOH concentrations until 0.4 M NaOH,
as only a small amount of sodium laurate is generated at
this concentration, whilst it increased monotonously as afunction of NaOH concentration from 0.4 to 0.8 M. While
the ηlow picked up at NaOH concentration of 0.8 M, past
0.8 M it decayed again suddenly. This suggests that some
kind of transition takes place in these emulsion systems at
this particular NaOH concentration. Most probably this can
be accounted for the formation of large number of vesicles,
and in particular for increase in vesicle number density as
a function of sodium laurate concentration. And, if this is
the case it is evident from these data that the ηlow of these
o/w emulsions increased with the increase in vesicle num-
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16 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
ber density, whereby the vesicle number density depended
largely on sodium laurate concentration.
These observations further suggest that sodium laurate–lauric
acid concentration ratio plays a strong role in imparting
viscosity, and thereby rheology properties to these o/w sys-
tems. What seems worth noting here is that both the ηlow
profile (Fig. 5) and images under cross polarizer (Fig. 2)show similar trends as far as emulsion texture is concerned,
clearly demonstrating that rheology properties that is be-
ing presented and discussed below depend not only on the
deformation of emulsified droplets but also on the deforma-
tion of the vesicle droplets present in these systems. And
in particular, the pronounced η and viscoelastic behavior
that these emulsions exhibit in the presence of 0.8 M NaOH
can probably be accounted for the size and population den-
sity of these vesicles. For note that as opposed to other
NaOH concentrations not only the average vesicle size was
smaller in the presence of 0.8 M NaOH but also the vesicle
population density was highest in this case.
As far as oil phase is concerned, except that system withmineral oil exhibited slightly higher values of ηlow compared
to system with paraffin oil throughout all sodium laurate
concentrations examined in this work, the fact that both oil
types examined here displayed similar trends, it is evident
that oil phase hardly play any notable role in rheology prop-
erties of these emulsion systems. The small differences in
ηlow between mineral oil and paraffin oil based systems can
probably be accounted for the fact that, while mineral oil
used in this work contained in it a complex mixture of both
the straight and branched chain naphthenic and aromatic hy-
drocarbons with 15 or more carbon atoms, the paraffin oil
contained in it a mixture of straight-chain hydrocarbons withan average chain length of 20–30 carbon atoms.
3.2.2. η(γ̇ ) Profiles
Fig. 6a shows η(γ̇) profiles of blank solutions consisting
of oil solubilized lauric acid. From the comparison of these
blank profiles with the η(γ̇) profile of sodium laurate–lauric
acid stabilized emulsions (Fig. 6b), it is evident that sodium
laurate–lauric acid concentration ratio plays a great role in
imparting viscosity to these w/o emulsions. This comparison
evidently suggests that the difference in η of systems with
and without sodium laurate–lauric acid is at least decade at
any given γ̇ , and that at any given γ̇ the η increased with
sodium laurate–lauric acid concentration ratio (in this case
NaOH concentration from 0.4 to 0.8 M), a situation which
corresponded to increase in vesicle population density as a
function of sodium laurate–lauric acid concentration ratio.
What is interesting here is that at any given γ̇ the η(γ̇)
profile of the emulsion with 0.6 M NaOH hardly differ from
the η(γ̇) profile of the emulsion with 0.8 M NaOH, except
that the η is slightly higher in case of 0.8 M NaOH at low
γ̇ domain. Probably it can be accounted for the fact that
vesicles number density per unit volume of sample is higher
in the presence of 0.8 M NaOH as opposed to 0.6 M and
other NaOH concentrations.
These profiles further suggest that irrespective of sodium
laurate–lauric acid concentration ratio all samples exam-
ined here displayed an exponentially decaying shear thin-
ning properties. It means that η in these flow profiles is
γ̇ dependent, and that the η(γ̇) relationship in these pro-
files are characterized by non-Newtonian flow behaviors,
with power law functions. Given the inverse relationshipbetween η and γ̇ , the exponent (n) of the power law were
derived from flow equation η = 1/γ̇ n [22], with the result
that the power law exponent decreased with the increase in
sodium laurate–lauric acid concentration ratios. Note these
sodium laurate–lauric acid concentration ratios corresponds
to NaOH concentration domain from 0.2 to 0.8 M (Fig. 7).
As far as the effect of oil phase is concerned mineral oil
exhibited higher rate of shear thinning compared to paraffin
oil at low γ̇ domain from 0.1 to 200 s−1, past which shear
rate the η was independent of oil type. This is evident from
both the exponent of power law and slopes of η–γ̇ profiles
as depicted in Fig. 8.
Another important property of these fatty acid salt/fattyacid stabilized o/w systems is that the slope of the lines of the
double logη(γ̇) profiles is a function of surfactant concentra-
tion, it increases with the increase in sodium laurate–lauric
acid concentration ratio (Fig. 8). Apparently, this serves as
a means of identifying the degree of shear thinning much in
the same way as power law index does, but with opposite
effect. The higher the slope of the line the greater is the de-
gree of shear thinning, the better emulsified is the emulsion.
Much like surfactant solutions and o/w emulsions sta-
bilized with other surfactants, sodium laurate–lauric acid
stabilized o/w emulsions combine shear thinning behavior
with pseudo-plastic behavior characterized by σ Y (Fig. 9).As expected the difference in σ Y between blank and sam-
ples with sodium laurate–lauric acid (NaOH in this case) is
huge. This figure further illustrates that, irrespective of the
0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
mineral oil
paraffin oil
P o w e r L a w
I n d e x ( n )
NaOH concentration (M)
Fig. 7. Power law exponent (n) as a function of sodium laurate–lauric
acid concentration (unless otherwise stated sodium laurate–lauric acid
concentration is indirectly measured in this work as a function of NaOH
concentration).
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 17
1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
s l o p e = - 0 .4 0 6 8 , n = 0 .5 9 3 2
s l o p e = - 0 .4 13 7 7 , n = 0 .5 8 6 2 3
s l o p e = - 0 .5 8 4 0 1, n = 0 .4 15 9 9
s l o p e = - 0 .8 8 8 1 , n = 0 .1 1 1 9
0.40M NaOH
0.60M NaOH
0.80M NaOH
1.00M NaOH
L o g ( η ,
P a s )
Log (γ , s-1
)
1.9 2.0 2.1 2.2 2.3 2.4 2.5
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
s l o p e = - 0 .3 7 5 7 6 , n = 0 .6 2 4 2 4
s l o p e = - 0 .4 1 5 2 2 , n = 0 .5 8 4 7 8
s l o p e = - 0 .8 4 4 2 9 , n =
0 .1 5 5 7 1
s l o p e = - 0 .8 0 8 9 8 , n
= 0 .1 9 1 0 2
0.80M NaOH (mineral oil) 0.80M NaOH (paraffin oil)
1.00M NaOH (mineral oil)
1.00M NaOH (paraffin oil)
L o g
( η ,
P a s )
Log (γ , s-1
)
(a)
(b)
Fig. 8. Double log η(γ̇) profiles of sodium laurate–lauric acid stabilized o/w emulsions as a function of NaOH concentration.
type of oil phase used, the σ Y of these emulsions increased
with the increase in sodium laurate–lauric acid concentra-
tion ratio. This further implies that structure, elastic property
and stability of these emulsions get enhanced and improved
over their viscous property with the increase in sodium
laurate–lauric acid concentration ratio (with the increase
in corresponding NaOH concentration from 0.4 to 0.8 M).
Note it decreased again when sodium laurate–lauric acid
concentration was further increased past 0.8 M NaOH. This
further suggests that the presence of vesicles like structure,
and in particular vesicles number density definitely plays a
great role in imparting both the solid-like elastic property
and thereby stability to these samples.
Higher value of σ Y indicates that higher resistance to ex-
ternal forces before the system start to flow. According to
Barry and Eccleston [3] this further indicates that sample in
the presence of 0.6–0.8 M NaOH possess a greater degree of
material structuring compared to samples with other NaOH
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
0
5
10
15
20
25
30
35
40
yield value of mineral oil emulsion
yield value of paraffin oil emulsion
σ y ,
P a
[NaOH], M
Fig. 9. Effect of sodium laurate–lauric acid concentration (NaOH con-
centration in this case) on yield stress (σ Y ).
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18 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
concentrations. As far as type of oil is concerned, this same
figure suggests that, the σ Y of these samples only slightly
depends on type of oil. As expected, systems with mineral
oil showing a slightly higher σ Y compared to systems with
paraffin oil.
3.2.3. Frequency dependence of dynamic moduliProfiles showing ω dependence of dynamic moduli at
sodium laurate–lauric acid concentration ratio that corre-
sponds to 0.8 M NaOH is given in Fig. 10a. These pro-
files suggest that both dynamic moduli are not high, that
they show strong ω dependence at low ω domain, whereas
0 2 4 6 8 10
0
50
100
150
200
250
300
ω, Hz
G ' , P a
storage modulus
0
50
100
150
200
250
300
G" ,P a
loss modulus
0 2 4 6 8 10
0
50
100
150
200
250
300
1.00M NaOH (G') 0.80M NaOH (G') 0.60M NaOH (G')
ω, Hz
G ' , P a
0.40M NaOH (G')
0
50
100
150
200
250
300
1.00M NaOH (G") 0.80M NaOH (G") 0.60M NaOH (G")
G" ,P a
0.40M NaOH (G")
(a)
(b)
Fig. 10. (a) ω Dependence of dynamic moduli (G and G ) of sodium laurate–lauric acid stabilized o/w emulsion in 0.80 M NaOH. (b) Effect of sodium
laurate–lauric acid concentration (NaOH concentration) on ω dependence of dynamic moduli of sodium laurate–lauric acid stabilized o/w emulsions. (c)
Effect of oil type on ω dependence of dynamic moduli.
they show very weak ω dependence at ω domain past 2 Hz.
These ω dependence of dynamic moduli further suggest that
G response of these samples is dominant over G response
throughout entire measured ω domain, implying that liquid-
like (viscous) property dominates over solid-like (elastic)
property in these emulsion samples.
Another important point worth mentioning here is that,unlike G response which increased with ω throughout all
measured ω domain, G response in fact decreased, though
slightly, past 7 Hz. Such a viscoelastic response is not un-
usual for surfactant stabilized o/w emulsions. As far as
effect of sodium laurate is concerned, as expected both
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 19
0 2 4 6 8 10
0
50
100
150
200
250
paraffin oil (Storage Modulus)
ω, Hz
G ' , P a
mineral oil (Storage Modulus)
0
50
100
150
200
250
paraffin oil (Loss Modulus)
G" ,P a
mineral oil (Loss Modulus)
(c)
Fig. 10. (continued ).
dynamic moduli increased with sodium laurate concentra-
tion from 0.4 to 0.8 M, whereas it decreased past 0.8 M
(Fig. 10b).
Much like η(γ̇) profiles which showed interesting behav-
ior in the presence of 0.8 M NaOH, what is noteworthy here
is that in the presence of 0.8 M NaOH the dynamic mod-
uli exhibit an interesting behavior in that they crossed over
around 2 Hz, which is not the case in the presence of other
NaOH concentrations. It means while G
is dominant overG at low ω domain, past 2 Hz G becomes dominant over
G in the presence of 0.8 M NaOH. This implies that at
0 20 40 60 80 100
0
50
100
150
200
250
300
0.80M NaOH
1.00M NaOH
0.40M NaOH
0.60M NaOH
G " ,
P a
ω2, Hz
2
Fig. 11. Plot of G as a function ω2 of sodium laurate–lauric acid stabilized
o/w emulsions.
this particular sodium laurate–lauric acid concentration ra-
tio these emulsions display excellent property pertaining to
emulsions used in pharmaceutical and cosmetic products.
The practical implication of this behavior is that emulsions
showing higher G response over G at low frequency do-
main spread easily when they are applied to human skin,
whereas emulsions show high stability and longer storage
life if they exhibit domination of G response over G at
higher frequency domain. Both properties are highly de-sirable when formulating emulsions for cosmetic applica-
tions. This implies that among other emulsions tested here
0 2 4 6 8 10
0
40
80
120
160
200
240
0.80M NaOH
1.00M NaOH
0.40M NaOH
0.60M NaOH
G " ,
P a
ω, Hz
Fig. 12. Plot of G as a function of ω of sodium laurate–lauric acid
stabilized o/w emulsions.
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20 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
samples stabilized with 0.8 M NaOH equivalent of sodium
laurate–lauric acid is more suited for cosmetics applications
from the point of both the storage stability and ease of
spreading.
As regards type of oil phase used, mineral oil based emul-
sions exhibited better viscoelastic responses compared to
paraffin oil based emulsion (Fig. 10c). The fact that the G
0 10 20 30 40 50 60 70 80 90
0
20
40
60
80
100
120
i
0.40M NaOH
G " , P a
G', Pa
0 50 100 150 200 250 300
0
40
80
120
160
200
240
ii
0.60M NaOH
G " , P a
G', Pa
0 20 40 60 80 100 120 140 160 180 200
0
20
40
60
80
100
iii
0.80M NaOH
G
" , P a
G', Pa
0 5 10 15 20 25 30 35 40 45
0
10
20
30
40
50
60
iv
1.00M NaOH
G
" , P a
G', Pa
100 120 140 160 180 200 220 240 260 280
120
140
160
180
200
220
240
i
mineral oil
paraffin oil
G " , P a
G', Pa
60 80 100 120 140 160 180 200 220 240 260
40
60
80
100
120
140
160
ii
mineral oil
paraffin oil
G " , P a
G', Pa
(a)
(b)
Fig. 13. (a) Cole–Cole plot of sodium laurate–lauric acid stabilized o/w emulsions as a function of NaOH concentration. (b) Effect of oil type on
Cole–Cole plot of sodium laurate–lauric acid stabilized o/w emulsions in the presence of NaOH concentration of 0.6 and 0.8M.
response of mineral oil based emulsions was higher than that
of paraffin oil based emulsions, it is evident that the degree
of elastic property of mineral oil based emulsions is higher
compared to similar paraffin oil based systems. This in turn
suggests that mineral oil based emulsions are more stable
and possess longer shelve life compared to paraffin oil based
emulsions. Also, as G
response of mineral oil based emul-
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B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 21
sions was higher compared to paraffin oil based emulsion
at all measured frequency domain, it is evident that mineral
oil based systems flow and spread more easily compared to
corresponding paraffin oil based emulsions.
Plot of G as a function of ω2 is given in Fig. 11, which
suggest that G is not a linear function of ω2 over all
NaOH concentration domains examined in this work, thatat any given frequency the G response increased from 0.4
to 0.8 M NaOH, past which it decreased as a function of
NaOH concentration.
Plot of G as a function of ω is given in Fig. 12, which
also suggests that G is not a linear function of ω either over
the same NaOH concentration domain, that G increased
with concentration from 0.4 to 0.8 M, past which it de-
creases as a function of NaOH concentration. Considering
these rheology properties it can be concluded that samples
under investigation possess complex flow and viscoelastic
1E-3 0.01 0.1 1 10
25
30
35
40
45
50
55
60
65
70
0.40M NaOH
0.60M NaOH
0.80M NaOH
1.00M NaOH
δ
( d e g r e e )
ω (Hz)
1E-3 0.01 0.1 1 10
24
28
32
36
40
44
48
52
56
(i) 0.60M NaOH
0.80M NaOH
δ
( d e g r e e )
ω, Hz
1E-3 0.01 0.1 1 10
30
32
34
36
38
40
42
44
46
48
(ii) mineral oil
paraffin oil
δ
( d e g r e e )
ω, Hz
(a)
(b)
Fig. 14. (a) Effect of NaOH concentration on frequency dependence of phase angle (δ). (b) Effect of type of oil on frequency dependence of phase angle (δ).
properties, resembling neither liquid-like nor solid-like flow
properties, nor they follow Maxwell and Kevin model type
viscoelastic behavior.
Fig. 13a depicts Cole–Cole plots of sodium laurate stabi-
lized o/w emulsions as a function of NaOH concentration.
These suggest that plot of G(G) deviates a lot from semi-
circled shaped Cole–Cole plot. This is a further indicationthat emulsions under investigation do not show Maxwell
type viscoelastic fluid flow behavior [12]. What is interest-
ing here is that in the presence of 0.8 M NaOH the curve
does tend to show a better fit to semicircular shape rep-
resenting Cole–Cole plot, implying that in the presence of
0.8 M NaOH these emulsions tend to be look more close
to Maxwell type viscoelastic behavior compared to other
NaOH concentrations.
Fig. 13b compares Cole–Cole plot of mineral oil and
paraffin oil based systems in the presence of 0.6 and 0.8 M
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22 B.B. Niraula et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22
NaOH. This suggests that type of oil does seem to play a
significant role in G(G) response of these emulsions. Min-
eral oil based system showing better semi-circled-like shape
compared to corresponding paraffin oil based system. This
in turn suggests that solid-like property of the mineral oil
based system is more dominating and prominent than the
solid-like property of the paraffin oil based system.Plot of δ as a function of ω is given in Fig. 14a. As de-
picted here, irrespective of NaOH concentration, δ of these
emulsions is very high, higher than 30◦ throughout entire
ω domain, suggesting that these emulsions are more liquid-
like in nature than solid-like. Interestingly, δ decreased with
ω. Much like other flow properties δ of these emulsions
highly depended on NaOH concentration. It decreased with
NaOH concentration from 0.4 to 0.8 M. Past which, it in-
creased again. Decrease of δ with NaOH concentration
means that the solid-like property of these emulsions can
be improved and enhanced over liquid-like property by in-
creasing NaOH concentration from 0.4 to 0.8 M. Evidently
this can be accounted for and is related to the presence of the maximum vesicle number density. On the contrary, past
0.8 M NaOH δ of samples again increased suggesting that
liquid-like property is enhanced over solid-like property
past 0.8 M of NaOH.
As far as the effect of oil type is concerned, irrespective
of oil type, δ decreased with frequency (Fig. 14b). Interest-
ingly, at sufficiently higher ω domain past 10 Hz it decreased
to 25◦, implying that viscoelastic property is frequency de-
pendent, and that in the presence of 0.6 and 0.8 M NaOH
solid-like property of these emulsions became dominant over
liquid-like property. As far as type of oil is concerned min-
eral oil based system showed much enhanced elastic char-acter at higher ω domain compared to corresponding paraf-
fin oil based system, as the former system showed lower δ
compared to similar system based on paraffin oil.
4. Conclusion
Light microscopy images of sodium laurate–lauric acid
stabilized o/w emulsions suggested that in addition to emul-
sified droplets these emulsion system contain surfactant
aggregates of vesicle type, whose size and number density
largely depend on sodium laurate–lauric acid concentra-
tion ratio. The vesicle population density increases with
sodium laurate concentration, being maximum at sodium
laurate concentration that corresponds to 0.8 M NaOH
equivalent.
Compared to other NaOH concentrations, at this NaOH
concentration sodium laurate–lauric acid stabilized emul-
sions displayed enhanced rheology properties as identified
by higher degree of shear thinning, higherη at any shear rate,
highest σ Y , higher dynamic moduli, and lowest phase angle.
This implies that in addition to emulsified droplets rheology
property of sodium laurate–lauric acid stabilized emulsions
depend highly on vesicles and their number density.
Acknowledgements
Authors extend their sincere thank to Ministry of Sci-
ence, Technology and Environment of Malaysia for kindly
providing financial assistance for this work under Grant No
09-02-03-9010-SR 0004/04.
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