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Langmuir 2008, 24, 7658-7662

Formation and Characterization of Microemulsions Containing
Polymeric Silicone
Suraj Chandra Sharma,† Koji Tsuchiya,† Kenichi Sakai,† Hideki Sakai,†,‡ Masahiko Abe,†,‡Shigeyuki Komura,§ Kazutami Sakamoto,† and Reiji Miyahara*,|Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo UniVersity of
Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, Institute of Colloid and Interface Science, Tokyo
UniVersity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan, Department of Chemistry,
Graduate School of Science and Engineering, Tokyo Metropolitan UniVersity, Tokyo 192-0397, Japan, andShiseido Research Center, 2-2-1 Hayabuchi, Tsuzuki-ku, Yokohama 224-8558, Japan
ReceiVed April 16, 2008. ReVised Manuscript ReceiVed June 2, 2008
We have prepared microemulsions consisting of water/[40 wt % polyoxyethylene (20 mol) glycerin isostearate(abbreviated as POE-GIS) + 60 wt % random copolymer of polyoxyethylene (POE, 38 mol)/polyoxypropylene (POP,
10 mol) pentaerythritol tetramethyl ether {abbreviated as PEPTME (38/10)}]/[polyoxyethylene (POE, 19 mol)/polyoxypropylene (POP, 19 mol) polydimethylsiloxane copolymer (abbreviated as POE/POP-PDMS)] and water/[40wt % POE-GIS + 60 wt % PEPTME (38/10)]/[95 wt % POE/POP-PDMS + 5 wt % oleic acid (abbreviated as OA)]
systems and characterized them with optical observation, rheometry, and freeze-fracture transmission electron
microscopy (FF-TEM) images. Bicontinuous and droplet-type O/W (oil-in-water) microemulsions are formed dependingon the volume fraction of water. The bicontinuous structure observed in the oil-rich region, upon successive dilution
with water, is transformed into a droplet-type microemulsion without phase separation.The prepared droplet-type
microemulsion containing polymeric silicone and random copolymer PEPTME (38/10) as a cosurfactant in the waterrich region has potential applications in cosmetics.

1. Introduction
Microemulsions are thermodynamically stable, isotropic,
microstructured solutions of oil and water stabilized by aninterfacial film of a suitable surfactant, usually in combination
with a cosurfactant.1–4 They can have characteristic properties
such as ultralow interfacial tension, large interfacial area, andthe ability to solubilize both aqueous and oil-soluble compounds.
The microemulsions can be of the droplet type, either with isolated
water pools dispersed in a continuous oil phase (water-in-oil
microemulsions, W/O) that usually occur in systems with highoil content or with isolated oil pools dispersed in a continuous
water phase (oil-in-water microemulsions, O/W), typicallyoccurring in water-rich region. Nondroplet-type microemulsions
feature continuous oil and water phases intertwined in dynamic
extended networks and are called bicontinuous microemulsions.
With progress in the study of microemulsions and theirincreased technical applications, knowledge of their microstructures has become more and more important. The uniquestability, solubilization, and optical clarity of microemulsions
make them very attractive for many industrial applications,for example, in the pharmaceutical fields as drug delivery
systems,5,6 in cosmetics as skin and body care products,7–9 in* Corresponding author. E-mail: reiji.miyahara@to.shiseido.co.jp.
†
Department of Pure and Applied Chemistry, Tokyo University of Science.
‡
Institute of Colloid and Interface Science, Tokyo University of Science.
§
Tokyo Metropolitan University.
|
Shiseido Research Center.

(1) Danielsson, I.; Lindman, B. Colloids Surf. 1981, 3, 391.
(2) Kunieda, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1983, 56, 980.
(3) Kunieda, H. J. Colloid Interface Sci. 1987, 116, 224.
(4) Abe, M.; Yamazaki, T.; Ogino, K.; Kim, M. J. Langmuir 1992, 8, 833.
(5) Araya, H.; Tomota, M.; Hayashi, M. Int. J. Pharm. 2005, 305, 61.
(6) Kim, C.-K.; Park, J.-S. Am. J. Drug DeV. 2004, 2, 113.
(7) Tokuoka, Y.; Uchiyama, H.; Abe, M.; Christian, S. D. Langmuir 1995, 11,725.
(8) Watanabe, K.; Noda, A.; Masuda, M.; Nakamura, K. J. Oleo Sci. 2004,
53, 537.

nanoparticles synthesis,10–12 and so on. Earlier papers mainly
described the formation of microemulsions with hydrocarbon
oils and alcohols with medium-length chains as cosurfactants.
However, because of their low surface tension and good
spreading capacity on the substrates that results in the ability
to protect skin and hair, silicone-based oils are receiving muchattention in the cosmetic field.13 The structure and dynamic
features of this oil differ from those of hydrocarbon oil in a
number of fundamental respects, giving rise to important
physical properties. The large Si-O-Si bond angle andabsence of substituents on the bridging oxygen atom permitsenhanced chain mobility, and the high methyl group density
imparts highly hydrophobic character.14 Generally, because
polymeric silicone chains spread, it is thought that the
preparation of O/W droplet-type microemulsions of polydimethylsiloxane compounds is a difficult task. Although
Aramaki et al. prepared a mixed micelle composed ofhydrophilic poly(oxyethylene)-polydimethylsiloxane and poly(oxyethylene)dodecylether,15,16 to our knowledge, there is noreport of the formation of droplet-type microemulsions ofhydrophobic polymeric silicone having a very high molecularweight.
(9) Garti, N.; Shevanchman, M.; Shani, A. J. Am. Oil Chem. Soc. 2004, 81,873.
(10) Malheiro, A. R.; Varanda, L. C.; Perez, J.; Villullas, H. M. Langmuir
2007 23, 11015.
(11) Debuigne, F.; Cuisenaire, J.; Jeunieau, L.; Masereel, B.; Nagy, J. B.
J. Pharm. Belg. 2000, 55, 59.
(12) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137.
(13) Blakely, J. M. The Benefits of Silicones in Facial and Body Cleansing
Products; Dow Corning Publication: Brussels, 1994.
(14) Steytler, D. C.; Dowding, P. J.; Robinson, B. H.; Hague, J. D.; Rennie,
J. H. S.; Leng, C. A.; Eastoe, J.; Heenan, R. K. Langmuir 1998, 14, 3517.
(15) Aramaki, K.; Olsson, U. J. Colloid Interface Sci. 2006, 300, 354.
(16) Aramaki, K.; Olsson, U. J. Colloid Interface Sci. 2007, 312, 52.

10.1021/la801205q CCC: $40.75  2008 American Chemical Society
Published on Web 07/04/2008

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Scheme 1. Schematic Molecular Structures of POE-GIS (a),PEPTME (38/10) (b), and POE/POP-PDMS (c)2.3. Rheometry. Samples for viscosity measurements were
homogenized and kept in a water bath at a specified temperature forat least 24 h to ensure equilibration before performing measurements.
The viscosity measurements were performed in a stress-controlled
rheometer, AR-G2 (TA Instruments), using cone-plate geometries(diameters of 60 mm for the low-viscosity sample and 40 mm forthe high-viscosity sample, each with a cone angle of 1°) with theplate temperature controlled by a Peltier unit. A sample cover provided
with the instrument was used to minimize the change in sample
composition by evaporation during the measurement.
2.4. FF-TEM. TEM observations of the sample were performed
using the freeze-replica method. The sample was rapidly frozen inliquid propane using a cryopreparation system (LEICA EM CPC,
LEICA microsystems), and the frozen sample was transferred intoa freeze-replica preparing apparatus (FR-7000A, Hitachi Science
Systems, Ltd.) and fractured with a glass knife at -120 °C. A replica
film was prepared by evaporating platinum-carbon at 45° and thencarbon at 90° on the fractured sample. The prepared replica film waswashed several times with acetone and distilled water after being
taken out of the freeze-replica preparing apparatus and transferred
onto a 300 mesh copper grid. The replica thus prepared was observedwith a transmission electron microscope (H-7650, Hitachi Science
Systems, Ltd.) at an accelerating voltage of 120 kV.

3. Results and Discussion

Miyahara et al. prepared a highly concentrated O/W droplettype microemulsion by using a random copolymer of polyoxyethylene/polyoxypropylene dimethylether (EPDME) as a cosurfactant.17,18 In this letter, we report a microemulsion that canoccur in a system containing water, polyoxyethylene (20 mol)glycerin isostearate, a random copolymer of polyoxyethylene(POE, 38 mol)/polyoxypropylene (POP, 10 mol) pentaerythritol
tetramethyl ether, which is an analog of EPDME, and apolyoxyethylene (POE, 19 mol)/polyoxypropylene (POP, 19 mol)polydimethylsiloxane copolymer with or without oleic acid.
Microemulsion characterization has been performed by phasebehavior, rheometry, and FF-TEM.

2. Experimental Section
2.1. Materials. A random copolymer of polyoxyethylene (POE,
38 mol)/polyoxypropylene (POP, 10 mol) pentaerythritol tetramethyl
ether, PEPTME (38/10), was kindly supplied by Nichiyu, Japan.
The number-average molecular weight of PEPTME (38/10) is 2300.
Polyoxyethylene (20 mol) glycerin isostearate, POE-GIS, andpolyoxyethylene (POE, 19 mol)/polyoxypropylene (POP, 19 mol)polydimethylsiloxane copolymer (POE/POP-PDMS) were purchasedfrom Nikko Chemical Co., Japan, and Dow Corning Toray Co. Ltd.,
Japan, respectively. The number-average molecular weight of POE/
POP-PDMS is 60 000. Oleic acid (OA) was a product of Nacalai
Tesque, Japan. All chemicals were used as received. Deionized(Millipore-filtered) water was used to prepare the samples. Schematic
molecular structures of POE-GIS, PEPTME (38/10), and POE/POPPDMS are shown in Scheme 1.
2.2. Phase Diagrams. Samples for phase diagram construction
were prepared by weighing the required amounts of reagents into
screw-capped test tubes and mixing using a vortex mixture. The
samples were kept in a water bath at 25 °C for equilibration. The
phase boundaries were established by the observation of the turbidityto-transparency transition and vice versa. The dotted lines weredrawn where the exact boundaries were not clear. Finally, phases
were identified by visual observation (through crossed polarizers).
(17) Miyahara, R.; Watanabe, K.; Ohmori, T.; Nakama, Y. J. Oleo Sci. 2006,
55, 403.
(18) Miyahara, R.; Watanabe, K.; Ohmori, T.; Nakama, Y. J. Oleo Sci. 2006,
55, 473.

3.1. Phase Behavior. The isothermal pseudoternary phase
diagrams constructed for the water/[40 wt % POE-GIS + 60 wt% PEPTME (38/10)]/(POE/POP-PDMS) system and the water/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/(95 wt %POE/POP-PDMS + 5 wt % OA) system at 25 °C are shown in
Figure 1a,b, respectively. The chemical structure of PEPTME(38/10) is similar to that of the polyoxyethylene chain inpolyoxyethylene nonionic surfactants. PEPTME (38/10) may
thus be considered to be a hydrophilic cosurfactant likepolyoxyethylene (POE)/polyoxypropylene (POP) dimethyl ether(EPDME)17,18 when the concentrations of both PEPTME (38/10) and polyoxyethylene nonionic surfactants are high and may
possibly be distributed in the polyoxyethylene chain of thepolyoxyethylene surfactant. Oleic acid is added to the oily phase(POE/POP-PDMS) to raise the curvature of an interfacial filmby an electrostatic repulsion of carboxylate groups. The mixture[40 wt % POE-GIS + 60 wt % PEPTME (38/10)] is insoluble
in both POE/POP-PDMS and (95 wt % POE/POP-PDMS + 5
wt % OA). For all [40 wt % POE-GIS + 60 wt % PEPTME(38/10)] to (POE/POP-PDMS) or (95 wt % POE/POP-PDMS +5 wt % OA) weight ratios R, the turbidity was noted for low
water content. As the amount of water increases, the systemreaches a single-phase microemulsion, indicated as I in the
diagrams. It is seen that the isostropic monophasic microemulsion
domains extend over a wide range in the diagrams. The area
occupied by the microemulsion region shrinks and extends sharply
toward the oil corner when POE/POP-PDMS is mixed with oleic
acid in a 19:1 (w/w) ratio (Figure 1b). Within the microemulsion
domain, the region with high oil content has a high viscosity.
The solutions in this region are isotropic at rest but are birefringent
when applying a shear, such as a sudden jerk. The microstructureof a microemulsion in the high oil content region is thought tobe bicontinuous and similar to that previously reported.19 Besides
a single-phase microemulsion domain, liquid crystal was also
observed. Additionally, a very small domain of a highly viscous
isotropic gel was encountered in an oleic acid-incorporated
system. Unfortunately, we could not identify the liquid crystal(LC) by both small-angle X-ray scattering and polarizing
microscopy. However, by judging the position of formation and(19) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc.,
Faraday Trans. 1990, 86, 2253.

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Figure 1. Pseudoternary phase diagrams of the water/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/(POE/POP-PDMS) system (a) and thewater/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/(95 wt % POE/POP-PDMS + 5 wt % OA) system (b) at 25 °C.

the physical appearance of the LC, it can be concluded that the
LC may be lamellar phase.
3.2. Rheometry. In microemulsions, viscosity is a macroscopic property that mainly depends upon the type and shape of
aggregates present, the concentration, and the interaction between
dispersed particles and hence can be used to obtain important
information concerning microstructural changes. For example,
the change in zero-shear viscosity provides valuable information
regarding the transition of rodlike to globular micelles.20 The
steady shear rate versus viscosity curves for the water/[40 wt %POE-GIS + 60 wt % PEPTME (38/10)]/(POE/POP-PDMS)
system with R ) 1 at different mixing fractions of water, expressedin weight fraction (W) at 25 °C, are shown in Figure 2. At aminimum water content of W ≈ 0.285, the Newtonian behavior
is limited to lower shear rate, and viscosity decreases graduallywith increasing shear rate. The decrease in viscosity withincreasing shear rate corresponds to shear-thinning behavior.
The dispersions are organized at a lower shear rate and suffer
fragmentation with increasing shear rate. With increasing watercontent up to W ≈ 0.305, the shear-thinning behavior is stillobserved, but the viscosity at the lower shear rate decreases.
However, with further increases in water content to W ≈ 0.391,(20) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388.

Figure 2. Steady shear rate versus viscosity curves for the water/[40 wt% POE-GIS + 60 wt % PEPTME (38/10)]/(POE/POP-PDMS) systemalong R ) 1 at various weight fractions of water, W: 0.285 ([), 0.305(O), 0.341 (2), and 0.391(]) at 25 °C.

the viscosity is nearly constant over a wide range of shear ratestudied, but a slight shear-thinning effect is still noticed, whichindicates that the system is becoming less structured. Ultimately,phase separation occurs on further dilution of the system withwater. Previous work on microemulsion rheology has indicatedthat bicontinuous microemulsions may be weakly shear thinning,Letters

Langmuir, Vol. 24, No. 15, 2008 7661

Figure 3. Variation of zero-shear viscosity as a function of the weightfraction of water for the water/[40 wt % POE-GIS + 60 wt % PEPTME(38/10)]/(POE/POP-PDMS) system with different values of R: 1 (9),1.86(b), and 4 (2) at 25 °C.

Figure 5. Freeze-fracture electron micrographs of microemulsions: (a)W ) 0.255, R ) 1.5; and (b) W ) 0.447, R ) 4 for the water/[40 wt% POE-GIS + 60 wt % PEPTME (38/10)]/POE/POP-PDMS system.
(c) W ) 0.291, R ) 2.33 and (d) W ) 0.448, R ) 4 for the water/[40wt % POE-GIS + 60 wt % PEPTME (38/10)]/(95 wt % POE/POPPDMS + 5 wt % OA) system.

Figure 4. Steady shear rate versus viscosity curves for the water/[40 wt% POE-GIS + 60 wt % PEPTME (38/10)]/(95 wt % POE/POP-PDMS+5 wt % OA) system along R ) 1 at various weight fraction of water,
W: 0.133 ([), 0.15 (O), 0.161 (2), 0.179 (]), and 0.195 (0) at 25 °C.

particularly if their low-shear viscosities exceed about 10 cP,21
which suggests that the microemulsion formed along R ) 1 isassumed to have a bicontinuous structure. We observed a very
weak shear-thinning effect at high shear rate for low water content
and Newtonian behavior (data not shown) over the wide rangeof shear rate studied up to 1000 s-1 for high water content alongR ) 1.86 whereas only Newtonian behavior was noticed at various
water contents over the entire range of shear rate studied alongR ) 4. The effect of water on the microstructures of the
microemulsion described in Figure 2 can be seen more clearlyin Figure 3 where zero-shear viscosity is plotted as a functionof the weight fraction of water at different values of R. The
values of zero-shear viscosity have been determined by extrapolating the viscosity to zero shear rate. With increasing watercontent, the zero-shear viscosity of the microemulsion decreases
continuously. The trend of decreasing zero-shear viscosity withincreasing water content suggests a structural change in the
microemulsion. In the bicontinuous region, the interconnectedchannels increase molecular interactions and hence increase inzero-shear viscosity for microemulsions with low water content.22
With continuous dilution, the bicontinuous structure graduallydisappears, and transition into the oil-in-water microemulsion(21) Chen, C.-M.; War, G. G. J. Phys. Chem. 1992, 96, 9492.
(22) Gatri, N.; Yaghmur, A.; Leser, M. E.; Clement, V.; Watzke, H. J. J. Agric.
Food Chem. 2001, 49, 2552.

occurs, which is reflected in a sharp decrease in the zero-shear
viscosity. The zero-shear viscosity continues to decrease as thenanodroplets decrease in number and size and become more andmore diluted with water. The progressive dilution decreases theinterdroplet interactions. We obtained a similar trend of decreasing
viscosity (data not shown) with increasing water content alongR ) 1.86 and 4 for the oleic acid-incorporated system. However,a stronger shear-thinning effect is observed along R ) 1, and theviscosity does not change with aqueous dilution as shown in
Figure 4. The incorporation of OA may raise the curvature of
an interface film, extend the microemulsion domain, and makethe droplet size small. Therefore, strong entanglement is expected,
leading to a viscosity increase. Further validation of the results
can be obtained using the FF-TEM technique.
3.3. FF-TEM Observation. Freeze-fracture electron microscopy (FF-TEM) is one of the most advanced and powerful
techniques used to characterize the size and shape of aggregatesin microemulsions. The micrographs obtained by the FF-TEM
technique are presented in Figure 5. A typical bicontinuous
microemulsion texture can be observed in Figure 5a for the water/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/(POE/POPPDMS) system at W ) 0.255 along R ) 1.5. Upon closer
inspection, the image of the microstructure of the microemulsion
shows a nearly zero mean curvature in the surfactant monolayers
between the oil and water channels. The bicontinuous structurechanges into oil-in-water droplets with increasing water content
in the microemulsion. Droplet-type structures having diametersof less than 100 nm can been seen in Figure 5b for the same
system at W ) 0.447 along R ) 4. Similar micrographs ofbicontinuous and droplet-type microstructures having diametersof around 100 nm for microemulsions (Figure 5c,d) can be seen
for the water/[40 wt % POE-GIS + 60 wt % PEPTME (38/10)]/(95 wt % POE/POP-PDMS + 5 wt % OA) system in low
and high water content regions, respectively. These micrographs
again reveal the transition from bicontinuous structure to oilin-water droplets with aqueous phase dilution. This result is inagreement with the viscosity measurement.

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4. Conclusions

and viscosity measurement. We believe that the preparedmicroemulsions using polymeric silicone oil would be
potentially useful in the cosmetics field as a result of excellent
spreading capacity on the substrate and the ability to protect
skin and hair.

In this letter, we report for the first time the formation ofa droplet-type microemulsion containing polymeric silicone
by using a random copolymer of polyoxyethylene (POE, 38mol)/polyoxypropylene (POP, 10 mol) pentaerythritol tetramethyl ether (PEPTME (38/10)) as a cosurfactant. The phase
diagrams of the pseudoternary water/[40 wt % GWIS-120 +60 wt % PEPTME (38/10)]/(POE/POP-PDMS) and water/[40wt % GWIS-120 + 60 wt % PEPTME (38/10)] /(95 wt %POE/POP-PDMS + 5 wt % OA) systems present a wide onephase microemulsion domain in which the structural transition
from bicontinuous structure to oil-in-water droplets occurs
continuously on dilution with the aqueous phase without phaseseparation. The transition from bicontinuous structure to oilin-water droplets is easily detected by FF-TEM micrographs

Acknowledgment. We are grateful to Professor Kenji Aramaki
of Yokohama National University for the use of the AR-G2
rheometer in his laboratory. S.C.S. is thankful to the Japan Society
for the Promotion of Science (JSPS) for financial support.
Supporting Information Available: Plot of zero-shear viscosity
versus weight fraction of water at R ) 1, 1.86, and 4 for the oleic
acid-incorporated system. This material is available free of charge via
the Internet at http://pubs.acs.org.
LA801205Q