O Microemulsion

Research Article pubs.acs.org/journal/ascecg

Formation of Concentrated Nanoemulsion by W/O Microemulsion Dilution Method: Biodiesel, Tween 80, and Water System Kun Tong,† Chunhua Zhao,‡ Zhicheng Sun,§ and Dejun Sun*,† †

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, Shandong 250100, China Oilfield Chemistry Research Institute, Division of Oilfield Chemistry, China Oilfield Services Limited, Yanjiao, Hebei 065201, China § Department of Chemistry and Biochemistry, University of California−Los Angeles, Los Angeles, California 90095, United States ‡

S Supporting Information *

ABSTRACT: In this work, we show the formation of concentrated green O/W nanoemulsion (dispersed phase mass fraction was up to 0.5) by diluting W/O microemulsion in the water/Tween 80/biodiesel system. The mechanism of the formation of nanoemulsions was examined and illustrated by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM). At high temperature, nanosized droplets formed spontaneously due to the surfactant migration and inversion upon dilution of W/O microemulsions, but these droplets were highly unstable. When cooled to room temperature, their stability was highly enhanced due to the decrease of collision frequency rate and the enhancement of stabilization of the oil/water interface. Even though, the Ostwald ripening still results in growth of droplets of the nanoemulsions after long-term storage, which limits the practical applications of nanoemulsions. W/O microemulsions are thermodynamic systems. Hence, W/O microemulsions that can form nanoemulsions by simple dilution of water can be used as an alternative to O/W nanoemulsion during storage and transport. Furthermore, biodiesel nanoemulsions could meet the requirements of green chemistry and engineering and be used as new green lubricants in water-based drilling fluid. KEYWORDS: W/O microemulsion dilution method, Nanoemulsion, Biodiesel, Green lubricant

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INTRODUCTION Nanoemulsions are emulsions with droplet diameter of nanometer scale, generally in 20−200 nm size range.1−3 Because of high kinetic stability and small droplets diameter, nanoemulsions are significant in practical applications including chemical,4 pharmaceutical,5 cosmetic,6 agrochemical,7 and oil recovery8 fields, etc. Nanoemulsions are not thermodynamically stable systems.9 To prepare nanoemulsions, it is necessary to provide external energy, overcoming the surface free energy to create droplets small enough.10 Generally, two approaches are used for preparing nanoemulsions: high energy methods11,12 and low energy methods.2,13 High energy methods utilize special equipment to supply a quantity of energy. The main variables that impact nanoemulsions characters formed by high energy methods are the energy intensity, the physicochemical properties of the oil and water, and the surfactant type and concentration.11 In contrast, the low energy emulsification methods make full use of the internal chemical energy of the system based on the phase behavior of surfactant, oil, and water systems.14 Generally, low energy emulsification methods can be classified as phase inversion and microemulsion dilution methods. Phase inversion methods make use of the chemical energy released during the emulsification process as a result of the change of surfactant spontaneous curvature, from negative to © XXXX American Chemical Society

positive (obtaining oil-in-water, O/W emulsions) or vice versa (obtaining water-in-oil, W/O emulsions). This change of curvature has been obtained by changing the composition at constant temperature15−17 (phase inversion composition method, PIC) or changing the temperature at constant composition18−21 (phase inversion temperature method, PIT). In these methods, phase transitions (lamellar liquid crystalline phase or bicontinuous microemulsions) during the emulsification process are critical in the formation of nanoemulsions.2,13,19 Microemulsion dilution methods are quite simple in practical implementation. Emulsification proceeds in the whole volume of a mixture; therefore, it can be scaled with ease.24,27 Commonly, oil-in-water (O/W) nanoemulsions are prepared by diluting an O/W microemulsion with water. In these cases, dilution of an O/W microemulsion with water induces part of the surfactant molecules to dissolve into water. The surfactants cannot maintain the low interfacial tension required (γ < 10−2 N·m−1) for thermodynamic stability and give rise to the formation of nanoemulsion droplets.22 Pons et al.23 reported that nanoemulsions could be formed by dilution of bicontinuous microemulsions, O/W microemulsions, or multiReceived: August 19, 2015

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DOI: 10.1021/acssuschemeng.5b00903 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering phase mixtures with water. Solans et al.13 claimed that small droplet-size nanoemulsions were always obtained by starting emulsification from O/W microemulsions. The nanoemulsion formed by this way was independent of the microemulsion composition and the dilution procedure used. In contrast, the properties of nanoemulsions obtained by diluting W/O microemulsions depend on both dilution procedures and composition of the starting microemulsions. Recently, Sole et al.24 reported that the nanoemulsions were prepared by stepwise addition of water over W/O microemulsions at which a direct microemulsion domain is crossed during emulsification. Dong et al.25,26 also prepared nanoemulsions by adding microemulsion into water in one step and found that transparent nanoemulsions formed when the initial concentrate was a bicontinuous microemulsion. However, these nanoemulsions formed by this method are prepared at relatively low mass fraction of the dispersed phase27 (0 < φ ≤ 0.1), which is a disadvantage for the practical applications.14,27 Recently, due to the requirements of green chemistry and engineering, it is necessary to prepare and use green nanoemulsions based on renewable and nontoxic oils. Biodiesel is a renewable clean bioenergy source and a promising oil phase substitute for classical n-alkanes in common microemulsion, which may be used to formulate more environmentally friendly industrial products like “green” cutting fluids, pesticides, cosmetics or pharmaceuticals.28−31 The advantages of biodiesel are its nontoxic, rapid biodegradability in soil, low vapor pressure and noninflammability, high flash point, etc. Therefore, the main objective of the current study was to investigate the formation of concentrated biodiesel nanoemulsions by microemulsion dilution method. In this work, concentrated green O/W nanoemulsions were prepared by a W/O microemulsion dilution method in the water/Tween 80/biodiesel system. This study explored the phase behavior of the initial concentrate, the surfactant concentration and the effect of the dilution temperature on the properties of concentrated nanoemulsions. More importantly, the mechanism of formation of nanoemulsions obtained by the dilution method was examined by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) in detail. We propose that our findings have significant value in actual applications, especially for new green lubricants in water-based drilling fluid.

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Droplet Size Determination. Nanoemulsion droplet size and distribution were measured by dynamic light scattering (DLS), using a Brookhaven BI-200SM research gonimeter. A 200 mW green laser (λ = 532 nm) with variable intensity was used, and measurements were carried out at room temperature with a scattering angle of 90°. The nanoemulsions were diluted about 1000 times with deionized water just before the measurements. The average diameter and polydispersity index were calculated from the intensity autocorrelation data with the cumulants method. The intensity−intensity time correlation functions were analyzed by the CONTIN method.32 Interfacial Tension Measurements. The interfacial tension between water and surfactant-in-oil solution was measured by the spinning drop interfacial tension meter of Model TX500C, which could be used in a wide range of measurements, 10−5−102 mN·m−1. Experiments were carried out with special care to avoid water evaporation. Cryogenic Transmission Electron Microscopy (cryo-TEM). Cryo-TEM images of the emulsions were obtained on a JEOL JEM1400 transmission electron microscope at a 200 kV accelerating voltage. The cryo-TEM samples were prepared in a controlled environment vitrification system (CEVS) at 25 °C. A micropipet was used to load 5 μL of the solution onto a TEM copper grid coated with a carbon support film. The excess solution was blotted with two pieces of filter paper, resulting in the formation of thin films suspended on the mesh holes. After about 10 s, the samples were quickly plunged into liquid ethane cooled by liquid nitrogen at −165 °C. The vitrified samples were then transferred to a cryogenic sample holder (Gatan 626) and examined with TEM at −174 °C. Small Angle X-ray Scattering (SAXS). X-ray scattering measurements were performed using a SAXSess MC2 high flux small-angle Xray scattering instrument (Anton Paar, Austria, Cu Kα, λ = 0.154 nm), equipped with a Kratky block-collimation system and using an image plate (IP) as the detector. The X-ray generator was operated at 40 kV and 50 mA. A standard temperature control unit (Anton-Paar TCS 120) connected to the SAXSess instrument was used to control the temperature and keep it at the desired level. Samples were transferred into standard quartz capillaries with a diameter of 1 mm. Both SAXS and WAXS scattering profiles were recorded simultaneously. An exposure time of 5 min was long enough to give a good signal-to-noise ratio. The scattering curve of pure water in the same type of capillary was recorded as a background. All data were normalized to the same incident primary beam intensity and corrected for background scattering from the capillary and water, according to the scattering of water in the wide-angle region.

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RESULTS AND DISCUSSION Formation of Concentrated Nanoemulsions by W/O Microemulsion Dilution Method. The microemulsion dilution method involves preparing a precursor microemulsion as the first step. The phase diagram of the water/Tween 80/ biodiesel at 70 °C is shown in Figure 1a. Liquid crystalline phases were observed in many regions of the diagram, identified as Mlc. A single microemulsion domain, W/O microemulsion, Om, is present at low water content. The nature of the different phases and their boundaries were identified by polarized light, conductivity measurements and transparency along water dilution paths with a fixed O/S ratio at 70 °C. When the O/S ratio was fixed at 1:1, nanoemulsions were formed by diluting W/O microemulsions with 0−18 wt % water in the precursor. As shown in Figure 1b, the droplet diameter of these nanoemulsions was the smallest, about 41 nm. With the increase of water mass fraction in precursor, the precursor became liquid crystal phase, whose viscosity increased rapidly, and the droplets of nanoemulsions became much large. These different phases in the phase diagram are consistent with changes in the surfactant hydration as a function of water

EXPERIMENTAL SECTION

Materials. The polyoxyethylene (20) sorbitan monooleate (Tween 80, chemically pure grade) was obtained from Sinopharm Chemical Reagent Co., Ltd. Biodiesel (Shandong Province, China) was used here. The components of this biodiesel are mainly with 16−18 carbon atoms (Supporting Information S1), as measured with an Agilent 7890 GC (Agilent Co.). All reagents were used as received without further purification. Water used in this work was deionized water. Phase Diagram Determination. All components were weighed, sealed in ampules, and homogenized with a magnetic stirrer. The samples were equilibrated at 70 °C. Optically anisotropic liquid crystalline phases were identified by using polarizing light microscopy (OptipHot-2, Nikon, Japan). The boundary lines were found by consecutive addition of water to mixtures of the Tween 80 and biodiesel. Preparation of Nanoemulsions. Nanoemulsions were prepared by diluting W/O microemulsions with water at 70 °C. During emulsification, W/O microemulsions were kept under continuous magnetic stirring. The influence of composition parameters, including the oil to surfactant weight ratio (O/S) and the dispersed phase mass fraction (φ), was investigated systematically. B


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Up to now, nanoemulsions formed by a microemulsion dilution method are generally prepared at a low mass fraction of the dispersed phase (0 < φ ≤ 0.1).25−27,33 However, in this work, concentrated nanoemulsions were successfully prepared by a W/O microemulsion dilution method. The dispersed phase mass fraction of the concentrated nanoemulsions varied from 0.1 to 0.5, which was higher than those of low mass fraction (0 < φ ≤ 0.1). In these dispersed phase mass fractions (0 < φ ≤ 0.5), nanoemulsions prepared always remained around 41 nm, indicating that the droplet size of the nanoemulsions was mainly governed by the size of the droplets formed spontaneously after diluting W/O microemulsions with water (Figure 2). This result was consistent with emulsions

Figure 1. (a) Partial phase diagram (T = 70 °C) of water/Tween 80/ biodiesel (Om, inverse micellar solution or W/O microemulsion; Mlc, multiphase region including liquid crystal phase; Em, multiphase region). (b) Evolution of the droplet diameter of nanoemulsions as a function of water mass fraction in precursor at O/S = 1:1, φ = 0.3.

Figure 2. Evolution of the droplet diameter and polydispersity index of nanoemulsions as a function of dispersed phase mass fractions of the finial nanoemulsions with O/S = 1:1.

produced by other low energy methods,21,25,34 where the key of nanoemulsions prepared by low-energy methods is the phase transitions of emulsions during the emulsification process. Therefore, the droplet diameters of nanoemulsions do not depend on the amount of water used once the nanoemulsions are formed. Excess water merely acts only as a dilution medium. Figure 3 shows the droplet size distribution and appearance of the nanoemulsions obtained with different O/S ratios. At a

in precursor.22 Generally, the optimum curvature of nonionic surfactant layers depends on hydration of the PEO, which can be increased by dilution with water. If there is no water in the precursor, the surfactants are dissolved in the oil phase and formed reversed micelles. When the reversed micelles are diluted with a large amount of water, the optimum curvature of the surfactants would be more hydrophilic. As a result, the surfactants would reverse and migrate toward the oil−water interface leading to the spontaneous increase in the surface area of the interface, resulting in formation of the small droplets. This is a complex process that may not be achieved adequately in a short period. Hence, the droplet diameter of the final nanoemulsions formed by diluting reversed micelles was a little larger than those formed by diluting W/O microemulsions. When water mass fraction of the precursor increases (about 0− 18 wt %), the optimum curvature would be around water, forming W/O microemulsions. The surfactants adsorbed on the oil/water interface could easily migrate and reverse to form small oil droplets when diluted with water. In addition, surfactants dissolved in the droplets may migrate through the oil/water interface to form small oil droplets.27 With further increase of the water mass fraction of the precursor (more than 18 wt %), the hydration of the PEO headgroups may drive the curvature from favoring W/O phases to O/W phases. The optimum curvature of the surfactants cannot allow them to form W/O microemulsions, and meanwhile insufficient amount of water cannot allow them to form bicontinuous microemulsions or O/W microemulsions.35 Hence, compared with nanoemulsions prepared by dilution of reverse micelles or liquid crystal phase, nanoemulsions with smaller droplets are produced by diluting W/O microemulsions.

Figure 3. Droplet size distributions of nanoemulsions at different oil/ surfactant mass ratios for samples with φ = 0.3. In small images, the visual aspect of the nanoemulsion is shown.

low O/S ratio (1:1), highly transparent nanoemulsions with droplet size of 40 nm and low polydispersity index are obtained. With the increase of O/S ratio from 1:1 to 4:1, the droplet size values increase from 40 to 156 nm due to the decrease of surfactant, which stabilizes the droplets. Similar results were observed by other authors but they used emulsifiable concentrates with no water in their composition.17,37,38 C


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the emulsions were highly unstable at elevated temperature. Generally, the increase of temperature reduces the viscosities of emulsion phases, and decreases the interfacial tension, thereby facilitating production of smaller droplets to form semitransparent nanoemulsions.41 However, the increase of storage temperature also accelerates the movement of droplets and lowers the stabilization of the oil/water interface, enhancing the coalescence of the droplets and thereby leading to the phase separation of the emulsions. Compared to the emulsion (φ = 0.3), the appearance of emulsion (φ = 0.1) just became much more turbid after 12 h storage at 70 °C. These results were caused by the decrease of collision frequency due to the lower dispersed volume fraction.42 These results demonstrated that the formation of emulsions with small droplets was not a thermodynamic process. The droplets formed due to the rapid increase of the hydration interaction of surfactants caused by the rapid addition of the water when diluted at 70 °C. This is a dynamical process, so the droplets are not stable at 70 °C. However, the stability of the droplets was enhanced when cooled to room temperature.43 Hence, the emulsions were stored at room temperature after being diluted with water at 70 °C. The morphology and size of emulsions occurred when W/O micoremulsons were diluted with water at 70 °C were observed by cryo-TEM. As shown in Figure 6, spherical droplets of

Hence, the droplet sizes can be precisely modulated by controlling the concentration of surfactants. Mechanism of the Formation of Concentrated Nanoemulsions by W/O Microemulsion Dilution Method. The nonionic surfactant (Tween 80) with the PEO chains was used in this study. The physicochemical and functional properties of nonionic surfactant change appreciably with temperature,39 which may have a great influence on the formation and properties of nanoemulsions. Therefore, the nanoemulsions were prepared by dilution of W/O microemulsions at different temperatures ranging from 30 to 80 °C at a fixed composition (O/S = 1:1, φ = 0.3). The droplet size of the final nanoemulsions was determined by DLS, as shown in Figure 4. Upon dilution with pure water at different temperatures, low-

Figure 4. Effect of dilution temperature on nanoemulsion droplet diameter and interfacial tension for samples with O/S = 1:1, φ= 0.3.

polydispersed nanoemulsions with the droplets in the range 42−110 nm were obtained. Obviously, with the increase of temperature, the droplet diameter of nanoemulsion decreases from 110 nm to about 42 nm. This is because the interfacial tension decreases rapidly with the increase of dilution temperature from 40 to 80 °C (Figure 4). In this temperature range, the oil−water interfacial tension became extremely low. This low interfacial tension favored the intermingling of the polar and nonpolar components and therefore facilitated the spontaneous formation of a very large oil−water interface.40 With the increase of temperature, the PEO becomes progressively dehydrated, promoting the adsorption of Tween 80 at the O/W interface. Furthermore, the increase of temperature leads to the increase in the rate of diffusion, which means faster adsorption of surfactants at higher temperatures. The more absorption of Tween 80 resulted in the greater reduction of interfacial tension. As a consequence, smaller droplets with narrow droplet size distributions were formed at high dilution temperatures. As shown in Figure 5, the emulsions were semitransparent exactly after dilution with water. After 12 h, the phase separation of emulsion (φ = 0.3) occurred, indicating that

Figure 6. Cryo-TEM images of samples occurred when W/O microemulsions were just dilututed at 70 °C with O/S = 1:1, (a) φ = 0.3 (b) φ = 0.1.

different sizes with diameters ranging from 5 to 35 nm were obtained in the system. When the W/O microemulsions were diluted with water at 70 °C, the O/W nanosized droplets formed spontaneously due to the rapid change of the surfactant hydration and the low interfacial tension. However, the formation of droplets was not a thermodynamic control process. The droplets, formed due to the surfactant migration and inversion within a short time, were polydisperse.

Figure 5. Photographs of biodiesel/Tween 80/water nanoemulsions after (a) 0 h (b) 12 h at 70 °C with different dispersed phase mass fractions with O/S = 1:1. D


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between the droplets is weak.44 It could be inferred that no obvious aggregation of droplets occurred. Generally, the scattering intensity profiles do not reveal intuitive structural information, which usually require some proper transformations of the SAXS profile. The pair distance distribution function (PDDF) p(r) can reveal valuable information about the shape of droplets, allowing more intuitive interpretation of the intensity profile, I(q).45 As shown in Figure 7b, the p(r) curves calculated from the IFT method (see the Supporting Information) have a slightly asymmetric distribution, revealing the droplet size distribution of the samples was not homogeneous.45,46 However, according to the cryo-TEM images (Figure 6), the droplets of samples at different temperatures were spherical. Hence, the nonstandard p(r) curves revealed the droplet size distribution of the samples was not homogeneous. The volume weighted size distributions were calculated by the IFT method (see the Supporting Information) from the scattering curves of different samples (Figure 7c).47 The curves reveal the size distribution of the samples at different temperatures. The area of the peak shows the volume fraction of droplets with different radius. In our experiments, the W/O microemulsions were just diluted by a large amount of water at 70 or 80 °C. With the addition of water, the surfactant hydration increases rapidly, and the surfactants start to inverse and migrate to form more oil/water interface. Nanosized spherical droplets formed spontaneously. But this process was not a thermodynamic control process and the diameter of the droplets was ploydispersed. At the same time, the volume fraction of small droplets was larger at 80 °C than at 70 °C. This result further evidence the previous statement that higher temperature leads to faster surfactant inversion and migration rate and lower interfacial tension. As mentioned before, Ostwald ripening was the most important factor for the nanoemulsion instability and militated when the droplets formed spontaneously. After the W/O microemulsions were diluted with water at high temperature, the samples cooled to room temperature. In this process, the small droplets gradually dissolved in the large droplets due to the Ostwald ripening. Therefore, the area of peak at small R value was much smaller at 25 °C than at high temperature. With the decrease of temperature, the rate of Ostwald ripening became much lower. Therefore, the concentrated nanoemulsions were formed finally. On the basis of these results from cryo-TEM and SAXS, the mechanism of the W/O microemulsion dilution method was proposed (Figure 8). The mechanism of W/O microemulsion dilution method can be considered as two parts. First, the W/O microemulsions were heated to decrease the interfacial intension of the system and then diluted with water at high temperature under stirring. Because of the addition of the large amount of water, the surfactant hydration increases rapidly. The surfactants at oil/water interface migrate and inverse forming nanosized droplets. In addition, surfactants dissolved in the W/

Figure 6a,b) also shows that there was little difference between the nanosized droplets formed at different water mass fractions. The droplet diameters of nanoemulsions did not depend on the amount of water used when the nanoemulsions formed. Excess water acted only as a dilution medium. SAXS measurements were performed to study the properties of emulsions at different temperatures. Figure 7a) shows SAXS

Figure 7. (a) SAXS profiles, (b) pair distance distribution function (PDDF) p(r), and (c) volume weighted size distribution of samples obtained when W/O microemulsions were just diluted at (■) 70 °C, (□) 25 °C cooled down from 70 °C, (▲) 80 °C, and (△) 25 °C cooled down from 80 °C, respectively.

spectra for samples at different temperatures. No obvious change in the shape or intensity of the SAXS was observed. As no platean or peak was observed at low q values, the interaction

Figure 8. Schematic of mechanism of W/O microemulsion dilution method to form nanoemulsions. E


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ACS Sustainable Chemistry & Engineering O microemulsions droplets may migrate through the oil/water interface to form O/W nanosized droplets. These results are in good agreement with the precious research described by Greiner et al.,48 who claimed that the nanoemulsions formed by the inversion of W/O microemulsions in a quiescent adjacent water phase was due to diffusion of water into the inverted micelles and the inversion of the surfactant. Pratt et al.49 also reported that the dispersion and formation of individual droplets were caused by the migration of the substances through the interface. Thus, the key of the formation of the nanoemulsion was the migration and inversion of surfactants once the W/O microemulsion was diluted with water. Meanwhile, the diameter of droplets was just controlled by the interfacial intension of the system despite of the mass fraction of the dispersed phase.41 Second, the emulsions were cooled to room temperature after W/O microemulsions were diluted. With the decrease of the temperature, the interfacial strength of the droplets increased, and the droplets became much more stable.42 Then, the nanoemulsions formed finally. Together with the formation of the nanoemulsions, slight Ostwald ripening and coalescence took place after the formation of nanosized droplets, resulting in slightly larger droplets at room temperature. Stabilization of Concentrated Biodiesel Nanoemulsions. Usually, the major destabilization mechanism of nanoemulsions is Ostwald ripening.50 It is the net transport of oil through the continuous phase from smaller droplets to larger droplets. Ostwald ripening primarily occurs via the molecular dissolution of oil in the continuous phase. In fact, Ostwald ripening is driven by the correlation of emulsion droplet solubility with its droplet diameter, as shown in the Kelvin eq 1 ⎛ 2γVm ⎞ ⎟ C(r ) = C(∞)exp⎜ ⎝ rRT ⎠

Figure 9. Droplet diameter versus time for samples prepared at different O/S ratios.

nanoemulsions have been reported during the last 15 years. This is first of all due to the fact that nanoemulsions undergo breakdown with time.3,27 Here, nanoemulsions could spontaneously form by simply diluting W/O microemulsions when necessary. Hence, W/O microemulsions could be an preferred alternative to nanoemulsions during long-term storage.24 W/O microemulsions are thermodynamically stable systems that can remain stable during transport and storage. Compared with O/ W nanoemulsions, the manufacturing, transport and storage cost of W/O microemulsions are decreased. Therefore, this finding is significant in practical applications. In this work, the biodiesel O/W nanoemulsions formed by a W/O microemulsion dilution method could be used as new green lubricants in water-based drilling fluid (see the Supporting Information).

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CONCLUSION In the water/Tween 80/biodiesel system, concentrated green nanoemulsions could be prepared by a W/O microemulsion dilution method. The droplet size of the nanoemulsions decreases dramatically with the increase of the surfactant concentration of the system or the dilution temperature. Cryo-TEM and SAXS were used to examine and illustrate the mechanism of formation of concentrated nanoemulsions by a W/O microemulsion dilution method. The formation of nanoemulsions could be considered as two parts. First, the interfacial tension of the system is very low at high temperature. The addition of large amount of water results in a rapid increase in surfactant hydration. Therefore, nanosized droplets formed spontaneously due to the surfactant migration and inversion. Second, the nanosized droplets became more stable due to the decrease of the rate of coalescence and Ostwald ripening when cooled to room temperature. Therefore, the nanoemulsions formed at last. This study contributes to a better understanding on the mechanism for nanoemulsion formation by dilution methods. In addition, the major mechanism for the instability of these nanoemulsions is Ostwald ripening, which limits the practical applications of nanoemulsions. However, diluting a W/O microemulsion to form a concentrated nanoemulsion could give important guidance for practical applications such as pharmaceutical, cosmetic, and agrochemical applications, as well as drilling fluid. A nanoemulsion is thermodynamically unstable, whereas a microemulsion is thermodynamically stable. Hence, a W/O microemulsion can be used as an alternative to O/W nanoemulsion due to low storage and transport cost. More importantly, biodiesel can successfully replace traditional oil to meet the requirement of green chemistry and engineering.

(1)

where C(r) is the solubility of the emulsified oil, C(∞) is the oil bulk phase solubility, r is the emulsion radius, γ is the interfacial tension, Vm is the oil molar volume, R is the gas constant, and T is the absolute temperature. Generally, if there is a linear relationship between the cube of the emulsion radius (r3) and time, Ostwald ripening would be the major destabilization mechanism (Lifshitz−Slesov−Wagner (LSW) Theory) and the slope of this linear relationship is the Ostwald ripening rate, ω ω=

dr 3 8 ⎡ C(∞)γVmD ⎤ = ⎢ ⎥ ⎦ dt 9⎣ ρRT

(2)

where D is the diffusion coefficient of the dispersed phase in the continuous phase and ρ is the density of the dispersed phase. Figure 9 shows the droplet diameter of nanoemulsions as a function of storage time at 25 °C. All curves exhibit a linear relationship between r3 and time, which shows that the main driving force for instability is Ostwald ripening. The Ostwald ripening rate can be calculated from the slope of the linear plots, which increases with the increase of the droplet diameter. The increase of Ostwald ripening rate may be due to the decrease of the concentration of surfactants in the interface and the increase of interfacial tension. Because of Ostwald ripening, the droplets of nanoemulsions will grow with time, even resulting in nanoemulsions breaking. Nowadays, only a few studies on particular applications of F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00903. Carbon number distribution of biodiesel; details for SAXS data evaluation; applications of biodiesel nanoemulsions formed by W/O microemulsion dilution method in water-based drilling fluid (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Dejun Sun. Tel.: +86-531-88364749. Fax: +86-531-88365437. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant 21333005). We acknowledge Dr. Qintang Li and Prof. Xiao Chen (Shandong University, China) for assistance with the study of small-angle X-ray scattering.

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REFERENCES

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