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Colloids and Surfaces A: Physicochem. Eng. Aspects 236 (2004) 7–22 Vesicles in fatty acid salt–fatty acid stabilized o/w emulsion—emulsion structure and rheology Boris B. Niraula 1 , Tiong Ngiik Seng, Misni Misran  Department of Chemistry, F aculty 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 emulsier 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 aggregate s of vesicle type is imminent in these conditions, both the size and number density of which is found to be a function of emulsier concentration ratio. The concentration of emulsier (sodium laurate–lauric acid) is being assessed in this work indirectly as a function of NaOH concentration, due to the problem associated with its quantication. 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 increa sed substantially . While vesicle population density decrease d past 0.8 M NaOH, their size increas ed with NaOH concentration from 0.8 to 1 M. These observations sugge sted 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  ˙ γ , 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( ˙ γ )  proles of sodium laurate–lauric acid stabilized o/w emulsion was not a linear function under the given sets of experimental conditions, at all conditions  η strongly depended on sodium laurate–lauric acid concentrations (concentration of NaOH in this case). What more is that at any given  ˙ γ , the η  was observe d 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 value s 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 concentrati on corresponding to 0.8 M NaOH is the opt imum conce ntr ati on at whi ch ma ximum numberof vesic lesare produced, andthat rhe ology proper ty of the se emuls ions depend not onl y on emulsion droplets but also on the vesicle number density and size. T est 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 conce ntration 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 arou nd 2 Hz, past whic h 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 ela stic property of these emulsions Corresponding author. Tel.:  +60-3-7967-4 079; fax:  +60-3-7967-4079.  E-mail addresses: [email protected] (M. Misran), borisnir@yaho o.com (B.B. Niraula). 1 Co-corresponding author. 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.101 6/j.colsurfa.20 04.01.002
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Page 1: Vesicles in Fatty Acid Salt–Fatty Acid Stabilized O-w

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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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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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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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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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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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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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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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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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