Preparation of D-limonene Oil-in-Water Nanoemulsion from an Optimum FormulationJiang Yang1, 2＊ , Wei Jiang2, Baoshan Guan2, Xiaohui Qiu2 and Yongjun Lu2
1 Department of Petroleum Engineering, Xi’an Petroleum University, Xi’an, Shaanxi, China2 Fracturing & Acidizing Technical Center, RIPED-Lanfang, PetroChina, China
1 INTRODUCTIONAlthough the Brownian motion of small droplet prevents
sedimentation or creaming and provides kinetic stability, nanoemulsion at diluted range with droplet size in the range of 20-200 nm is a thermodynamic unstable system. In contrast, microemulsion at nanometer size is a thermo-dynamic stable system1）, and is formed at much higher sur-factant concentration. However, the use of high surfactant concentration limits its use in industries. Preparation of nanoemulsion often requires high energy shear, high-pres-sure homogenization, or ultrasound. High energy input breakup the large droplets into small ones. The composi-tion, order of addition and path of dilution will affect the droplet size of nanoemulsion. Nanoemulsions can be also prepared at lower energy mixing by selection proper sur-factant and physical chemical preparation methods. For example, nanoemulsion was prepared by phase inversion temperature（PIT）method with fatty alcohol ethoxylate nonionic surfactant2－5）. In PIT methods, nanoemulsion is formed by rapidly heating or cooling through the hydro-philic-lipophilic balance（HLB）temperature. The emulsion is not stable with low interfacial tension at HLB tempera-ture. In PIT process, the fatty alcohol ethoxyate become li-pophilic with increasing temperature as dehydration of polyoxyethylene chains. Water-in-oil（W/O）microemulsions coexist with excess oil phase at higher temperature. At low
＊Correspondence to: Jiang Yang, Department of Petroleum Engineering, Xi’an Petroleum University, Xi’an, Shaanxi, 710065, China.E-mail: [email protected] July 9, 2014 (received for review February 26, 2014)Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 onlinehttp://www.jstage.jst.go.jp/browse/jos/ http://mc.manusriptcentral.com/jjocs
temperature, oil-in-water（O/W）microemulsions coexists with excess oil phase. At intermediate temperature, HLB temperature, the ultralow interfacial tension exists in a bi-continuous, D phase microemulsion containing comparable amount of water and oil phases coexists with both excess water and oil phases, where spontaneous curvature becomes close to zero. Bicontinuous phase is passed during the process of changing from W/O emulsion at high tem-perature to O/W emulsion at low temperature, or vice versa from O/W to W/O emulsion. However, the PIT method re-quires high surfactant concentration. It also can not be used in temperature insensitive surfactant system such as ionic and sugar based nonionic surfactants5）.
The solubilization of the oil in a zero curvature micro-emulsion can also facilitate formation of bluish transparent oil-in-water nanoemulsion at constant temperature6－9）. Solans’s group8, 9） has studied these processes that involve change the surfactant film spontaneous curvature by step-wise addition of water to a solution of the surfactant in oil. The phase behavior of oil-water-surfactant is important to find the optimum dilution path in formation of nanoemul-sions.
The applications of nanoemulsions have been found in food10）, pharmaceutical11）, cosmetic12）, oil and gas indus-tries13, 14）. Recently, nanoemulsion has been found in emerging applications as flowback aid to cleanup the in-
Abstract: D-limonene in water nanoemulsion is prepared from an optimum formulation by low energy process at room temperature. The phase behavior of d-limonene/isotridecanol ethoxylate-6/ isopropyl alcohol /water system is systematically investigated to identify the optimum formulation. The microstructure of intermediate phases has been characterized by optical microscope and small angle X-ray diffraction. The microstructure of formulation concentrate has been further determined by means of electrical conductivity. The droplet size of nanoemulsion has been determined by light scattering and correlated with their microstructure. The results show that d-limonene nanoemulsion with droplet size of ca. 40 nm is obtained via the addition of the optimum formulation, which is a microemulsion, directly into water. This process involves composition change from a bicontinuous structure.
jected fluids in tight gas reservoir, which has less adsorp-tion of fluids on the formation and higher contact angles15, 16）. No systematic studies have been done on the formation of nanoemulsion using green natural ingredient, d-limenone, which is of great interests to the scientists and engineers exploring flow back aids of unconventional gas, flavor and fragrance additives.
In this report, we describe a study of phase diagram and formation of nanoemulsion of d-limonene, nonionic surfac-tant, solvent, and water system. D-limonene is selected as model oil as it is environmental friendly natural oil extract-ed from orange peels. Isopropanol is selected as co-solvent which is a commonly used in the oil industries. The nano-emulsion was prepared by optimum composition passing through bicontinous microemulsion regions from phase be-havior studies. The transition of microstructure is charac-terized by electrical conductivity. The droplet size and size distribution of nanoemulsion was also characterized. Hence, instead of using high energy sonication and micro-fluidization of d-limonene to form nanoemulsion17）, stable d-limonene nanoemulsion was prepared by the addition of an optimum microemulsion composition directly into water by gentle mixing at room temperature. The studies provide guidance to prepare d-limonene nanoemulsion system by low energy for practical applications.
2 EXPERIMENTAL PROCEDURES2.1 Materials
Technical grade d-limonene was obtained from Florida Chemical Inc, USA. Isotridecanol ethoxylates-n（i-C13EOn, n＝4, 6, 10）, were obtained from Sasol. The structure of surfactant and oil is shown in Scheme 1. Isopropanol（IPA）was ＞98％ in purity, and received from Sinopharm Chem-cial Co, China. Water was deionized.
2.2 MethodsThe phase diagram was determined by mixing of surfac-
tant, solvent and water at different weight ratio. The samples were fully mixed with a vibromixer and equilibrat-ed at 25℃. The phase boundary lines between liquid
crystal and isotropic phase were determined by consecu-tive addition of one component to mixtures at 5％ incre-ment. The two or multiphase phase can be detected by centrifugation of mixture and observation of liquid crystal birefringence through cross polarizer. For Winsor phase screening, samples were first gently rotated for 24 hours and then equilibrated for another 24 hours. In viscous liquid crystal phase, the samples were centrifuged to move bubble and equilibrated for 24 hours, and the microstruc-ture was characterized by polarized optical microscope and small angle X-ray diffraction（Anton Par SAXSess）. Nano-emulsion droplet size was measured by Zetasizer nano2S（Malvern）. Conductivity was determined with a Conduc-
timeter Shengci DDS-11A, with a Pt/platinized electrode. The samples were prepared with an electrolyte solution（10 mM）.
3 RESULTS AND DISCUSSION3.1 Selection of Surfactant with Optimum HLB
From molecular geometrical consideration, surfactant with bulky hydrophobic structure is more efficient in forming the microemulsion for maximum in solubilization of oil, which is good for bicontinous microemulsion and lower interfacial tension in formation of nanoemulsions18）. For examples, silicone nanoemulsion was made by uses of optimum highly branched trimethylnonyl ethoxylates19）, and branched hexadecyl benzene sulfonate has the lowest interfacial tension20）. Hence, we selected branched i-C13EOn（Scheme 1）as the model surfactants to emulsify.
In order to find out optimum HLB surfactant for the for-mation of d-limonene nanoemulsion, different ethoxylate surfactant at fixed concentration was firstly screened for formulation of Winsor III phase21）. Winsor III phase is a mi-croemulsion in equilibrium with both excess water and oil. Winsor I phase is a microemulsion（or swollen micelles）in equilibrium with excess oil. Winsor II phase is a microemul-sion（or swollen micelles）in equilibrium with excess water. The ultralow interfacial tension exists in Winsor III phase. In order to prepare nanoemulsion in diluted range, Winsor III microemulsion phase is needed to be present and passed. The optimum HLB surfactant for formation of Winsor III microemulsion is i-C13EO6 as shown in Table 1.
3.2 Phase Diagram of d-limonene/i-C13EO6/isopropanol/water
To study systemically the phase structure related to for-mation of nanoemulsion, four-component phase diagram was studied for optimum surfactant, i-C13EO6, isopropa-nol, d-limonene and water. The i-C13EO6 and isopropanol fixed at 4/1 ratio were combined as one component. The pseudo-ternary phase diagram of d-limonene/i-C13EO6/isopropanol/water is shown in Fig. 1. The nonionic surfac-Scheme 1 Structure of surfactants and oil studied.
Preparation of D-limonene Oil-in-Water Nanoemulsion from an Optimum Formulation
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1135
tant（i-C13EO6）, isopropanol, and d-limonene are miscible with each other and form one single phase. D-limonene belongs to cyclic terpene with six member ring structure（Scheme 1）. D-limonene is immiscible with water. Isopro-panol was used as co-surfactant. Addition of water to i-C13EO6/ isopropanol/d-limonene mixture forms inverse micelle or water in oil（W/O）microemulsion, L2, below 20％ water. A typical appearance of a sample, A1, in the L2 area is shown in Fig. 1. Addition of more water in range of 30-40％ forms viscous liquid gel phase（Lα）. A typical ap-pearance of composition A2 within gel phase is shown bire-fringence between cross polarizer in Fig. 1, which is typical liquid crystal optical behavior. The liquid crystal phase is lamellar liquid crystal（Lα）which is characterized by flow-able and coarse mosaic texture as shown in Fig. 2 under polarized microscope. The lamellar structure is further confirmed for their long-range ordering in small angle X-ray scattering peaks ratio of 1:2:3, which higher order of reflections are simple multiples of the first order one in a lamellar phase22）. Further addition of water enters oil-in-water（O/W）or bicontinuous, L1, microemulsion range. A typical appearance of a sample, A3, in the L1 area is shown in Fig. 1. Mutiphases M, region containing water, oil and
liquid crystal will be also entered for certain ratio of oil and water. The typical appearances of samples, B and C, in the M area are shown in Fig. 1. The multiphase may have dif-ferent amount of oil depending on water and oil ratio.
Microemulsion phase transition can be understood by concept of critical packing parameter23）, p:
p＝v/al ［1］
where v is volume of surfactant, a is effective head group, l is length of surfactant. Steric chain-chain and oil penetration interaction affect v and l. When there is excess of oil and solvent in upper right corner of the phase diagram, the penetration of oil and solvent makes p＞1, which gives inverted micelles and form W/O microemul-sion. P value that approach one with less oil and solvent gives a lamellar liquid crystalline phase or a bicontinous microemulsion in the middle areas. A more flexible surfac-tant film favors the microemulsion over liquid crystal. Further addition of water with less oil and surfactant pene-tration will make p＜1, the system will give spherical mi-celles and form O/W structure microemulsion.
The phase behavior in presence of oil and alcohol is better explained by intrinsic curvature of the interfacial region, called the Winsor R ratio21）. R ratio is ratio of cohe-sive energy of all molecules in oil phase and aqueous phase:
R＝ACO/ACW ［2］
Where ACO indicates the interaction between the surfac-tant adsorbed at the interface and the oil phase per unit area of interface, and where ACW indicates similar interac-tion for the water phase. For R＜1（Winsor I）, excess of oil
Table 1 Optimum HLB surfactant for formation of Winsor III (d-limo-nene oil/water ＝ 1/1, surfactant 2％, isopropanol 0.6％).
Surfactant i-C13EO4 i-C13EO6 i-C13EO10HLB value 9.4 11.4 13.7
Phase 2 phase-Winsor II 3 phase-Winsor III 2 phase- Winsor I
Fig. 2 The optical texture of lamellar liquid crystal re-gion under crossed polarized microscope.
J. Yang, W. Jiang, B. Guan et al.
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1136
phase equilibrates with oil interior micelles. For R＞1（Winsor II）, excess of water phase equilibrates with water interior reverse micelles. In intermediate state, R＝1（Winsor III）, both water and oil excess phases, bicontinu-ous structure, exists which the curvature is zero. Depended on relative amount of surfactant, water and oil or tempera-ture for nonionic surfactant, a change in R ratio happens and the diagram is also changed, and passed through R＝0, where the ultralow interfacial tension exists.
3.3 Association Microstructure Studied by Electrical Con-ductivity
The conductivity of the system is a useful tool to probe the microstructure of the fluids, and extensively studied by Smith and Solans group8, 24）, where linear nonionic surfac-tant and hydrocarbon were used in the studies. To enhance sensitivity of electrical conductivity, samples were made with aqueous NaCl solution instead of pure water. It has been shown25, 26） that phase behavior is not significantly changed with a low concentration of electrolyte in the system. The conductivity of the variable water fraction at 3 different ratios of d-limonene/（i-C13EO6＋isopropanol）is shown in Fig. 3, which correspond to the compositions along 3 lines in Fig. 1. The conductivity is low when oil is external phase and water is internal phase for water-in-oil microemulsion. The conductivity is also low in lamellar liquid crystal phase. As water fraction is further increased and enters the 2 phase areas with O/W microemulsion, the conductivity is increased drastically. Conductivity starts to increase at lower water fraction 40％ for higher ratio of d-limonene/（i-C13EO6＋isopropanol）, 3/2, at the point when it enters 2 phase areas containing water external phase emulsion. The corresponding samples appear as more turbid emulsion above 50％ water which is shown in Fig.
4a. For lower ratio of d-limonene/（i-C13EO6＋isopropa-nol）＝2/3, conductivity increases at higher water fraction 60％. The corresponding samples appear as turbid emul-sion above 60％ water which is shown in Fig. 4c. The more water can be solubilized into lamellar liquid crystal region with relative higher ratio of surfactant. In between, con-ductivity increases at middle water fraction 50％ for 1/1 ratio of d-limonene/（i-C13EO6＋isopropanol）. The corre-sponding samples appear as all relatively clear which is shown in Fig. 4b. It is noted that the slope of increase in conductivity is the highest for this 1/1 ratio of d-limonene/（i-C13EO6＋isopropanol）, which indicates a bicontinuous microemulsion coexist, in which the microemulsion in equilibrium simultaneously with excess water and hydro-carbon phase. The similar trend was observed in decane-C12EO4-H2O system8）. Along optimum oil/surfactant ratio line, addition of water increases droplet number and reduces the distance of the droplet, which makes the droplet gradually interlinking and clustering each other and forms bicontinuous structure. The percolation conduc-tion phenomenon occurs27）.
3.4 Preparation and Droplet Size Distribution of Nano-emulsion
The phase change and formation of emulsions at differ-ent ratio of d-limonene/（i-C13EO6＋isopropanol）, 3/2, 1/1, 2/3, were evaluated with addition of water to d-limonene and surfactant solution. The dilution lines are shown in Fig. 1. The photographs of the phase at each step are shown in Fig. 4, which correspond to the points marked in the phase diagram from 10％ to 80％ water fraction at 10％ incre-ment in Fig. 1. Photographs of further stepwise dilution from 98％ to 99.9％ water fraction are shown in Fig. 5.
It can be seen in Fig. 5b that bluish clear nanoemulsion up to 99％＋water can be formed from dilution of micro-emulsion at d-limonene/（i-C13EO6＋isopropanol）＝1/1. The results are consistent with conductivity studies which shows bicontinuous microemulsion exists at dilution path of d-limonene/（i-C13EO6＋isopropanol）＝1/1. The zero spontaneous curvature of the surfactant is reached within this bincontinuous structure. Hence, ultralow interfacial tension is passed and forms nanoemulsion by dilution with gentle mixing8, 19）. The system enters into multiphase turbid macroemulsion at dilution ratio of d-limonene /（i-C13EO6＋isopropanol）3/2 or 2/3, where no such bicontinu-ous phase exists at such ratio. The appearances of the 99.8％ and 99.9％ water fraction samples as clear in the photographs of Fig. 5a and 5c are actually due to oil sepa-rated out on the top of solution. It appears as transparent for such small amount of oil（＜0.1％）.
For practical oilfield industrial application, addition of microemulsion concentrate directly into large volume of water with gentle stirring was studied. The initial micro-emulsions at ratio of d-limonene /（i-C13EO6＋isopropa-
Fig. 3 Electrical conductivity of emulsion/micro-emulsion by addition of 10 mM NaCl brine to surfactant-oil-solvent mixture at different oil/surfactant ratio (alone lines Fig. 1).
Preparation of D-limonene Oil-in-Water Nanoemulsion from an Optimum Formulation
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1137
nol）, 1/1, with different water fraction of 10％, 60％ and 90％ were added to water（up to 99.8％ water fraction）with gentle mixing. The concentration of surfactant and d-limonene is 0.08％ and 0.1％ respectively in nanoemulsion with 99.8％ water. The droplet size distribution of nano-emulsion was determined by dynamic light scattering as shown in Fig. 6.
As it can be seen from phase diagram in Fig. 1, the mi-croemulsion forms water in oil microstructure when the water fraction is 10％. The microemulsion changes to bi-continous microstructure when the water fraction is 60％. The microemulsion changes oil-in-water microstructure when the water fraction is 90％. The nanoemulsion with average droplet size of 40 nm was obtained when initial mi-croemulsion is within bicontinous structure. The size dis-tribution（20-80 nm）of nanoemulsion prepared from bicon-tinous microemulsion is also the narrowest as shown in Fig. 6b. The nanoemulsions prepared by dilution from W/O or
O/W microemulsion have average droplet size 100 nm in di-ameter, while their size distribution is also much broader（20-300nm）as shown in Fig. 6a and 6c.
In the formation of d-limonene nanoemulsion process, addition of water changes the phase behavior from a bicon-tinuous single phase（stable）microemulsion or liquid crystal to a 2-phase behavior that results in separation of droplets since it is no longer a stable phase. The formation of smaller nanoemulsion involves homogenous nucleation of oil from bicontinuous microemulsion range upon dilution and mixing with water7）. Hence, the smallest size nano-emulsion can be prepared from d-limonene /（i-C13EO6＋isopropanol）, 1/1 at water fraction 60％, which is in pre-in-verted bicontinuous microemulsion single phase. The na-noemulsion prepared by this method is kinetically stable without changing in droplet size for months. In contrast, nanoemulsion with relative larger droplet size was formed by dilution at other higher or lower initial water fraction of
Fig. 4 Appearance of samples at the dilution path of different oil/surfactant ratio. The labels show water fraction from 10% to 80% wt.
J. Yang, W. Jiang, B. Guan et al.
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1138
O/W or W/O microemulsion. Although they have the same oil/surfactant/solvent ratio, initial W/O microemulsion is not within bicontinuous structure, and initial O/W micro-emulsion is partially in equilibrated with bicontinuous mi-croemulsion. The existing oil or water droplets may act as nuclei and trigger heterogeneous nucleation, which form in droplets with larger sizes and polydispersity28）.
Fig. 5 The nanoemulsion or emulsion prepared from diluting different oil/surfactant ratio. The labels show water fraction from 98% to 99.9% wt.
Fig. 6 Droplet s ize of nanoemulsion crash di -lu ted f rom the ra t io of d- l imonene / ( i -C13EO6+isopropanol) , 1/1, from (a) water fraction at 10% wt, (b) water fraction at 60% wt, and (c) water fraction at 90% wt.
Preparation of D-limonene Oil-in-Water Nanoemulsion from an Optimum Formulation
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1139
3.5 Association Microstructure Studied by Small Angle X-ray Diffraction
In order to understand the molecular interaction that affects nanoemulsion formation from associated surfactant structure, the interaction of oil and surfactant in lamellar liquid crystal phase was further studied by small angle X-ray diffraction. The small angle x-ray scattering results give the interlayer spacing d（Fig. 7）. The interlayer spacing from surfactant with water, IPA and hydrocarbon was shown in Fig. 8. An increased ratio of d-limonene/sur-factant led to an increasing slope of the plot, and gave slightly higher interlayer spacing extrapolated to zero water content.
Hence, the hydrophobic molecule, d-limonene, must
locate in C range of lamellar structure in Fig. 7. The water molecules locate in A range. Obviously, the optimum sur-factant and oil ratio in the middle favors the transformation of structure to form nanoemulsion during the dilution.
The studies of emulsion and nanoemulsion range with small angle X-ray diffraction don’t show any meaningful diffraction peaks due to weak diffraction. Further studies with small angle neutron diffraction will be used7）.
4 CONCLUSIONThe d-limonene nanoemulsion was prepared from an
optimum formulation at room temperature. The optimized formulation was determined from phase behavior of d-lim-onene/isotridecanol ethoxylate-6/isopropyl alcohol/water. The nanoemulsion at extremely low surfactant and oil frac-tion can be directly formed from crash dilution of optimum microemulsion concentrate into large volume of water. The optimum microemulsion concentrate contains bicontinuous structures, and was obtained by stepwise addition of water into solution of surfactant, solvent and d-limonene mixture.
ACKNOWLEDGMENT The author acknowledgements the supports of the
project by the National Hi-Tech Development（863）Plan Project（2013AA064801）and National Natural Science Foundation of China（Grant No 51174163）
References1） Friberg S. Microemulsions. Theory and Practice（ed.
Prince, L. M.）, Academic, New York（1977）.2） Shinoda, K.; Saito, H. The Stability of O/W type emul-
sions as functions of temperature and the HLB of emulsifiers: The emulsification by PIT-method. J. Col-loid Interface Sci. 30, 258-263（1969）.
3） Izquierdo, P.; Feng, J.; Esquena, J. The Influence of Surfactant Mixing Ratio on Nano-emulsion Formation by the PIT Method. J. Colloid Interface Sci. 285, 388-394（2005）.
4） Morales, D.; Solans, C.; Gutierrez, J. M. Oil/Water Droplet Formation by Temperature Change in the Wa-ter/C16E6/Mineral Oil System. Langmuir 22, 3014-3020（2006）.
5） Salager, J. L.; Forgiarini, A.; Marquez, L.; Peña, A.; Piz-zino, A.; Rodriguez, M. P.; Rondón-González, M. Using Emulsion Inversion in Industrial Processes. Adv Col-loid Interf Sci. 108-109, 259-272（2004）.
6） Fernandez, P.; Andre, V.; Rieger, J.; Kuhnle, A. Nano-emulsion Formation by Emulsion Phase Inversion.
Fig. 7 The lamellar liquid crystal contains three zones: A: water layer plus polar groups; B: hydrocar-bon chain methylene groups; C: hydrocarbon chain terminal methyl group and space between them.
Fig. 8 The interlayer spacing versus volume ratio of water/ (D-limenonene+i-C13EO6+IPA) in la-mellar liquid crystal region.
J. Yang, W. Jiang, B. Guan et al.
J. Oleo Sci. 63, (11) 1133-1140 (2014)
1140
Colloids Surface A 251, 53-58（2004）.7） Wang. L.; Mutch, K. J.; Eastoe, J.; Heenan, R. K.; Dong,
J. Nanoemulsions Prepared by a Two-Step Low-Ener-gy Process. Langmuir 24, 6092-6099（2008）.
8） Forgiarini, A.; Esquena, J.; Gonzalez, C.; Solans, C. Formation of Nano-emulsions by Low-Energy Emulsi-fication Methods at Constant Temperature. Langmuir 17, 2076-2083（2001）.
9） Solans, C.; Izquierdo, P.; Nolla, J; Azemar, N.; Garcia-Celma, M. J. Nano-emulsios. Cur. Opin. Colloid Inter-face Sci. 10, 102-110（2005）.
10） Jafari, S. M.; He, Y.; Bhandari, B. Optimization of Nano-emulsions Production by Microfluidization. Eur. Food Res. Tech. 22, 733（2007）.
11） Zhou, H.; Yue, Y.; Liu, G. Preparation and Character-ization of a Lecithin Nanoemulsion as a Topical Deliv-ery System. Nanoscale Research Lett. 5, 224-230（2010）.
12） Sonneville-Aubrun, O.; Simonnet, J. T.; L’Alloret, F. Nanoemulsions: a New Vehicle for Skincare Products. Adv. Colloid Interface Sci. 108, 145-149（2004）.
13） Pursley, J. T.; Penny, G.; Holcomb, D. Microemulsion Additives Enable Optimized Formation Damage Repair and Prevention. SPE International Symposium and Exhibition on Formation Damage Control, Paper No. 86556（2004）.
14） Rickman, R. D.; Jaripatke, O. Optimizing Microemul-sion/Surfactant Packages for Shale and Tight-Gas Res-ervoirs, SPE Deep Gas Conference and Exhibition, Pa-per No. 131107（2010）.
15） Zelenev, A. S.; Zhou, H.; Ellena, L. B.; Penny, G. S. Mi-croemulsion-Assisted Fluid Recovery and Improved Permeability to Gas in Shale Formations. SPE Interna-tional Symposium and Exhibition on Formation Damage Control, Paper No. 127922（2010）.
16） Champagne, L. M.; Zhou, H.; Zelenev, A.; Lett, N. The Impact of Complex Nanofluid Composition on Enhanc-ing Regained Permeability and Fluid Flowback from Tight Gas Formations and Propped Fractures. SPE In-
ternational Symposium and Exhibition on Formation Damage Control, Paper No. 151845（2012）.
17） Li, P. H.; Chiang, B. H. Process optimization and stabil-ity of d-limonene-in-water nanoemulsions prepared by ultrasonic emulsification using response surface meth-odology. Ultrasonics Sonochemistry 19, 192（2012）.
18） Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution, 2nd Eds, John Wiley & Son, Chichester, pp. 144（2003）.
19） Gee, R. P. Oil-in-Water Microemulsions from Associa-tion Structures of Surfactant, Water and Aminosilicone Polymer Oil. Colloids Surface A 137, 91-101（1998）.
20） Morgan, J.; Schechter, R. S.; Wade, W. H. Recent Ad-vances in the Study of Low Interfacial Tensions, in Improved Oil Recovery by Surfactant and Polymer Flooding, edited by Shah, D. O. and Schechter, R. S. Academic Press, New York, pp. 109（1977）.
21） Winsor, P. Solvent Properties of Amphiphilic Com-pounds, Butterworth, London（1954）.
22） Evans, D. F.; Wennerstrom, H. The Colloidal Domain, p.249, VCH publishers, Inc.（1994）.
23） Israelachvii, J. Intermolecular and Surface Forces, p.370, Academic Press（1992）.
24） Smith, S. H.; Sampath, R.; Dadyburjor, D. B. Tempera-ture Dependence of Emulsion Morphologies and the Dispersion Morphology Diagram. J. Phys. Chem. 100, 17558-17562（1996）.
25） Eicke, H. F.; Meier, W.; Hammerich, H. On electric con-ductivity of infinitive clusters in water-in-oil micro-emulsions. Langmuir 10, 2223-2227（1994）.
26） Bostock, T. A.; McDonald, M. P.; Tiddy, G. J.T. Surfac-tant in Solution; Mittal, K. L.; Lindman, B. Eds. Ple-num Press, New York, Vol. 3, pp. 1805-1820（1984）.
27） Lagourette, B.; Peyrelasse, J.; Boned, C.; Clausse, M. Percolative Conduction in Microemulsion Type Sys-tems. Nature 281, 60-62（1979）.
28） Vincent, B.; Kiraly, Z.; Obey, T. M. Modern Aspects of Emulsion Science, Binks, B. P. Ed. The Royal Society of Chemistry; Cambridge, UK. pp. 100-114（1998）.
