lamellar explicação

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9
Journal of Oleo Science
Copyright ©2013 by Japan Oil Chemists’ Society
J. Oleo Sci. 62, (1) 9-16 (2013)
The viscosity stability of O/W emulsion containing 
α-gel through an ionic-complex system
Makoto Uyama＊ , Kaori Ikuta, Takashi Teshigawara, Kei Watanabe and Reiji Miyahara
Shiseido Research Center (Shin-Yokohama), Shiseido Co. Ltd., 2-2-1 Hayabuchi, Tsuzuki-ku, Yokohama, Kanagawa 224-8558, Japan
1 INTRODUCTION
Various active ingredients, which often are electrolytes, 
are used in cosmetic products for moisturizing and whiten-
ing the skin. When these electrolytes are added to an oil in 
water（O/W）emulsion system, stability generally deterio-
rates remarkably because of the increasing hydrophobici-
ties of the surfactants in the system, particularly in cases 
of nonionic surfactants, and their cloud points decrease. To 
reconcile the stabilities of O/W emulsion systems contain-
ing electrolyte-type active ingredients, a system containing 
an α-crystalline phase（α-gel）, which consisted of water, 
higher alcohols, and the anionic surfactant, sodium N-stea-
royl-N-methyl taurate（SMT, Fig. 1（a））, was developed by 
Konno and Watanabe1, 2）.
An α-gel（α-type hydrate crystal）is one of the hydrated 
crystalline state of surfactants and lipids that appears as a 
highly viscous gel3, 4）. The long-periodic structure of an 
α-gel has a repeating bilayer structure, with its packed
（sub-cell）structure of hydrophobic groups forming a hex-
agonal sub-lattice and its hydrophilic groups holding a 
large amount of water.
α-Gel is often confused with a lamella liquid crystalline 
phase, because both have a repeated bilayer structure. 
However, unlike an α-gel, the hydrophobic groups of a 
＊Correspondence to: Makoto Uyama, Shiseido Research Center (Shin-Yokohama), Shiseido Co. Ltd., 2-2-1 Hayabuchi, Tsuzuki-ku, 
Yokohama, Kanagawa 224-8558, Japan
E-mail: makoto.uyama@to.shiseido.co.jp
Accepted July 14, 2012 (recieved for review May 22, 2012)
Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online
http://www.jstage.jst.go.jp/browse/jos/　　http://mc.manusriptcentral.com/jjocs
lamella liquid crystalline phase are in a molten（liquid）state 
with no specific sub-lattice structure. The two forms can 
be easily distinguished by X-ray scattering measurements. 
In the wide-angle X-ray scattering pattern of an α-gel, a 
sharp peak is observed at 15 nm－1（Bragg angle of 21°）, 
which corresponds to the diffraction of the sub-lattice. In 
contrast, no peak is evident for the sub-lattice structure of 
lamellar liquid crystalline phases.
Abstract: Although many active ingredients are used in cosmetic products for moisturizing and whitening 
the skin, they are often electrolytes, and the stabilities of oil in water (O/W) type emulsion formulae 
containing electrolytes are generally difficult to control. To solve this problem, formulae containing an 
α-crystalline phase (α-gel) consisting of water, higher alcohols, and anionic surfactants such as sodium 
N-stearoyl-N-methyl-taurate (SMT) have been developed. However, in spite of their excellent salt tolerance, 
these formulae have poor viscosity stability under non-electrolyte conditions, and the viscosity decreases 
over time. To obtain adequate viscosity stability, the required electrolyte concentration is approximately 
1wt%, which is somewhat high for cosmetic applications. To replace the salts, distearyl dimethyl ammonium 
chloride (DSAC), a cationic surfactant, with an opposite electric charge to SMT, was used in O/W emulsion 
formulae, resulting in improved viscosity stability at a lower concentration than that of salts. The 
stabilization mechanism with DSAC was found to be different from that of salts.
Key words: α-Crystalline phase, α-gel, Emulsion, Sodium N-stearoyl-N-methyltaurate, Distearyl dimethyl ammonium 
chloride, DSC, SAXS, cryo-SEM
Fig. 1　 Chemical structures of SMT (a), DSAC (b), and 
C22TAC (c).
M. Uyama, K. Ikuta, T. Teshigawara et al.
J. Oleo Sci. 62, (1) 9-16 (2013)
10
Konno has reported that an O/W emulsion showing ex-
cellent long-term viscosity stability can be obtained with 
SMT as an anionic surfactant in the α-gel structure of the 
emulsion, even with electrolytes, unlike other anionic sur-
factants1）. However, approximately 1wt％ NaCl was needed 
to obtain long-term viscosity stability, because under low- 
or non-electrolyte conditions, remarkable viscosity deterio-
ration was observed over time. Although the α-gel system 
with SMT showed outstanding salt tolerance, its excellent 
viscosity stability could be lost in non- or low-concentration 
electrolyte conditions over time. Moreover, the use of 1 
wt％ NaCl is marginally intolerable for preparing cosmetic 
formulae.
This study investigated materials that can be used with 
SMT at lower concentrations to enhance the long-term vis-
cosity stability of O/W emulsions.
In particular, this study focused on the following two 
points. First, materials should be cationic, to weaken the 
electrostatic repulsion of SMT and neutralize its electric 
charge. Second, they should be hydrophobic surfactants, 
because unlike water-soluble NaCl, they are integrated into 
the α-gel structure of the system at concentrations lower 
than that required by NaCl.
Distearyl dimethyl ammonium chloride（DSAC, Fig. 1
（b））5－9）, a cationic surfactant, is a strongly hydrophobic 
double-alkyl-chain-type surfactant, which is insoluble in 
water. DSAC is thought to be easily incorporated into the 
α-gel structure because of its straight alkyl chains, and to 
interact electrostatically with SMT in the α-gel. Using cryo-
genic scanning electron microscopy（cryo-SEM）, Yamagu-
chi et al. studied changes in the structure of assembly in 
the ternary 1-hexadecanol, octadecyl trimethyl ammonium 
chloride, and water system, altering the molar ratio of 
1-hexadecanol to octadecyl trimethyl ammonium chloride, 
as well as the preparation temperature10, 11）. Nagahara et al. 
observed the cationic assembly in the system composed of 
behenyl trimethyl ammonium chloride, behenyl alcohol, 
and water, with or without amido propyl betaine laurate 
also using cryo-SEM12－14）.
This study shows（1）the improvement of the long-term 
viscosity stability of an O/W emulsion system containing an 
α-gel, and（2）a possible mechanism through an SMT/DSAC 
ionic complex.
2 EXPERIMENTAL
2.1 Materials
SMT and behenyl alcohol were purchased from Nikko 
Chemicals（Tokyo, Japan）. DSAC, behenyl trimethyl am-
monium chloride（C22TAC, Fig. 1（c））, and stearyl alcohol, 
were purchased from Sanyo Chemical Industries（Kyoto, 
Japan）, Toho Chemical Industry（Tokyo, Japan）, NOF cor-
poration（Tokyo, Japan）, respectively. Other materials used 
in this study were cosmetic grade and used as received.
2.2 Methods
2.2.1 Preparations of O/W emulsions
Formulae of O/W emulsion sample used in this study are 
provided in Table 1. O/W emulsion samples were prepared 
Table 1　Basic formulae of O/W emulsions (wt%).
Ingredients
NaCl
added*
DSAC
added**
C22TAC
added***
Water phase
　Sodium N-stearoyl-N-methyltaurate (SMT)
　Humectant
　Xanthan gum
　Citrate buffer
　Sodium chloride
　Disodium EDTA
　Preservative
　Water
Oil phase
　Stearyl alcohol
　Behenyl alcohol
　Hydorocarbon Oil
　Silicone Oil
　Distearyl Dimethylammonium chloride (DSAC) 
　Behenyl trimethyl ammonium chloride (C22TAC) 
0.2
13
0.1
0.1
0 or 1
0.1
proper wt.
up to 100
0.3
1.1
2
3
－
－
0.2
13
0.1
0.1
－
0.1
proper wt.
up to 100
0.3
1.1
2
3
0－0.1
－
0.2
13
0.1
0.1
－
0.1
proper wt.
up to 100
0.3
1.1
2
3
－
0－0.1
*Formula added with NaCl.
**Formula added with distearyl dimethyl ammonium chloride (DSAC).
***Formula added with behenyl trimethyl ammonium chloride (C22TAC).
The viscosity stability of O/W emulsion containing α -gel through an ionic-complex system
J. Oleo Sci. 62, (1) 9-16 (2013)
11
by adding the oil phase heated at 85℃ to the water phase 
heated at 80℃ and subsequent emulsification using a ho-
mogenizer（T.K. ROBOMIX®）（PRIMIX, Osaka, Japan）. 
Emulsion particle sizes were observed withthe optical mi-
croscope and made below 5 μm. The samples were then 
cooled at 40℃ in an ice/water bath.
2.2.2 Viscosity measurements
The prepared sample was stored at room temperature, 
and viscosity was measured by a rotary viscosity meter（B-
type, VDA2）（Shibaura systems, Tokyo, Japan）at 1, 15, and 
30 days after preparation. For the measurements, rotor No. 
3 was used at 12 rpm at 30℃ for 1 min.
2.2.3 Small-angle X-ray scattering（SAXS）measurements
SAXS measurements were performed with a SAXSess 
camera（Anton Paar, Graz, Austria）. Cu-Kα（λ : 1.542 Å）ra-
diation was used. The scattering intensity of each sample 
was measured at 25℃ for 10 min with a cyclone imaging 
plate detection system（Perkin-Elmer, Waltham, MA, USA）
and analyzed by SAXSQuant software（Anton Paar）. Emul-
sion samples for SAXS measurements were ultracentri-
fuged at 65,000 rpm for 7 hours at 25℃, using a himac 
CP100wx（Hitachi Koki, Tokyo, Japan）before SAXS mea-
surements.
2.2.4 Differential scanning calorimetry（DSC）
DSC measurements were performed using a DSC Q2000
（TA Instruments, Delaware, USA）, equipped with a refrig-
erated cooling system（RCS 90）（TA Instruments）, over a 
temperature range of 25 to 85℃ at a heating rate of 1℃/
min. Tzero pans and Tzero hermetic lids（TA Instruments）
were used, and the sample weight was adjusted to approxi-
mately 10 mg.
2.2.5 Electron microscopy and image analysis 
Cryo-SEM can fix and observe the actual state of a col-
loidal material, such as a cream product, while preserving 
its α-gel structure.
Under conventional SEM operating conditions, α-gel 
structural information would be lost due to possible gel 
structure coalescence at ambient temperature. To obtain 
the cryo-SEM images of prepared samples, a small amount 
of sample was put onto a specimen holder and plunged into 
liquid nitrogen. The frozen specimen on the holder was 
transferred to a microscope cold stage at －70～－90℃. 
Then, the surface of specimen was fractured on the stage 
and observed with a scanning electron microscope（Hitachi 
S-3000N）（Hitachi High-Technologies, Tokyo, Japan）with 
an acceleration potential of 5 kV.
3 RESULTS AND DISCUSSION
3.1 Effect of DSAC on the viscosity of O/W emulsions
Base formulae used in this study are shown in Table 1. 
Although the concentrations and the types of humectants 
varied to some extent, the variations were confirmed to 
have no influence on the viscosity stability of the sample 
emulsion in a separate experiment（data not shown）. Simi-
larly, the influence of humectants on the viscosity stability 
was also hardly taken into consideration in a previous 
report1）. Xanthan gum（0.1 wt％）, a thickening agent, was 
also confirmed to have practically no effect on the viscosity
（data not shown）. Citrate buffer（citric acid and sodium 
citrate）was used to control the emulsion pH between 5.8 
and 6.2.
Cationic surfactants are known to attenuate the electric 
charge of SMT. We expected that a small amount of DSAC 
would affect the viscosity of O/W emulsion samples more 
strongly than salts, because DSAC has two straight alkyl 
chains that are easily incorporated in the α-gel, and can ef-
ficiently shield the electric charge of SMT in the α-gel.
Figure 2 shows the viscosity changes upon addition of 
DSAC. The viscosities of samples containing 0, 0.02, and 
0.05 wt% DSAC decreased from 3560, 3920 and 4080 mPa・
s at 1 day to 2950, 3210, and 3950 mPa・s at 30 days post-
preparation, respectively. Decreases in the viscosities of 
samples containing 0, 0.02, and 0.05 wt% DSAC for 30 days 
were found to be 610, 710, and 130 mPa・s, respectively. 
On the other hand, the viscosities of samples containing 
Fig. 2　 Time-course of the viscosity of O/W emulsion 
samples containing DSAC or NaCl. Plots repre-
sent viscosity of samples containing (●) 0 wt% 
DSAC; (▲), 0.02 wt%; (■), 0.05 wt%; (◆), 
0.07 wt%; and (×), 0.10 wt% DSAC. The stars 
(＊) represent the viscosity of the sample con-
taining 1.0 wt% NaCl.
M. Uyama, K. Ikuta, T. Teshigawara et al.
J. Oleo Sci. 62, (1) 9-16 (2013)
12
0.07 and 0.1 wt% DSAC increased from 3960 and 2530 
mPa・s at 1 day to 4130 and 2900 mPa・s at 30 days post-
preparation, respectively, indicating that DSAC was able 
not only to suppress spontaneous viscosity decreases but 
also to increase the viscosity of the samples in a dose-de-
pendent manner. The minimum concentration of DSAC for 
obtaining the suppression effect was 0.05 wt％, which was 
quite lower than those of salts. This result indicated that 
the systems containing 0.07 and 0.10 wt% DSAC were in 
condensed states, because van der Waals attractions 
became more dominant than electrostatic repulsions. 
Therefore, the optimal concentration of DSAC in this 
system was 0.05 wt％.
On the other hand, the viscosities of sample containing 
1.0 wt% NaCl were found to be 4050 mPa・s at 1 day and 
3770 mPa・s at 30 days after preparation, and yielding a de-
crease in the sample viscosities of 280 mPa・s, over 30 days. 
This result indicated that 0.05 wt% DSAC suppressd the 
spontaneous viscosity decrease more than 1.0 wt% NaCl.
3.2 Effect of C22TAC on the viscosity of O/W emulsions
C22TAC has a single straight alkyl chain and is water 
soluble（＞50℃）, in contrast to DSAC. Yet, C22TAC is also 
a cationic surfactant that was expected to form an ionic 
complex with SMT.
Figure 3 shows the results of the viscosity change upon 
addition of C22TAC. Spontaneous viscosity decrease was 
slightly suppressed only in the sample containing 0.100 
wt% C22TAC, indicating that a larger amount of C22TAC 
than DSAC was required to suppress the spontaneous de-
crease in viscosity. It may be possible to include C22TAC in 
an α-gel that consists of water, higher alcohols, and SMT 
but probably a large portion of C22TAC exists as micelles 
in the system12, 15）.
3.3 Mechanism for the improvement in the viscosity sta-
bility of O/W emulsions by addition of DSAC
3.3.1 Phase behavior and interlamellar distance
SAXS measurements were performed on O/W emulsion 
samples with or without DSAC（Fig. 4）. The samples were 
the same samples used in the viscosity measurement ex-
periments in Fig. 2. Samples were ultracentrifuged before 
SAXS measurement to condense the α-gel, because the 
α-gel concentration was low and resulted in insufficiently 
Fig. 4　 SAXS patterns of ultracentrifuged O/W emul-
sion samples containing DSAC having pos-
sible interlamellar distances of structures and a 
sample of hydrate crystal of higher alcohol. The 
samples were the same samples used in the vis-
cosity measurement experiments (Fig. 2). Pat-
tern (a) shows a sample containing 0 wt%; (b), 
0.02 wt%; (c), 0.05 wt%; (d), 0.07 wt%; and (e), 
0.10 wt% DSAC. Pattern (f) shows a sample of 
the hydrate crystal of the higher alcohol. Down-
ward arrow indicates the peak originating from 
the hydrated crystal of the higher alcohol.
Fig. 3　 Time-course of the viscosity of O/W emulsion 
samples containing C22TAC. Plots represent 
viscosity of samples containing (●) 0 wt% 
C22TAC; (▲), 0.045 wt%; (■), 0.063 wt%; 
(◆), 0.085 wt%; and (×), 0.100 wt% C22TAC.
The viscosity stability of O/W emulsion containing α -gel through an ionic-complex system
J. Oleo Sci. 62, (1) 9-16 (2013)
13
intense signals. A diffraction pattern derived from a lamel-
lar structure was observed for all samples. Generally, the 
addition of electrolytes, such as NaCl, to this sort of emul-
sion system reportedly allows the hydrophilic groups of 
surfactants to dehydrate, resulting in the shrinkage of the 
interlamellar distance of the lamellar structures1, 16, 17）. Al-
though DSAC is not a salt, shrinkage of the interlamellar 
distance was observed in a dose-dependent manner. The 
interlamellar distances of samples containing 0, 0.02, 0.05, 
0.07, and 0.10 wt% DSAC were 23, 22, 21, 21, and 19 nm, 
respectively. Generally, oppositely charged surfactants 
form an ion complex, allowing the shrinkage of the interla-
mellar distance18, 19）. However, when DSAC was added to 
the system at more than0.10 wt％, a peak different from 
that of a lamellar structure was observed（the arrow in Fig. 
4（e）, q＝1.07 nm－1）. The peak was speculated to originate 
from a hydrate crystal of a higher alcohol（Figs. 4（e） and
（f））. However, a peak shape change was not observed 
clearly in the wide-angle upon the addition of DSAC, prob-
ably because the intensity was weak（data not shown）.
3.3.2 DSC Measurements
To confirm the inclusion of DSAC in the α-gel, DSC mea-
surements of the samples were performed20－23）. The DSC 
data for the samples used in Fig. 2 are shown in Fig. 5. 
Single endothermic peaks were observed in the samples 
containing 0－0.05 wt% DSAC. Upon increasing DSAC 
from 0.05 to 0.10 wt%, the peaks moved toward lower tem-
peratures, and other peaks appeared in the lower tempera-
ture region（～59℃ and ～61℃）. An endothermic peak 
that appeared in this region was probably derived from the 
melting of the higher alcohol hydrate crystal that was ex-
cluded from the α-gel, rather than the melting of the DSAC 
hydrate crystal, because（1）the possible electrostatic and 
hydrophobic interactions between SMT and DSAC were 
very strong, impeding the independent precipitation of 
DSAC, and（2）the melting point of the hydrate crystal
（Krafft point）of DSAC is known to be 37℃5）.
To prepare an O/W emulsion formula having excellent 
viscosity stability and a single α-gel structure, the ratio of 
SMT and DSAC was very important, and our findings indi-
cated an optimal ratio of DSAC to SMT of approximately 4 
to 1（25 wt％）. This ratio does not change dramatically, 
even when converted into a molar ratio, because the mo-
lecular weight of SMT and DSAC are 427.62 and 585.60, 
respectively.
3.3.3 Cryo-SEM observations
Figure 6 shows the cryo-SEM photographs of the control 
O/W emulsion sample（Fig. 6（A））, a sample containing 1.0 
wt％ NaCl（Fig. 6（B））, a sample containing 0.05 wt％ 
DSAC（Fig. 6（C））, and a sample containing 0.10 wt％ 
C22TAC（Fig. 6（D））10－14）, 24, 25）. All samples were observed 
seven days after preparation. To prevent samples from be-
coming difficult to freeze or easy to melt, humectants were 
removed from the basic formula. Konno reported that after 
the addition of 0.5 wt％ NaCl to an O/W emulsion sample, 
no layer structures were observed1）. In this study, stratified 
vesicle structures having 3 or 4 layers around emulsion 
particles were observed in the control（Fig. 6（A））. 
Fig. 5　 DSC thermograms of O/W emulsion samples 
containing DSAC. The samples were the same 
samples used in the viscosity measurement ex-
periments (Fig. 2). Line (a) shows the thermo-
gram of the sample containing 0wt%; (b), 0.02 
wt%; (c), 0.05 wt%; (d), 0.07 wt%; and (e), 0.10 
wt% DSAC.
Fig. 6　�Cryo-SEM photographs of O/W emulsion 
samples containing SMT. The magnifications of 
photographs were 500, and cryo-SEM observa-
tion was performed at –70 – –90 ℃. Photograph 
(A) is the control sample; (B), sample contain-
ing 1.0 wt% NaCl; (C), sample containing 0.05 
wt% DSAC; (D), sample containing 0.10 wt% 
C22TAC. All samples were observed 7 days af-
ter the preparation. The arrows in photograph (C) 
and (D) indicate plate-like structures.
M. Uyama, K. Ikuta, T. Teshigawara et al.
J. Oleo Sci. 62, (1) 9-16 (2013)
14
However, the addition of 1.0 wt％ NaCl gave a slightly 
stratified vesicle structure（Fig. 6（B））. The interlamellar 
distance was compressed by the addition of NaCl, and the 
stratified vesicle structure disappeared in cryo-SEM. On 
the other hand, in the system containing DSAC, no strati-
fied vesicle structure was observed completely, but rather, 
a plate-like structure was observed（arrows in Fig. 6（C））. 
Yamagata et al. studied the effects of temperature on the 
development of the internal structure of the cetyltrimeth-
ylammonium chloride/cetyl alcohol/ water system13）. Using 
cryo-SEM, they discussed the system’s internal structure 
using the terms lamellar structure（plate-like lamellar 
structure in this report）and multilamellar vesicles. The 
cause of the improvement of the viscosity stability of the 
O/W emulsion sample upon addition of DSAC was thought 
to be due to this plate-like structure. In the case of 
C22TAC, a plate-like structure was also observed, as in the 
case of DSAC, but the structure was sparse and caves were 
notably large（Fig. 6（D））.
As illustrated in Fig. 7, electrolytes such as NaCl might 
be distributed according to the Boltzmann distribution 
from the surface of the α-gel to bulk water26－28）, conse-
quently, a high concentration of salt was necessary to sta-
bilize the viscosity of the system. On the other hand, DSAC 
might be included in the α-gel, interacting with SMT di-
rectly, and therefore, it was speculated that a smaller 
quantity of DSAC than that of salt would improve the vis-
cosity stability. Unlike salt, which attenuated electrostatic 
repulsion and reduced the interlamellar distance, DSAC 
was able to neutralize the charge of SMT directly in the 
α-gel, resulting in a change in the α-gel microstructure.
Figure 8 shows the schematic illustration of a possible 
mechanism explaining DSAC effect. After being added to 
an emulsion system, distearyl dimethyl ammonium（DSA）
ion was incorporated into the α-gel through the hydropho-
bic interaction of its alkyl chains with SMT, forming an ion 
complex with SMT through an electrostatic interaction. 
Then, entropy（ΔS）was increased by release of the sodium 
ion（the counter cation for SMT）from the surface of the 
α-gel to bulk water, resulting in a decrease in the Gibbs 
Free Energy（ΔG＝ΔH－TΔS）. Therefore, this process was 
speculated to be spontaneous and entropy-driven29－32）. Al-
though there was an entropic decrease upon incorporation 
of the DSA ion in the α-gel, a favorable change in Gibbs 
energy might be observed due the strong, unfavorable hy-
drophobic interaction of DSA ion with bulk water. In addi-
tion, the entropy of the chloride ion（the counter anion for 
DSA ion）increased when the DSA ion formed an ion 
complex with SMT. Therefore, being included in the α-gel, 
DSAC was speculated to be in a favorable energy balance 
state. Hydration and dehydration might also significantly 
affect the entropy（ΔS）of the system31）. The incorporation 
of DSAC in the α-gel allowed the small sodium counterion 
of SMT to be exchanged for a bulky DSA ion, and this re-
placement was thought to allow the microstructure of the 
α-gel to assume a plate-like lamellar structure instead of 
the curved structure formed in a stratified vesicle. This mi-
crostructure change was speculated to contribute to the 
improvement of the viscosity stability and the increase of 
the storage modulus value upon addition of DSAC. Because 
the friction between stratified vesicles produced viscosity 
without DSAC, the inter-vesicle contact areas were small, 
and the structure collapsed easily when shear stress was 
applied to the system. On the other hand, with DSAC, the 
structure became difficult to collapse even under a high 
shear stress, because the friction between plate-like struc-
tures was strong due to the large contact areas.
Fig. 7　�Schematic illustration of the micro layer struc-
ture of an α-gel in O/W emulsion samples. 
Illustration shows differences in the working 
mechanism between NaCl (A) and DSAC (B) 
for contributing to the formation of the structure.
Fig. 8　�Schematic illustration of microstructural change 
of an α-gel in an O/W emulsion induced by 
DSAC.
The viscosity stability of O/W emulsion containing α -gel through an ionic-complex system
J. Oleo Sci. 62, (1) 9-16 (2013)
15
4 Conclusions
In this report, DSAC was used instead of salts to solve 
the problem of the viscosity stability of O/W emulsions 
containing SMT. A small amount of DSAC improved the 
viscosity stability and more effectively than salts. The 
mechanism of the stabilization by DSAC was speculated to 
differ from that of salts.
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