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Colloids and Surfaces

A: Physicochemical and Engineering Aspects 180 (2001) 4153

Surfactant-free O/W emulsion formation of oleic acid andits esters with ultrasonic dispersion

Keiji Kamogawa a,b, Hidetaka Akatsuka c, Mitsufumi Matsumoto c,Shoko Yokoyama d, Toshio Sakai c, Hideki Sakai b,c, Masahiko Abe b,c,*

a Ele. & Sec. Ed. Bureau, The Ministry of Education Science, Sports and Culture, Science, Kasumigaseki, Chiyoda,

Tokyo 100-0013, Japanb Institute of Colloid and Interface Science, Science Uni6ersity of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

c Faculty of Science and Technology, Science Uni6ersity of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japand

Kyoritsu College of Pharmacy, 1-5-30 Shibakoen, Minato, Tokyo 105-8512, Japan

Received 14 January 2000; accepted 5 September 2000

Abstract

Dispersibility and stabilizing factors for surfactant-free O/W emulsion were investigated with oleic acid (OA) an

its esters, focusing on the effects of their weak polarity, molecular length and branched chain structure, in compariso

to normal hydrocarbons. The droplet size distributions obtained by the dynamic light scattering method appeared t

be discrete but almost singly peaked except for OA. For OA monoesters, the droplet growth was found to b

continuous and retarded as the ester chain length increased, in contrast to the discrete, fast growth in OA dispersion

In the case of glycerol trioleate (GTO), a branched ester, aqueous dispersions of extremely fine droplets could bprepared and the number distribution of droplet diameters showed a single peak in the nanometer range. This hig

dispersibility remained unchanged for about a year after preparation to give the dispersions a good stability. Chang

of the observed x potential, Fourier transform-infra red (FT-IR) spectrum, fluorescence spectrum of probes indicate

that a particular carboxyl acid group network is formed in the droplet sphere to make it more stable than expecte

while the interior of oil droplets is hydrophobic. The x potential change, in particular, was found to be high

correlated with these of the carboxyl CO stretching frequency and the reciprocal droplet diameter. The dropl

stability evaluated by increase in the diameter revealed a biphasic growth consisting of fast and slow modes. The fa

growth at early stages (in hours) observed for OA and methyl oleate (MO) was found to proceed by the Ostwa

ripening mechanism through rate analysis. On the other hand, the slow growth at later stages (in days) found for th

other esters showed a semi-logarithmic dependence on the oil viscosity. This seems to be caused by an Arrhenius-typ

activation factor in the stepwise flocculation/coalescence rate. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Surfactant-free emulsion; Oleic acid and its esters; Growth mechanism

www.elsevier.nl/locate/colsur

* Corresponding author. Tel.: +81-471-248650; fax: +81-471-248650.

E-mail address: [email protected] (M. Abe).

0927-7757/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved.

P I I : S 0 9 2 7 - 7 7 5 7 ( 0 0 ) 0 0 7 5 8 - 5


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K. Kamogawa et al. /Colloids and Surfaces A: Physicochem. Eng. Aspects 180 (2001) 415342

1. Introduction

When an oil-in-water type (O/W) emulsion isprepared using two hardly miscible components,e.g. water and a hydrocarbon, a suitable surfac-tant is usually used to enhance emulsification andmake the emulsion formed stable. O/W type

emulsions are widely employed in various prod-ucts such as medicines like highly nutritiousagents and lipid microspheres [1,2], cosmetics [3],and foods [4]. While ultrafine particles of inor-ganic and organic substances are usually preparedin the presence of surfactant [5], the removal ofadsorbed surfactant molecules from the surface ofultrafine particles is difficult. Correspondingly, in-troduction of surfactant-free emulsions is desiredsince the droplets are composed of oil moleculesalone. Transient dispersions are familiar to us, aswell known by oil-and-vinegar sauce, solvent ex-

traction, etc. Droplets in such emulsion are ex-pected to have a clear surface and the interiorwith the highly hydrophobic nature of oil proper.

In view of these, we have prepared droplets inwater of aromatic hydrocarbons like benzene [6],straight chain hydrocarbons like n-hexane andn-hexadecane [7], and branched chain hydrocar-bons like squalane by ultrasonic irradiation andexamined the dispersibility and the stability of oildroplets formed. We then found that dispersionsof such oil droplets are regularly characterized bydiscrete size distributions with a certain degree of

stability [8]. The behavior may be classified into(1) benzene type giving small-sized, unstabledroplets, (2) n-hexane type giving middle-sized,stable droplets and (3) n-hexadecane type givingsmall-sized, stable droplets. In addition, mixing ofn-hexadecane with benzene was found to suppressdroplet growth remarkably, despite the fact thatthe interfacial tension (a) increased on mixing.This would mean that a decrease in a is notnecessarily coupled with the dispersion stability ofsurfactant-free emulsions. Among the other prop-

erties, the branched structure possessed bysqualane and surface charging supposed for n-hexadecane seem to be promising factors [9]. It isthus worth studying the dispersibility and thestability of such oils that have a branched struc-ture as well as a polar group instead of inducedcharges.

As to the kinds of oils to be used, fatty acid

and their glycerol esters have weakly pol

groups, in addition to their long hydrophob

chains and branched structure. They are too muc

hydrophobic to be regarded as surfactant as rep

resented by a very low HLB value of 1 an

solubility in water. Such a low HLB value mean

that they may act only as weak emulsifiers suiable for W/O dispersion. In a dynamic sens

however, we can expect a rather high stability

water of their droplets. In addition, oleic acid

present as a component in complex lipids, main

phospholipids of biomembranes, and thus it pla

an important role in various tissues and organ

while diversely changing its chemical and ster

structure and molecularly assembling mode [10

In the present study, we examine the dispersibilit

and growth characteristics of methyl oleate, dec

oleate, oleyl oleate, glycerol trioleate, and ole

acid as the control to find the conditions nece

sary for preparing surfactant-free emulsion of

long term dispersion stability.

2. Experimental

2.1. Materials

Methyl oleate (MO, NOF Corp., 99.7% purity

decyl oleate (DO, NOF Corp., 98.5% purity

oleyl oleate (OO, NOF Corp., 91.6% purity), glyerol trioleate (GTO, NOF Corp., 97.0% purity

oleic acid (OA, NOF Corp., 99.7% purity) we

used as oils as supplied. The residual impuriti

were estimated to be unsaturated fatty acids wit

different chain lengths or their esters. Diole

esters might be present at about 3.0% of th

glycerol esters, which is sufficiently low not t

activate the interface. Distilled and deionized w

ter for injection (Ohtsuka Pharmaceutical Co

was employed without further purification. Pyren

(Wako) and 8-anilino-1-naphthalene sulfonic ac(ANS, Wako) were used as fluorescence probes

2.2. Preparation of oil droplet dispersions

Preparation of oil droplet dispersions was pe

formed in the following way. A mixed liquid o


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water and oil at a given mixing ratio (50 ml)

was pretreated by stirring for 1 min in a

vortex mixer and it was then ultrasonically irra-

diated for 8 min on an ultrasonic cleaner (Bran-

sonic 220, 125W, Smith-Kline Co.). The

exposure time of 58 min was necessary for the

size distribution in the nmmm region to appear

steadily.

2.3. Measurement of oil droplet size distribution

Measurement of oil droplet size distribution

was performed periodically by a dynamic light

scattering method (homodyne method) using an

NICOMP 380 ZLS (Particle Sizing System Co.).

The oil concentration was 0.5 mM, the light

source was a diode pump solid state laser

(DPSS laser) with a wavelength of 535 nm, and

the measuring angle was 90.

2.4. Interfacial tension and x potential

measurement

The surface properties of oil droplets were

evaluated with interfacial tension and x poten-

tial. The interfacial tension was obtained with a

platinum Wilhelmy plate in Model CBVP-Z in-

terfacial tension meter (Kyowa Interface Science

Co. Ltd.), where samples were kept at 30C be-

fore the measurements. Data were stored whenthe value became invariant for more than 10

min.

x Potential was measured on oil droplets in

dispersions prepared by the method described in

2.2 with the laser Doppler method (heterodyne

method). This method gives the electrophoretic

mobility and x potential of fine particles moving

in an electric field based on the Doppler shift, a

frequency shift of laser light scattered by the

particles. The oil concentration and the light

source were the same as those in the dropletsize distribution measurements. The laser output

power was 10 mW, the scattering angle was

19.8, and the electric field strength was 5.0 V

cm1. Sensitivity test was done for the system

of n-hexadecane droplets in water, which was

found to be highly stable. It gave a value of

70 mV, in agreement with the literature da

for docosane particles [11].

2.5. FT-IR spectrum measurement

Fourier transform-infra red (FT-IR) spectru

measurement was conducted on oil droplet di

persion using the ATR method at an oil concentration of 10 mM to further evaluate the surfac

properties of the droplets. The apparatus use

was a spectrophotometer JIR-5300 (JEOP Co

equipped with an ATR cell (Model TNL-13

Tunnel Cell) (JEOP Co.). The cell is made

zinc selenide (ZnSe), an IR-transparent materi

(crystal).

2.6. Fluorescence spectrum measurement

Fluorescence spectrum measurement wmade using two fluorescence probes, pyrene an

ANS, to investigate the environment of o

droplets. The apparatus employed was

fluorospectrophotometer RF-5000. (Shimadzu).

2.6.1. Measurement using pyrene

Fluorescence spectrum measurement usin

pyrene as the probe was carried out on a dispe

sion of oil droplets containing 1 mM pyren

prepared according to the procedures de

cribed in Section 2.2. The oil concentration the dispersion was varied from 0.1 to 3

mM. The ratio, I1/I3, for pyrene monomer w

calculated from the fluorescence spectrum o

tained with freshly prepared dispersion (Ex 33

nm).

2.6.2. Measurement using ANS

In spectral measurements using ANS as th

probe, the fluorescence spectrum of ANS di

solved in oil at 0.1 mM was compared with tha

in droplets of oil containing 0.1 mM ANS dipersed at 5 mM in water (Ex 337 nm)

Preparation of oil droplet dispersion and me

surements of droplet size distribution, x poten

tial, and fluorescence spectrum were performe

at 30C. FT-IR spectra were measured at 2292C.


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3. Results and discussion

3.1. Initial dispersibility and growth

Fig. 1 shows a typical size distribution of oleic

acid (OA) droplets dispersed in water.

OA droplets were found to have a discrete size

distribution immediately after preparation of dis-persion with two size classes, S and M, the former

Fig. 2. (A) Stepwise coalescence model and (B) critical flocc

lation/coalescence model.

Fig. 1. Particle size distribution of OA droplets in water in

number% (solid curve) and in volume% (broken curve).

Droplets were prepared at 0.5 mM concentration. The arrows

S, M and L denote small (S), middle (M) and large (L) classes,

respectively.

appearing at 70100 nm, and the latter at 300

700 nm. These peaks were distinguishable in e

ther number% or volume%, with a highsensitivity for small or large droplets, respectivel

As time elapsed, the droplets grew in such a wa

that the height of peak changes while the pea

positions for S and M classes remain almo

unchanged and L class appeared at 1000 250

nm 3 days later. Extremely large droplets abov

30005000 nm observed 10 and 60 min aft

presentation, seem to be partially coalesced drop

which disappear through creaming and will not b

discussed further. We also noticed that the S-cla

size of OA droplets is much smaller than 200 nmthe size found for oil droplets by mechanic

dispersion with 0.1 mol l1 of SDS [12]. Th

growth of OA droplets is much faster than that o

n-hexadecane (C16H30) droplets [7], which have

comparable size to OA with respect to carbo

number. As a whole, the behavior of OA drople

much resembled that found for benzene drople

[6], over that for n-hexadecane droplets, even th

acid has a carboxylic acid group and a long alk

chain. This resemblance suggests the presence of

critical flocculation/coalescence scheme in O

droplet growth, as previously assumed for be

zene droplets [6,8]. Basically, small oil droplets a

expected to grow through several processes, in

cluding coalescence and the Ostwald ripening.

As shown in Fig. 2(A), the former is unde

stood to proceed by stepwise collision proces


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resulting in gradual increase in the droplet size as

has been found elsewhere. When droplets are

sufficiently resistive to collision or collision-in-

duced coalescence, however, a certain degree of

flocculation would be required to open an effec-

tive fusion path to coalescence as shown in Fig.

2(B). This critical flocculation/coalescence scheme

can be related with discrete distribution andgrowth of droplet size, which are characteristic for

the surfactant-free hydrophobic oil droplets [8].

Methyl oleate droplets (MO), by contrast, had

almost a single peak of S class in their size

distribution (Fig. 3(a)) and grew more rapidly.

Thus, the droplets showed a growth process in

which their size distribution changed continu-

ously, keeping the bimodal distribution in S an

M classes. Both of S class and M class droplets o

decyl oleate (DO) grew in size from about 90 an

400 nm to about 130 and 700 nm, respectivel

Such continuous growth is rarely observed fo

surfactant-free emulsions though it is quite popu

lar in the surfactant-assisted emulsions [13]. Suc

continuous size drift suggests a significant contrbution to droplet growth of stepwise flocculatio

and coalescence referred in Fig. 2. Correspon

ingly, this hybrid profile is somewhat differe

from the characteristics of normal hydrocarbo

droplets.

In Fig. 3(b and c) are shown the droplet siz

distributions for DO (b) and OO (c), both havin

a longer alcoholic alkyl chain than methyl oleat

Freshly prepared droplets of these two esters als

showed a single peak in S class in their siz

distributions as in the case of MO droplets. The

growth also followed a similar path, continuous

changing their size distribution from the one in

class to the other in M class. Droplets in L siz

class were negligibly few in the number% distribu

tion. While it took only 23 days for M

droplets to grow two to three times in size, 2

weeks were needed for DO and OO droplets t

become two to three times larger in size. In othe

words, the rate of droplet growth slows down a

the ester chain length increases.

Fig. 3(d) shows the size distribution for GT

droplets, which demonstrates that the S class sizdroplets observed immediately after dispersio

preparation remain unchanged for at least a yea

This is the longest period during which oil dropl

dispersion keeps its initial stability in our surfa

tant-free emulsion study. Such a long term stabi

ity would be due partly to the fact that GTO is

branched ester with long hydrophobic group

whose branched structure extends to the who

molecule and partly to a high viscosity of th

ester as mentioned later.

The peak diameter for S class droplets wplotted against time for esters in Fig. 4. Since th

growth continued monotonously for a week, th

size of MO droplets reached 500 nm, which corr

sponds to M class size. From the curves, th

growth speed was roughly estimated for S cla

droplets as 60, 10, 4 , and 0.5 nm per day for MO

Fig. 3. Particle size distribution of oil droplets in water in

number % for (a) MO, (b) DO, (c) OO, and (d) GTO.

Droplets were prepared at 0.5 mM concentration. The arrows

S, M and L denote small (S), middle (M) and large classes (L),

respectively.


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Fig. 4. Changes in droplet diameter as a function of time. ()

MO, () DO, (2) OO and () GTO. Data were taken fromFig. 3 as the average in number % distribution.

mPas) it failed to give sufficiently small drople

in water, suggesting no direct correlation betwee

droplet dispersibility and viscosity for the aci

This would mean that the initial dispersibility

determined by the specific nature of carboxyl

acid group at the interface or mechanically r

stricted by the high viscosity and a given cavita

tion field in the ultrasonic bath.

3.2. Surface properties

In view of this, we attempted to see if an

relation exists between the dynamic stability of o

droplets and their surface properties in the growt

process. Fig. 6 shows the x potential of o

droplets dispersed in water as a function of tim

for the five oils, indicating that oil droplets have

x potential of 33 to 30 mV soon after dispe

sion preparation.Note here that oil droplets are negative

charged though no electrically charged third sub

stance is present in the dispersion. The initi

levels were deep enough to allow better dispersio

stability for the droplets. As to why the x poten

tial of oil droplets is negative, the following even

that are possible to happen on the droplet surfac

would be responsible.

DO, OO, and GTO esters, respectively. No sig-

nificant difference in interfacial tension (a) was

found among the oils used in the present work.

The avalues for OA/water and GTO/water were

in agreement with the literature values of 15.6 and

23.2 mNm1, respectively [14]. The values were

high enough (1330 mNm1) to prevent sponta-

neous emulsification. On the other hand, the vis-

cosity increased with increasing alkyl chain lengthof alcohol group in an increasing order of MO

(4.88 mPas) BDO (12.7 mPas) BOO (24.0

mPas) BGTO (56.0 mPas). Hence, there is a

close relationship between droplet dispersibility,

i.e. the smallness in droplet size, and viscosity for

the four esters studied. Droplet size is thought to

be determined by interfacial tension (a) and vis-

cosity (p) of the fluid under a shear stress [15,16],

occasionally by the ratio of a/p [17]. This rela-

tionship was confirmed recently for surfactant-

free droplets of n-alkanes in water [8]. The initial

droplet size in Figs. 1 and 3 was then plotted

against a/p in Fig. 5, as is proposed for surfac-

tant-added emulsions.

The initial size was independent of or slightly

decreasing with a/p for the oils under the present

condition. Although OA has a high viscosity (23.0

Fig. 5. Dependence of initial peak size of droplets on a/p

pure oil. () OA, () MO, () DO, (2) OO and () GT

The sizes were taken from Figs. 1 and 3.


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Fig. 6. Changes in x potential of oil droplets dispersed inwater as a function of time. () OA, () MO, () DO, (2)

OO, () GTO.

appreciable growth of OA and MO. On the othe

hand, the drift of x potential value might b

related with the diameter increase. Fig. 7(a) show

the plot of x potential change against diamet

has no linear correlation but hyperbolic one. Th

plot was then made against the reciprocal diam

ter to yield a linear relation as shown in Fig. 7(b

This linear relationship does not seem to arisfrom conservation of the total charges on th

decreasing total droplet surface area. Since th

droplet growth at a constant total volume shou

decrease the total surface area by 1/diameter, th

charge will be concentrated on the droplet surfac

to raise the negative x potential in proportion t

diameter. A plausible source for changing x p

tential is variable charges induced on the surfac

by the intrinsic droplet properties. The linear rel

tionship may then suggest a geometrical expan

sion or cracking at the droplet surface, whic

would magnify adsorption probability of the r

sponsible ions onto the surface. This change in

potential with time would then be better related t

the structural changes of oil droplets.

3.3. FT-IR spectra

FT-IR spectrum measurements were conducte

on aqueous dispersions of OA, MO and GTO i

order to examine the surface of oil droplets

detail with emphasis on the state of carboxyl

acid groups (Fig. 8).The spectra are shown in the region from 165

to 1850 cm1 since peak shifts were observed i

the absorption range corresponding to the C

stretching vibration. When the spectra for oi

alone (i) were compared with those temporal one

for their droplets in water (iiiv), the peak at 174

cm1 for glycerol trioleate (a) was found to shi

by 5 cm1 to the high wave number side for i

droplets immediately after dispersed in water (i

and no further change was observed in the spe

trum even after a week (iv), while an upward shiof absorption peak was also observed for ole

acid (b) and methyl oleate (c) from 1708 (i) t

1712 cm1 (ii) and from 1740 (i) to 1744 cm

(ii), respectively, but the shifts gradually dimin

ished as time elapsed, making the absorptio

peaks return to the initial positions. Since th

1. Carboxylic acid groups that have a permanent

dipole and are weakly polar (C+O) orient

on the surface of oil droplets to induce a

negative charge around them.

2. Hydroxyl ions generated in the dissociation of

water are stabilized on the droplet surface due

to the difference in dielectric constant between

the oil droplet and aqueous phase [18], and

3. The permanent and induced dipoles are or-

dered following the structuring of hydrophobi-

cally hydrated water molecules.

Comparison of x potential changes with time

among the oils revealed that GTO droplets with a

good stability keep a highly negative x potential

of33 mV over a week while the x potentials of

OA and MO droplets with a poor stability rise

from a similar value with time up to 10 and

18 mV for the former and the latter, respec-

tively. These initial x potential values in Fig. 6 are

so similar to each other that they can not be

correlated with the difference in the growth speed

evaluated from the initial slope in Fig. 4, where

OA exhibits a pronounced slope of Dd/Dt, much

larger than that for MO. The initial x potential

value is then no useful for predicting the fast or


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absorption due to the CO stretching vibration of

oleic acid generally appears at 1708 cm1 for its

dimers and it shifts to a higher wave number of

1760 cm1 for its monomers [19], the observed

upward shift of the absorption peak would be

caused by breakdown of CO/CO interaction

between neighboring molecules when the oil is

dispersed as droplets in water. Hence, a particularorder, which differs from the structure of

oil itself, is presumably formed in droplets for the

oleic acid esters. Invariance of the upward shift

suggests that formation of the particular order

should be a stabilizing factor and the order is

protected by the presence of alcoholic groups. The

return of the peak to the initial position observed

for OA and MO would be brought about by

diminution of the particular structure throug

molecular diffusion, droplet coalescence, wat

penetration and carboxylic acid group migratio

to the surface, etc. toward a reestablishment

the molecular arrangement in the bulk of o

Then, an attempt was made to correlate th

CO shifts directly to the x potential changes Fig. 9.

A correlating curve was obtained for each

OA and MO. Thus, the two curves similarly ha

a negative slope and the upper one tended t

converge to 17 mV. The similarity suggests th

the drift ofx potential is mainly controlled by th

state of the carboxylic acid groups.

Fig. 7. Correlations of x potential (a) with droplet diameter () fitted to a hyperbola and (b) with reciproca1 diameter () fitt

to a line at corresponding time. Data were taken from Figs. 4 and 6.


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Fig. 8. IR spectra in the carboxyl CO stretching region for (a) GTO, (b) OA and (c) MO. (i) Oil phase, (ii) oil-droplets just aft

preparation, (iii) after 3 days and (iv) after 7 days.

3.4. Fluorescence probe

The particular structuring related with the car-

boxylic acid group may also be responsible for the

hydrophobicity or fluidity of the droplet. The

hydrophobic environment of oil droplets was then

examined using pyrene, a fluorescence probe (Fig.

10).

The ratio of the intensity of band 1 to that of

band 3, I1/I3, both bands being characteristic of

the vibrational structure of fluorescence for

monomeric pyrene molecules, is an index for the

hydrophobicity of the microenvironment sur-

rounding pyrene molecules [20,21]. Pyrene fluores-

cence intensity measurements on droplets of OA,

MO, and GTO yielded a constant value (0.75

0.9) of the ratio for the three oils above an

extremely low oil concentration of 0.3 mmol l1.

The values for dispersions of cyclohexane and

benzene, both are nonpolar oils, are 0.8 and 1.0,

respectively [22], and 1.0 1.2 for SDS micelles

[21,2325]. The values obtained for the oils in this

work locate between those for SDS and dodecane

(0.6) environment [25]. A slight degree of hydra-

tion might be suggested for the dispersed oil

droplets in view of the I1/I3 values of 0.49, 0.53

and 0.52 for bulk OA, MO and GTO, respec-

tively. The values for droplets obtained above are

indicative of a rather hydrophobic environment o

these droplets, much more hydrophobic than th

of micellar systems. An interesting fact is that O

has the lowest value, implying a higher hydropho

bicity of OA droplet. This might result in the fa

break down of the particular structure.

Fig. 9. Correlation between the carboxyl CO frequency and

potential at various times. Data were taken from Figs. 6 an

8 for OA (), MO () and GTO ().


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Fig. 10. Temporal change of I1/I3 value in the monomer

fluorescence of pyrene dissolved in oil droplets at 1.0 mM

concentration. OA (), MO () and GTO (). Dotted lines

indicate the values for pyrene in water and those in SDSmicelles.

higher value. Hence, we can use the wavelength

the fluorescence intensity maximum and the quan

tum yield of ANS to estimate the polarity of th

environment in which the dye is present. Mor

over, it is known that even though the dye

dissolved in a polar solvent its fluorescence spe

trum makes a blue shift and its quantum yie

rises if the viscosity of the solvent is high [26ANS fluorescence measurements revealed that th

intensity peak of the dye makes a blue shift

about 60 nm when the dye is in OA, MO, an

GTO from that in water. Similar measuremen

were done for droplets of the three oils containin

0.1 mM ANS. A small blue shift of 1 nm wa

found for OA and MO dispersions and a shift o

7 nm and a slight increase in intensity we

observed for GTO dispersion. The noticeable shi

and the larger peak height for GTO dispersio

indicate a higher adsorpability of the probe to th

droplet surface.

3.5. Growth rate analysis

Based on the experimental findings mentione

so far, the growth of oil droplets is quite likely t

be explainable by the diminution of the particula

structure coupled with the state of carboxylic ac

groups. An examination was then conducted t

know how the particular structure diminishes, o

the basis of the mechanisms mentioned prevously. In general, two growth mechanisms a

possible when an emulsion breaks and separat

While fluorescence probe technique is quite sen-

sitive, the sensitivity is site-selective, and hence, it

is regulated by the dissolution characteristic of the

probe. We, therefore, used ANS as another probe,

focusing on its behavior on the droplet surface.

Fig. 11 shows the fluorescence spectra of ANS in

water, oil and dispersion. When the polarity of

solvent decreases the wavelength at the maximum

fluorescence intensity of ANS generally makes a

blue shift by about 60 nm at its maximum and the

quantum yield rises up to an about 200 times

Fig. 11. Fluorescence emission spectra of 0.1 mM ANS at 30C. (a) ANS in oil and water phase, (b) ANS in oil droplets. Wate

(), OA (------), MO ( ) and GTO ( ).


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into two liquids physicochemically (1) flocculation

and coalescence of droplets and (2) molecular

diffusion. The latter is called the Ostwald ripening

[27,28] and caused by the increasing solubility of a

substance with decreasing droplet size. Thus,

molecules of the substance of smaller particles

dissolve in the dispersion medium and are ab-

sorbed into large particles through diffusion,thereby causing a change in particle size distribu-

tion. It can be confirmed with drifts in size in the

opposite directions for smaller and larger droplets

as shown in Fig. 1. For OA dispersion, while M

class droplets grew in size from 300 to 400 nm

and L class droplets at 3.5 mm disappeared, S

class droplets shrank from 100 to 50 nm within 1

h. On the other hand, for MO dispersion, S and

M class droplets grew from 100 to 150 nm and

from 300 to 400 nm, respectively, within 3 h. The

Ostwald ripening is then highly probable in the

growth of OA droplets. This scheme was applied

as below for the fast mode of growth while coales-

cence model was tested for the slow mode of

growth.

3.6. Fast growth mode

Lifshitz et al. [29,30] put forward a theory

(LSW theory) for the Ostwald ripening of fine

particles as expressed by the following equation:

=dr3

dt =8DC

kM

9z2RT (1)

where D is the diffusion coefficient of the dis-

persed phase in water, C

the solubility of the

dispersed phase in water, g the interfacial tension

between water and the dispersed phase, M and z

are the molecular weight and density of the dis-

persed phase, respectively, and r is the mean size

of all droplets in the dispersed phase. Kabalnov

[31] and Taylor [32,33] have succeeded in plausi-

bly explaining the Ostwald ripening of liquid

droplets based on this equation. If the dropletgrowth in the present study obeys the equation,

the cube of the mean droplet size should increase

linearly with time. In order to check this point,

the cube of mean radius in volume% representa-

tion was plotted against time for poorly stable

OA and MO droplets and the most stable GTO

Fig. 12. Plot of the cube of droplet radius as a function

time. The averaged radius were obtained as those provided

the volume% representation. () OA, () MO and () GT

droplets (Fig. 12). The plots were made in thperiod of 3 h within which droplets of OA anMO were found to grow to give those in M sizclass. The value of r3 linearly increased rathsteeply with time for OA droplets while the increase in r3 with time for MO droplets wslightly steep and the value of for the ester wamerely one fortieth of that for OA. Almost nchange with time in r3 was found for GT

droplets. This represents a difference in the rate othe Ostwald ripening between OA and the esterThe difference can be partially ascribable to thlower solubility (C

) of the latter. C

was est

mated from the total organic carbon (TOC) concentration and turbidity changes (Fig. 13).

TOC concentration (a) estimated for saturateoleic acid solution was 3 ppm, i.e. 1105 ml1 when the amount introduced to water w3105 mol l1. In addition, the turbidity of thdispersion (b) started to increase at the introduce

amount of 2105

mol l1

, supporting thsolubility of 1105 mol l1. While sucestimation was limited for GTO, the turbidistarted to rise at a lower concentration than Oto suggest a smaller value of the solubility. Thripening rate according to Eq. (1) was almonegligible for GTO as seen in Fig. 12.


12/13


Fig. 13. Solubility analysis of OA in water and GTO in water. (a) TOC plot vs. the provided concentration for OA () and (

turbidity vs. the provided concentration for OA () and GTO (). The arrows from the bottom abscissa indicate an interpolate

position as the solubility point of OA.

3.6.1. Slow growth mode

As mentioned previously, the slow growth rate

can be characterized with Dd/Dt as in Fig. 3. In

view of the time needed for the slow rate to be

observed, this slow mode seems to be related with

the flocculation/coalescence mechanism. The slope

ofDd/Dt obtained from Fig. 4 was plotted against

viscosity in Fig. 14 in a semi-logarithmic manner.

A good correlation was seen between them, only

with slight deviation for MO. This type of correla-

tion seems to suggest an Arrhenius-type depen-

dence of flocculation/coalescence rate on

viscosity, such as the case of deformation energy

necessary for droplets at the transition state to

coalesce. The slight deviation in Fig. 14 should

have resulted from the Ostwald ripening analyzed

in Fig. 12, which also be operative even in M

class. The extrapolation for MO indicates that 13

nm per day or 21% of the apparent growth speed

would be due to flocculation/coalescence and the

remaining Ostwald ripening.

The growth process for droplets of OA and the

esters are regulated by the fundamental rate fac-

tors including their molecular solubility and vis-

cosity, etc. Electrostatic repulsion due to x

potential would effectively prevent flocculation/

coalescence as long as the interaction occurs for

similarly sized droplets. However, the repulsio

seems not preventive enough against molecul

processes such as molecular deposition by th

Ostwald ripening and water penetration relatin

to the higher hydrophobicity of OA droplet. I

addition, even though the repulsion should pr

Fig. 14. Dependence of Dd/Dt for the slow growth mode o

the viscosity of the oil. Data were taken from the slope in Fi

4. () MO, () DO, (2) OO, () GTO.


13/13


vent collision-controlled coalescence, it is not oper-

ative enough in much slower flocculation/coales-

cence determined by the Arrhenius-type activation

factor, exp(DE*/RT). Nevertheless, x potential

seems to be a significant indicator for droplet

growth through its 1/d dependence, in which

droplets with varying x potentials are less stable.

4. Conclusions

From the results mentioned so far, OA and the

four esters were found to form surfactant-free

emulsion with different characteristics. OA can be

regarded as an oil giving benzene-like droplets that

are discretely sized and form unstable dispersion in

spite of its C18 chain length. On the contrary, the

dispersion stability of the four oleic acid esters

becomes higher in accordance with their alcoholicalkyl chain length and branched structure. In

particular, GTO droplets of good dispersibility and

stability could be prepared in the absence of

surfactant. This stability is ascribable to formation

of a particular structure around the carboxylic acid

groups. Although x potential value is not predictive

of droplet stability, it is a good indicator for droplet

growth providing a high correlation with reciprocal

diameter. We propose here that particular structure

formation as well as high oil viscosity are the

promising factors for oil droplets to remain stable

in such surfactant-free emulsion against floccula-tion/coalescence.

Acknowledgements

The authors are grateful to the Cosmetology

Research Foundation for its financial support in

this work.

References

[1] H. Sato, S. Ogoshi, S. Usui, Graphic Presentation of High

Calorie Transfusing Liquids, second ed., Igaku-shoin,

1982, p. 44.

[2] M.Y. Levy, W. Schutze, C. Fuhrer, S. Benita, J. Microen-

capsulations 11 (1994) 79.

[3] A.L.L. Hunting, Cosmetics Toiletries 101 (1986) 49 6

[4] M. Endo, H. Sagitani, J. Jpn. Oil Chem. Soc. 40 (1991) 13

[5] M. Abe, H. Nishino, K. Ogino, Sekiyu Gakkaishi 32 (198

151.

[6] K. Kamogawa, T. Sakai, N. Momozawa, M. Shimaza

M. Enomura, H. Sakai, M. Abe, J. Jpn. Oil Chem. Soc. 4

(1998) 159.

[7] K. Kamogawa, M. Matsumoto, T. Kobayashi, T. Sak

H. Sakai, M. Abe, Langmuir 15 (1999) 1913.

[8] K. Kamogawa, M. Abe, Surfactant-free emulsions, iEncyclopedia of Emulsion Technology, Marcel Dekker,

press.

[9] K. Kamogawa, M. Matsumoto, T. Sakai, H. Sakai, M

Abe, in preparation.

[10] M. Suzuki, Fragrance J. 23 (1995) 105.

[11] D.E. Dunstan, D.A. Saville, J. Chem. Soc. Faraday Tran

88 (1992) 2031.

[12] T. Yamamoto, H. Sakai, Y. Sakai, F. Harusawa, K

Kamogawa, M. Abe, Mater. Technol. 18 (2000) 53.

[13] C.M. Miller, J. Venkatesan, C.A. Silbi, E.D. Sudol, M

EL-Aasser, J. Colloid Interf. Sci. 162 (1994) 11.

[14] Y. Minegishi, T. Takeuchi, H. Arai, J. Jpn. Oil Chem. So

20 (1971) 160.

[15] E.S.R. Gopal, in: P. Sherman (Ed.), Emulsion SciencAcademic Press, London, 1969.

[16] P. Walstra, in: P. Bechap (Ed.), Encyclopedia of Emulsi

Technology, vol. 1, Marcel Dekker, New York, 1983.

[17] T. Horiuchi, Research and Development for Nov

Emulsifiers and New Emulsifier Technology, CMC, Toky

1998.

[18] R.S. Schechter, A. Garcia, J. Lachaise, J. Colloid Inte

Sci. 204 (1998) 398.

[19] T. Shimanouchi, Analyses of IR Spectra, Nankodo, 198

pp. 100103.

[20] K. Kalyanasundaram, J.K. Thomas, J. Am. Chem. Soc.

(1986) 288.

[21] R.J. Hunter, Foundations in Colloid Science, vol.

Clarendon Press, Oxford, 1986, p. 601.[22] T. Sakai, K. Kamogawa, N. Momozawa, H. Sakai, M

Abe, submitted.

[23] A. Ito, K. Kamogawa, H. Sakai, Y. Kondou, N. Yoshin

M. Abe, J. Jpn. Oil. Chem. Soc. 45 (1996) 479.

[24] R.G. Alargova, I.I. Kochijashky, R. Zana, Langmuir

(1998) 1575.

[25] S. Zhang, J.F. Rusring, J. Colloid Interf. Sci. 182 (199

558.

[26] Y. Saito, Y. Kondo, M. Abe, T. Sato, J. Jpn. Oil Chem

Soc. 11 (1994) 325.

[27] R. Buscall, S.S. Davis, D.C. Potts, Colloid Polym. Sci. 2

(1979) 636.

[28] T. Shimanouchi, Analyses of IR Spectra, Nankodo, 198

p. 263.[29] I.M. Lifshitz, V.V. Slezov, J. Phys. Chem. Solids 19 (196

35.

[30] C. Wagner, Z. Electrochem. 65 (1961) 581.

[31] A.S. Kabalnov, Langmuir 10 (1994) 680.

[32] P. Taylor, Colloids Surf. A Physicochem. Eng. Aspects

(1995) 175.

[33] P. Taylor, Adv. Colloid Interf. Sci. 75 (1998) 107.

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