ARTICLE pubs.acs.org/IECR
Microfluidic Extraction of Docosahexaenoic Acid Ethyl Ester: Comparison between Slug Flow and Emulsion Eiji Kamio,† Yu Seike,‡ Hidekazu Yoshizawa,‡ Hideto Matsuyama,† and Tsutomu Ono‡,* †
Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan ‡ Department of Environmental Chemistry and Materials, Okayama University, 3-1-1, Tsushima-naka, Okayama, Okayama 700-8530, Japan ABSTRACT: Microfluidic liquidliquid extraction of the ethyl ester of docosahexaenoic acid (DHA-Et) with silver ions was investigated using slug flow and a water-in-oil (W/O) emulsion as specific flow patterns possessing large interfacial areas. To compare the extraction rates of both systems, slug flow and a W/O emulsion with similar specific interfacial areas were prepared. Extraction rates for the systems were the same at 298 K. However, at 268 K, slug flow had a faster extraction rate than the W/O emulsion. Theoretical calculation follows the results for both systems at 298 K and for the slug flow system at 268 K, suggesting that the emulsion stabilizer inhibits complex formation between DHA-Et and silver ions at the O/W interface. A reduction in extraction rate was observed in the slug flow system at 268 K when the emulsion stabilizer was used. Slug flow can be prepared without stabilizer, offering advantages over emulsions in microfluidic extraction.
’ INTRODUCTION Liquidliquid extraction utilizing a microfluidic device has attracted increasing interest in recent years and could be useful for applications in areas as diverse as micrototal analysis systems (μ-TAS),13 nuclide separation systems,4 and mini- or microchemical plants.57 Specific flow patterns comprising two immiscible fluids, such as parallel laminar flow, emulsion, and slug flow, can be easily generated using microfluidic devices. The flow patterns are usually well-defined, and thus the interfacial areas of the flows are uniform, allowing simple and precise modeling of several chemical processes. Among the flow patterns above, emulsions and slug flow offer promising advantages in microfluidic liquidliquid extraction systems because of their large interface-to-volume ratios. This enhances extraction rates and allows a reduction in the size of the microfluidic extraction unit, which is essential for microchemical plants. In addition to high interface-to-volume ratios, emulsions and slug flow offer another interesting and characteristic property that enhances extraction rates in that they minimize the diffusion distance not only because of the narrow microchannel but also by internal circulation in the compartmentalized fluids. In addition to the above advantages, the microfluidic device itself has one further very important advantage: the temperature of the system can be rapidly and precisely controlled.810 Because the size of the unit is quite small, the energy input required for achieving and maintaining a desired temperature can be reduced. Therefore, a microchemical plant consisting of microfluidic devices is environmentally friendly and is also suitable for highly exothermic reactions and treatment of biomolecules, which are easily denatured at high temperature. In view of these advantages, we have investigated extraction of ethyl esters of N-3 polyunsaturated fatty acids (PUFA-Ets) using a microreactor.10,11 N-3 polyunsaturated fatty acids (PUFAs), for instance docosahexaenoic acid (DHA) and eicosapentaenoic r 2011 American Chemical Society
acid (EPA), are effective in maintaining the normal functioning of the brain and nervous system.12,13 For use as pharmaceutical products, they must be separated from other PUFAs and saturated fatty acids, and this must be done at low temperature because PUFAs are sensitive to oxidation. When a PUFA is oxidized, toxic oxidation products such as hydroperoxides and aldehydes, which can induce various diseases, including cancer, diabetes, and rheumatoid arthritis, are formed. Liquidliquid extraction of PUFAs with silver ions at low temperature offers an advantage among several possible purification techniques. In previous investigations, it was found that extraction of the ethyl ester of docosahexaenoic acid (DHA-Et, a model PUFA-Et) with silver ions was an exothermic reaction that progressed faster at lower temperature.11,14 Therefore, extraction of PUFA-Et with silver ions at low temperature has the potential to overcome oxidation during the purification process. A microfluidic device is an excellent tool for developing a suitable system for purification of PUFAs, and we previously demonstrated that DHA-Et could be effectively extracted by slug flow.10,11 The final objective of our research is to develop a microchemical plant in which PUFAs are effectively extracted and separated. Although we demonstrated the advantage of slug flow for the extraction of PUFA-Ets, we could not declare that slug flow offers the most effective flow pattern. As mentioned above, because emulsions also have a large interface-to-volume ratio, they should also be effective for the extraction of PUFA-Ets. Indeed, the advantages of using emulsions for liquidliquid extraction have been reported in many papers.5,1519 However, there has been no investigation comparing the extraction Received: October 31, 2010 Accepted: April 26, 2011 Revised: March 18, 2011 Published: April 26, 2011 6915
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efficiencies of slug flow and emulsion systems under identical conditions. Here, we investigate liquidliquid extraction of DHA-Et with silver ions, using slug flow and an emulsion and compare their extraction efficiencies or the purpose of identifying the best flow pattern for a microfluidic extraction device for extraction and separation of PUFA-Ets. The information obtained in this work is expected to be useful for the extraction of not only PUFA-Ets but also other chemical species.
’ EXPERIMENTAL SECTION Reagents. DHA-Et (>98%) was kindly supplied by Bizen Chemical Co., Ltd. (Okayama, Japan). Sorbitan monooleate (Span 80), which is an oil-soluble surfactant and effective emulsion stabilizer, was purchased from Aldrich. Other chemicals, such as silver nitrate, sodium nitrate, and n-heptane were of guaranteed grade and purchased from Wako Pure Chemical Industry Co., Ltd. (Osaka, Japan). The organic phase was prepared by dissolving DHA-Et in n-heptane. In some cases, Span 80 was also dissolved in the organic phase. Because it is necessary for the concentration of a surfactant to be somewhat higher than its critical micelle concentration (CMC) to keep an emulsion droplet stable in a microfluidic channel,21 the concentration of Span 80 was fixed at 2.0 wt %, which is higher than its CMC (1.8 105 mol/dm3 = 1.13 102 wt %22). The aqueous phase was prepared by dissolving silver nitrate and sodium nitrate in ultrapure water, with the ionic strength maintained at 4.0 103 mol/m3. In this study, we investigated the extraction behavior under two different silver ion concentrations: 2.0 103 mol/m3 at 288 and 298 K and 1.0 103 mol/m3 at 268 K. The use of differing temperatures for the two silver ion concentrations was necessary, due to experimental limitations. At 298 K, DHA-Et is barely extracted using an aqueous phase at 1.0 103 mol/m3 of silver ions.11 We therefore set the silver ion concentration at 2.0 103 mol/m3 at 298 K. On the other hand, the initial extraction rate is too rapid to follow at an aqueous phase silver ion concentration of 2.0 103 mol/m3 at 268 K.11 Therefore, we set the silver ion concentration to 1.0 103 mol/m3 at 268 K. Extraction of DHA-Et with Slug Flow Prepared with a Microreactor. The experimental setup for DHA-Et extraction with slug flow is described elsewhere.10,11 In this study, a glass chip with a T-shaped microchannel (MP4B-15, Toso Co., Ltd.) was used to prepare a slug flow. The inlets of the glass chip were connected to syringe pumps (BS-MD1001, Bioanalytical Systems, Inc.) with poly(tetrafluoroethylene) (PTFE) tubing, through which the aqueous and organic phases were fed. A PTFE tube of i.d. 5 104 m was connected behind the glass chip. The slug flow created in the glass chip was passed through the PTFE tube and DHA-Et was extracted from the organic phase to the aqueous phase. For the first 120 s, the solutions were passed through a PTFE tube of i.d. 2 103 m to reach a steady flow. After 120 s, the latter tube was closed and the stream was redirected to flow into the syringe through a poly(etheretherketone) (PEEK) tube of i.d. 1.3 104 m. Because the inner diameter of the PEEK tube (i.e., the inner volume) was small, the residence time of both phases was very short. Therefore, it could be considered that there was scant extraction of DHA-Et into the aqueous phase in the PEEK tube. As soon as the slug flow passed into the syringe, the organic and aqueous phases were quickly separated from one another. The aqueous phase was continuously removed from the bottom of the syringe using an HPLC pump so that little or no DHA-Et would be extracted in the syringe. After a
Figure 1. (a) Illustration of experimental setup for extraction of DHA-Et using a W/O emulsion. The width and depth of channels of the glass chip used to create the W/O emulsion were 4.40 104 and 2.00 104 m, respectively. (b) Schematic of the union tee separating the organic phase from the W/O emulsion. φ1 and φ2 were 1.6 103 and 5.0 104 m, respectively.
desired time, 5 107 m3 of the organic phase was collected and the concentration of DHA-Et was measured using a gas chromatograph equipped with a flame ionization detector (GC-14A, Shimadzu Co., Ltd.). In this study, all experiments were carried out twice to confirm reproducibility. The T-shaped glass chip, the PTFE tube behind the glass chip, the PEEK tube, and the syringe were housed in a constant temperature chamber (SU-261, ESPEC Co., Ltd.). The residence time was controlled by changing the length of the PTFE tube behind the T-shaped glass chip. Extraction of DHA-Et with W/O Emulsion Prepared with a Microreactor. The experimental setup for DHA-Et extraction with the W/O emulsion is shown in Figure 1. A glass chip with a Y-shaped microchannel (NH3-4Y, Toso Co., Ltd.) was used to prepare the W/O emulsion. The inlets of the glass chip were connected to syringe pumps with PTFE tube through which the aqueous and organic phases were fed. Behind the glass chip, a PTFE tube with i.d. 5 104 m was connected. The W/O emulsion created in the glass chip was fed into the PTFE tube, in which DHA-Et was extracted from the organic phase to the aqueous phase, with the residence time controlled by changing the tube length. Behind the PTFE tube, a union tee was connected. Because the W/O emulsion was stable and thus the phase separation of the aqueous and organic phases was not rapidly achieved, the syringe separator used for slug flow system could not be used. Therefore, the union tee was used to collect only the organic phase of the W/O emulsion. The layout and dimensions of the union tee are also shown in Figure 1. After a desired time, 5 107 m3 of the organic phase was collected and the concentration of DHA-Et was measured using the gas chromatograph. The Y-shaped glass chip, the PTFE tube behind the glass chip, and the union tee were housed in a constant temperature chamber. All experiments were carried out twice to confirm reproducibility.
’ THEORETICAL SECTION To analyze the uptake curves, we improved the theoretical model derived in our previous research.11 In the improved 6916
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considered to describe microfluidic extraction.7,11,20,23,24 Such internal circulation will also be generated in emulsion droplets flowing in the microchannel,25 even though the wall film does not exist. In the derivation of the theoretical model for microfluidic extraction with slug flow and a W/O emulsion, the effect of internal circulation was incorporated by assuming a thin boundary layer. In addition, the wall film was expressed as a thinner layer with thickness δorg,2, as shown in Figure 2c. We thus derived the theoretical model to predict the uptakes of DHA-Et for the W/O emulsion and slug flow systems according to the two-film model involving interfacial complexation, considering the following stepwise processes: (I) diffusion of DHA-Et from the bulk organic phase to the interface across the organic film, (II) complex-formation of DHA-Et and silver ions at the interface, and (III) diffusion of the extracted complex from the interface to the center of the aqueous droplets. Diffusion of DHA-Et through the Organic Film to the Interface. Using a linear driving force approximation, the mass transfer rate of DHA-Et through the organic film, Worg, can be expressed as follows: ! A1 Dorg A2 Dorg Worg ¼ þ ð½DHA-Etb ½DHA-ETi Þ ð1Þ δorg, 1 δorg, 2 where δorg,1 and δorg,2 are the mean thicknesses of the organic boundary layer and the organic wall film, respectively; Dorg is the diffusion coefficient of DHA-Et in each organic film; subscripts “b” and “i” denote the bulk phase and near the interface, respectively; the overbar denotes the organic phase; and A1 and A2 are the areas of the interfaces corresponding to the organic films with thicknesses δorg,1 and δorg,2, respectively. For the W/O emulsion, A2 is zero because most surfaces of the W/O emulsion droplet do not approach the inner surface of the channel. Complex-Formation Reaction of DHA-Et with Silver Ion near the Interface. Considering the stepwise complex-formation reaction, the rate of complex-formation of DHA-Et with silver ions near the interface, Wcom, can be written as follows: Wcom ¼ ðA1 þ A2 Þðk1 ½DHA-Eti ½Agþ i k2 ½comi Þ m
Figure 2. Schematic representations of DHA-Et concentration around the O/W interface of a single compartment (a) and effective O/W interface of the W/O emulsion (b) and slug flow (c).
model, more realistic descriptions for liquid films are considered and more convincing values of the diffusion coefficient are used for calculation. Derivation of the theoretical model is briefly described below. In the theoretical development, we considered the single flow units as shown in Figure 2 (schematic representations of DHA-Et concentration around the O/W interface of a single compartment (a) and the effective O/W interface of the W/O emulsion (b) and slug flow (c)). Derivation of the Theoretical Model. Mass transfer rates for liquidliquid extraction are often characterized conveniently in a two-film model, involving a complex formation at the liquid liquid interface. As described in our previous paper,11 it was suggested that the extraction behavior of DHA-Et with silver ions in the microfluidic device could be characterized by the two-film model involving interfacial complexation. However, it was also suggested that the inherent characteristics of slug flow, such as the existence of a liquid wall film and internal circulation, must be
ð2Þ
where “com” denotes the extracted complex; m is the apparent reaction order; and k1 and k2 are the apparent forward and reverse reaction rate constants, respectively. For the W/O emulsion, A2 is zero. The silver ion concentration can be considered to be constant because the amount of silver ions is significantly larger than that of DHA-Et under all of the experimental conditions in this study. That is, [Agþ]i = [Agþ]i,eq = [Agþ]b,0, where subscripts “eq” and “0” denote the equilibrium and initial states, respectively. Next, we consider the extraction equilibrium. The concentrations of chemicals near the interface at equilibrium should be the same as those in the bulk phases. In addition, Wcom is zero. Hence from eq 2, k2 ¼
k1 ½DHA-Etb, eq ½Agþ b, 0
m
½comb, eq
ð3Þ
The [com]b,eq term in eq 3 can be expressed as [DHA-Et]b,eq and [Agþ]b,0 with the following extraction equilibrium relationship: DHA-Et þ nAgþ a com; Kex Kex ¼ 6917
½comb, eq ½DHA-Etb, eq ½Agþ b, 0 n
ð4Þ ð5Þ
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where Kex is the extraction equilibrium constant based on eq 4 and n is the number of Agþ coordinated with a DHA-Et, which corresponds to the number of unsaturated double bonds in DHA-Et. Substituting eqs 3 and 5 into eq 2, Wcom is rewritten as follows: ! ½comi m þ Wcom ¼ ðA1 þ A2 Þk1 ½Ag b, 0 ½DHA-Eti Kex ½Agþ b, 0 n
Table 1. Parameters Used for Calculation of DHA-Et Uptakes with Microcompartmentalized Fluids value slug flow δaq
W/O emulsion
unit
2.5 104
2.5 104
m
4
δorg,1
2.5 10
2.5 104
m
δorg,2 r
1.06 106 2.5 104
1.65 104
m m
Diffusion of Extracted Complex in the Aqueous Droplets.
A1
2πr2
4πr2
m2
We express the mass transfer rate of the extracted complex in the aqueous droplets, Waq, as a linear driving force approximation.
A2
πrLaq
ð6Þ
Waq
ðA1 þ A2 ÞDaq ¼ ð½comi ½comb Þ δaq
ð7Þ
where Daq is the diffusion coefficient of the extracted complex in the aqueous compartments and δaq is the thickness of the aqueous boundary layer generated in the aqueous compartments by internal circulation. In eq 7, A2 is again zero for the W/O emulsion. Rate Equation for Overall Process. By assuming a quasisteady-state for the overall DHA-Et extraction process, it is considered that eqs 1, 6 and 7 are equal and also equal to the extraction rate of DHA-Et, -rDHA-Et. Thus, rDHA-Et can be written as follows: rDHA-Et ¼
½DHA-Etb
! ½comb n = þ Kex ½Ag b, 0
δorg, 1 δorg, 2 δaq 1 1 þ þ ðA1 δorg, 2 þ A2 δorg, 1 ÞDorg ðA1 þ A2 Þk1 ½Agþ b, 0 m ðA1 þ A2 ÞDaq Kex ½Agþ b, 0 n
Here, we consider the mass balance of DHA-Et. Because the amount of DHA-Et distributed in an aqueous phase and that adsorbed on the interface are much smaller than the amount of DHA-Et in an organic phase and the amount of extracted complex, the mass balance of DHA-Et can be written as follows: ½comb ¼ ð½DHA-Etb, 0 ½DHA-Etb Þ
Vorg Vaq
ð9Þ
where Vorg and Vaq are the volumes of organic and aqueous phases, respectively. By substituting eq 9 into eq 8, rDHA-Et can be expressed as follows: rDHA-Et ¼ Vorg ¼
d½DHA-Etb dt
ð1 þ βÞ½DHA-Etb β½DHA-Etb, 0 R
ð10Þ
where R¼
δorg, 1 δorg, 2 1 þ ðA1 δorg, 2 þ A2 δorg, 1 ÞDorg ðA1 þ A2 Þk1 ½Agþ b, 0 m Vorg δaq 1 1 ; β¼ þ Kex ½Agþ b, 0 n Vaq ðA1 þ A2 ÞDaq Kex ½Agþ b, 0 n ð11Þ
m8.5 mol1 s1
k0
2.88 10
Ea
5.43 10
ΔH
122.0
122.0
kJ mol1
ΔS
795.5
795.5
J K1 mol1
m n
2.5 6
2.5 6
4
2.88 10
23
5.43 10
4
kJ mol1
In addition, k1 in eq 11 can be written as a function of T by the following the Arrhenius equation: Ea k1 ¼ k0 exp ð12Þ RT where Ea and k0 are the overall activation energy and frequency factor, respectively, and were determined previously.11 Kex in eq 11 can also be expressed as a function of T by the following equation:
!
ð8Þ
m2
0 23
log Kex ¼
ΔSo ΔH o 1 2:303R 2:303R 3 T
ð13Þ
where R is the gas constant; ΔS and ΔH are entropy and enthalpy, respectively, for the extraction of DHA-Et with silver ions, as determined previously.10 Integrating eq 10 under the initial condition [DHA-Et]b = [DHA-Et]b,0 at t = 0, we derive the following equation: ½DHA-Etb ¼
β þ expfð1 þ βÞt=ðRVorg Þg ½DHA-Etb, 0 1þβ ð14Þ
We simulated the relationship between [DHA-Et]b and t for the microfluidic extraction utilizing the W/O emulsion and slug flow using eq 14. The parameters used in the calculation are listed in Table 1. To increase the validity of the simulation, reliable values of diffusion coefficients were used in the calculation, calculated for DHA-Et and extracted complex using the equation proposed by Wilke and Chang.26 Over the temperature range used in this study, Daq was calculated as 1.58 1010 to 3.32 1010 m2/s and Dorg was 1.12 109 to 1.25 109 m2/s. A wall film thickness, δorg2, of 1.06 μm was used, as calculated according to Bretherton’s law27 and approximately equal to the values reported in several papers.20,28 Among the parameters listed in Table 1, the boundary layer thicknesses δorg1 and δaq were the fitting parameters, set at 25 μm in accordance with our previous study.11
’ RESULTS AND DISCUSSION Comparison of Slug Flow and the W/O Emulsion. Examples of slug flow and the W/O emulsion created in this study are shown in Figure 3 panels a and b, respectively. These were 6918
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Table 2. Properties of Slug Flow and the W/O Emulsion Droplets and Experimental Conditions Used To Compare the Two Microcompartmentalized Fluids Properties of Slug Flow and the W/O Emulsion Droplet parameter slug flow W/O emulsion Laq (m)
4.5 104
Lorg (m) d.m. (m)
8.6 104 3.3 104 8.76 10
11a
Vorg (m )
1.70 10
10a
A (m2)
1.09 106a
3.42 107
A/Vorg (m )
6.45 10
9.09 103b
[SPAN80] (wt %)
0
3
Vaq (m ) 3
1
parameter
3a
2.0
Experimental Conditions Figure 4 a
3
1.88 1011
Figure 4 b
[Agþ]b,0 (mol/m )
1000
ionic strength (mol/m3)
4000
4000
[DHA-Et]b,0 (mol/m3)
10
10
2000
aq/org flow ratio (μL/μL)
50/100
50/100
T (K)
268
288
a
Figure 3. Slug flow (a) and W/O emulsion (b) prepared using the microreactor.
created with ultrapure water and n-heptane. Blue dye (Oil blue N) was dissolved in n-heptane to observe each phase easily. The length of each phase of the slug flow could be controlled by changing the flow rates of both phases. The coefficient of variance (CV) of the length of aqueous and organic phases for all slug flows was about 10%. Although the slug flows were created without emulsion stabilizer, they were stable within the PTFE tube. In addition, even though the compartments of the slug flow were small, for example, 4.5 104 m for the aqueous slug and 8.6 104 m for the organic slug, they could be easily and rapidly separated to lower (aqueous) and upper (organic) blocks (hereafter, separation of small compartmentalized segments to aqueous and organic blocks is referred to as O/W phase separation). The rapid O/W phase separation allowed us to use a simple minisettler in the microfluidic extraction unit. Alternatively, the membrane separator proposed by Kralj et al. is a promising device for O/W phase separation from a slug flow.29 In any case, it can be reliably stated that slug flow is easily handled and useful for microfluidic extraction. The size of droplets in the W/O emulsion was also controllable by changing the flow rates of both phases. In our investigation, the size of emulsion droplet could be controlled in the range of 3 105 to 3.3 104 m. The CV value of the droplet size was below 5%. The geometry of the W/O emulsion droplet was more well-defined than that of the slug flow but emulsion stabilizer was necessary to keep the W/O emulsion stable in the PTFE tube. The use of emulsion stabilizer was effective in stabilizing the W/O emulsion droplets but made O/W phase separation difficult. Even if the size of the water droplet was 3.3 104 m, it could not be readily separated from the organic phase and separation became more difficult as the size of the droplet became smaller. Considering the O/W phase separation properties, it is considered that slug flow would be more easily handled than the emulsion.
These values were calculated on the assumption that an organic wall film thickness of 1.06 μm was present around the aqueous slugs. b Vorg calculated from aq/org flow ratio (= 50/100) was used for calculation.
Subsequently, we compared the extraction efficiencies, such as the amount and rate of DHA-Et extraction for slug flow and the W/O emulsion. Properties of the slug flow and W/O emulsion and the experimental conditions used are listed in Table 2. The major difference between these was that the W/O emulsion and the slug flow were prepared with and without Span 80, respectively. Another minor difference was in the interface-to-volume ratio, whereby that of the W/O emulsion was 1.5 times larger than that of slug flow. Figure 4 panels a and b show the uptakes of DHA-Et with slug flow and the W/O emulsion. The theoretical uptakes of DHA-Et, calculated from eq 14, are also shown in Figure 4 by solid (for slug flow) and broken (for W/O emulsion) curves. As shown in Figure 4, the equilibrium concentrations of DHA-Et resulted in both slug flow and W/O emulsion systems being almost identical. They were also the same as those predicted from the theoretical calculation. Comparing the experimental data and calculated results, it was found that the theoretical uptakes followed the experimental data except in the case of the W/O emulsion at 268 K. Focusing on the result for the W/O emulsion at 268 K, as shown in Figure 4a, the experimental uptake exhibited a slower extraction rate than the calculated rate. Moreover, it was slower than that for slug flow. This was unexpected because the W/O emulsion used had a somewhat larger interface-to-volume ratio than the slug flow. As mentioned above, the differences in properties between the W/O emulsion and the slug flow were the interface-to-volume ratio and existence of Span 80 in the organic phase. Therefore, it was suggested that Span 80 affected the extraction rate for the W/O emulsion system at 268 K. Effect of Emulsion Stabilizer on Microfluidic Extraction. To confirm the effect of Span 80 on the extraction rate, we investigated DHA-Et extraction for the slug flows prepared by using the organic phases with and without Span 80. The properties of the slug flow and the experimental conditions are 6919
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Figure 4. Uptake curves of DHA-Et with the slug flow and W/O emulsion. The properties of the slug flow and W/O emulsion droplet and the experimental conditions of this investigation are summarized in Table 2. Calculated lines denote (a,b) not considering and (c,d) considering Span 80 adsorption on the O/W interface.
summarized in Table 3. The experimental uptakes are shown in Figure 5 panels a and b, along with the calculated results for slug flows with and without Span 80, shown by broken and solid lines, respectively. As shown in this figure, the experimental and theoretical uptakes were in good agreement, except for the case of slug flow with Span 80 at 268 K. This tendency was the same as for the W/O emulsion system described above and can be explained by adsorption of the emulsion stabilizer on the O/W interface. Emulsion stabilizer, which is a type of surfactant, adsorbs on the O/W interface and decreases the interfacial free energy. During the initial stage of extraction, some areas on which no surfactant adsorbs (hereafter referred to as free interface) may remain, and the incubation of the droplet along the channel leaves time for the surfactant to adsorb at the O/W interface. In the literature,21 it has been shown that the buildup of surfactant at the interface occurs in microfluidic channels on a time scale on the order of the typical time scale of droplet manipulation (mss). In addition, in the same literature, it is also shown that the surfactant does not accumulate uniformly on the surface of the droplets. Therefore, the available area for extraction decreases with the addition of the surfactant. However, it is hard to estimate the ratio of free interface to the total interface. Therefore, we assumed that the ratio of free interface reaches about 75% at 298 K when Span 80 is added; that is, Afree|298 = 3/4Atotal, where Afree|298 and Atotal are the areas of free and total interfaces, respectively. We now consider the effect of temperature on the free area. Naturally, the amount of surfactant adsorbed increases with decreasing temperature. That is, surfactants adsorb more densely at lower temperature. Therefore, the region occupied by a unit surfactant molecule becomes smaller at lower temperature. Accordingly, the ratio of free interface must become smaller at lower temperature. Estimating the ratio of the areas covered by a
Table 3. Properties of Slug Flow and Experimental Conditions for the Investigation of the Effect of Span80 Properties of Slug Flow parameter
value
Laq (m)
7.1 104
Lorg (m)
6.0 104 1.38 1010a 1.19 1010a
3
Vaq (m ) Vorg (m3)
1.50 106a
A (m2) 1
A/Vorg (m )
3.27 103a
[SPAN80] (wt %)
0 or 2.0
parameter
Experimental Conditions Figure 5a
3
Figure 5b
[Agþ]b,0 (mol/m )
1000
2000
ionic strength (mol/m3)
4000
4000
[DHA-Et]b,0 (mol/m3)
10
10
aq/org flow ratio (μL/μL)
50/100
50/100
T (K)
268
298
a
These values were calculated on the assumption that an organic wall film of thickness 1.06 μm was present around the aqueous slugs.
unit surfactant molecule at 268 K to that at 298 K from the data given in the literature,30 results in a value of about 4/5. This ratio can be simply regarded as the ratio of the free interface at 268 K to that at 298 K because the size of the surfactant molecule itself does not change. From the above considerations, the effective interfacial area at 268 K, A|268, becomes A|268 = 4/5A|298 = 3/5 6920
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Figure 5. Effect of Span 80 on DHA-Et uptake with slug flow. The properties of slug flow used for this investigation and experimental conditions are summarized in Table 3. Calculated lines denote (a,b) not considering and (c,d) considering Span 80 adsorption on the O/W interface.
Atotal = 0.6Atotal. The effective interfacial area at 288 K, A|288, was estimated as 0.71Atotal. Taking A|268 = 0.6Atotal, A|288 = 0.71Atotal and A|298 = 0.75Atotal as effective interfacial areas at 268, 288, and 298 K, respectively, the uptakes for both the W/O emulsion and slug flow created using an organic phase containing Span 80 were calculated. The results are shown in Figures 4(c), 4(d), 5(c) and 5(d). The theoretical uptakes for both systems corresponded well with the experimental data. Contribution of Interface-to-Volume Ratio on Microfluidic Extraction. Because an emulsion stabilizer is unnecessary in the slug flow system, rapid extraction and easy O/W phase separation can be achieved. That is, slug flow has more advantages than emulsion when their interface-to-volume ratios are almost the same. However, as shown in Figure 6, in which predicted uptake curves for different sized W/O emulsions and slug flows are shown, if we use an emulsion consisting of smaller sized droplets, the extraction rate becomes quite fast. If smaller emulsion droplets can be easily and rapidly coalesced and separated from the continuous phase after the extraction, an emulsion should be an excellent tool for microfluidic extraction. There have been some approaches to achieve O/W phase separation of emulsion in microfluidic devices.5,15,24,31 For example, application of an electric field was investigated to achieve rapid agglomeration and coalescence of emulsion droplets.15,31 Electrocoalescence is indeed a promising technology for continuous O/W separation but the device becomes complex. A simple device for easy and rapid O/W phase separation of the emulsion is worth investigating and developing. In the meantime, for slug flow, Figure 6 shows an interesting extraction property, shown in the result for the slug flow consisting of long aqueous and organic phases. Even if the length of the aqueous phase reaches 1.0 102 m, the extraction rate barely decreases. This property is peculiar to slug flow; the extraction rate for the W/O emulsion indicates monotonous
decrease with increasing size of the aqueous droplets. The plots in Figure 6 show the experimental data obtained under the corresponding conditions (length of aqueous and organic phases of 9.8 103 m and 5.6 103 m, respectively). The data are in good agreement with the predicted uptake curve. The reason the extraction rate did not change for various lengths of the aqueous and organic phases in the slug flow is because of the interface-to-volume ratio. The relationship between interface-to-volume ratio and the length of the aqueous phase slug is shown in Figure 7a,b. Figure 7 panels a and c show the relationship between the interface-to-volume ratio and the size of the W/O emulsion droplet. Here, the volume fraction of aqueous phase to organic phase, that is, their flow ratio, was kept constant. As we can see, the interface-to-volume ratio for not only slug flow but also W/O emulsion reaches a constant value with increasing slug length or size of a W/O emulsion droplet. However, the interface-to-volume ratios at the plateau region (L > 0.01 m for slug flow and droplet diameter . 0.1 m for the W/O emulsion) are different. The interface-to-volume ratio of slug flow asymptotically decreases to about 4000, because of the presence of the organic wall film in the slug flow. When the length of the slug increases, the interfacial area of the side of the aqueous slug becomes larger than that of both ends and thus becomes dominant in the total interfacial area. Under this condition, the interface-to-volume ratio is expressed by the following limit function: ! ( ) 2πðr δorg ÞL þ 2πðr δorg Þ2 A ¼ lim lim L f ¥ Vorg Lf¥ πðr δorg Þ2 Lðvorg =vaq Þ ¼ 6921
vaq 2 4000 r δorg vorg
ð15Þ
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Figure 6. Predicted uptake curves for different sized slugs (a) and W/O emulsion droplets (b). The length of the aqueous slug and the diameter of the W/O emulsion droplet are shown in each legend. In the calculation, the volume of the aqueous phase was twice that of the organic phase. Plots in panel a are the experimental results using slug flow; the lengths of the slugs were 8.8 mm for the aqueous slug and 4.3 mm for the organic slug.
Therefore, when the length of the slug increases, the extraction rate becomes almost constant and is relatively fast. Such long slugs should be useful in practical microfluidic extraction, because the O/W phase separation should become easier for longer slugs. On the other hand, the specific area of the W/O emulsion asymptotically decreases to 0. For a W/O emulsion, because the interfacial area is a quadratic function of the radius of an aqueous droplet and the volume of the organic phase is a cubic function, the interface-to-volume ratio is inversely proportional to the radius of an aqueous droplet. Therefore, when the size of an emulsion droplet becomes larger, the interface-to-volume ratio asymptotically decreases to zero: ! ( ) A 4πr 2 ¼ lim lim r f ¥ Vorg r f ¥ ð4πr 3 =3Þðvorg =vaq Þ ! 3 vaq ¼ lim 0 ð16Þ r f ¥ r vorg As the result, the extraction rate would also decrease with increasing aqueous droplet size. Therefore, in our opinion, large sized emulsions are not useful for practical microfluidic extraction.
’ CONCLUSIONS The microfluidic extraction of DHA-Et with silver ion was investigated using slug flow and a W/O emulsion as specific compartmentalized flow patterns. The slug flow was
Figure 7. Relationship between interface-to-volume ratio and length of the aqueous slug (a, b) or diameter of the W/O emulsion droplet (a, c).
controllable in size and stable in a PTFE tube without emulsion stabilizer. Because no emulsion stabilizer was used, separation of the aqueous and organic phases was rapid and easy. On the other hand, the W/O emulsion could not be stabilized in the PTFE tube without Span 80 (an emulsion stabilizer) and thus, rapid separation of the aqueous and organic phases was difficult. In addition, because Span 80 strongly adsorbed on the O/W interface at 268 K during extraction, the effective interfacial area for mass transfer was decreased. As a result, the extraction rate decreased when Span 80 was used. Therefore, slug flow is more advantageous from the points of view of easy handling and rapid extraction rate than an emulsion. In addition, from the theoretical calculation, it was predicted that the extraction rate for slug flow was virtually unchanged even if the lengths of the aqueous and organic phases were large. The theoretical analysis was confirmed by experiments. From this investigation, we conclude that slug flow is more useful than an emulsion for microfluidic extraction. 6922
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’ AUTHOR INFORMATION Corresponding Author
*Address: Department of Environmental Chemistry and Materials, Okayama University, 3-1-1, Tsushima-naka, Okayama, Okayama 700-8530, Japan. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors express their thanks to the Bizen Chemical Co., Ltd. for providing DHA-Et. This work was supported in part by the Cooperation of Innovative Technology and Advanced Research in Evolutional Area (CITY AREA), based on funds provided by the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) and administered by the Okayama Prefecture Industrial Promotion Foundation. ’ NOTATION A1 = area of interface for which the organic film has a thickness of δorg,1 (m2) A2 = area of interface for which the organic film has a thickness of δorg,2 (m2) com = extracted complex Daq = diffusion coefficient of extracted complex in aqueous laminar film (m2/s) Dorg = diffusion coefficient of DHA-Et in organic laminar film (m2/s) Ea = activation energy for forward reaction of complex formation (kJ/mol) k0 = frequency factor for forward reaction of complex formation (m8.5/(mol2.5s)) k1 = apparent rate constant for forward reaction of complex formation (m8.5/(mol2.5s)) k2 = apparent rate constant for backward reaction of complex formation (m/s) Kex = extraction equilibrium constant ((m3/mol)6) m = reaction order with respect to silver ions (-) n = number of Agþ ions coordinated with a DHA-Et molecule to form extracted complex (-) r = for slug flow: radius of PTFE tube behind a microchip (m), For emulsion: radius of W/O emulsion droplet (m) R = gas constant (J/(K 3 mol)) rDHA-Et = overall extraction rate of DHA-Et (mol/s) T = temperature (K) t = time (s) Vaq = volume of aqueous phase (m3) Vorg = volume of organic phase (m3) Waq = mass transfer rate of extracted complex through the aqueous film (mol/s) Wcom = complex formation rate of DHA-Et and silver ions at the interface (mol/s) Worg = mass transfer rate of DHA-Et through the organic film (mol/s) ΔH = enthalpy for PUFA-Et extraction (J/mol) ΔS = entropy for PUFA-Et extraction (J/(Kmol)) Subscripts
0 = initial state b = bulk phase eq = equilibrium state i = near interface org = organic phase
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Greek Letters
δaq = thickness of aqueous boundary layer (m) δorg,1 = thickness of organic boundary layer at both ends of slugs and around emulsion surface (m) δorg,2 = thickness of organic wall film for slug flow (m)
’ REFERENCES (1) Tokeshi, M.; Minagawa, T.; Kitamori, T. Integration of a microextraction system on a glass chip: Ion-pair solvent extraction of Fe(II) with 4,7-diphenyl-1,10-phenanthrolinedisulfonic acid and tri-noctylmethylammonium chloride. Anal. Chem. 2000, 72, 1711–1714. (2) Minagawa, T.; Tokeshi, M.; Kitamori, T. Integration of a wet analysis system on a glass chip: Determination of Co(II) as 2-nitroso-1naphthol chelates by solvent extraction and thermal lens microscopy. Lab Chip 2001, 1, 72–75. (3) Yeung, S. W.; Hsing, I.-M. Manipulation and extraction of genomic DNA from cell lysate by functionalized magnetic particles for lab on a chip applications. Biosens. Bioelectron. 2006, 21, 989–997. (4) Hotokezaka, H.; Tokeshi, M.; Harada, M.; Kitamori, T.; Ikeda, Y. System for high-level radioactive waste using microchannel chip-extraction behavior of metal ions from aqueous phase to organic phase in microchannel. Prog. Nucl. Energ. 2005, 47, 439–447. (5) Benz, K.; Jackel, K.-P.; Regenauer, K.-J.; Schiewe, J.; Drese, K.; Ehrfeld, W.; Hessel, V.; Lowe, H. Utilization of micromixers for extraction processes. Chem. Eng. Technol. 2001, 24, 11–17. (6) Burns, J. R.; Ramshaw, C. The intensification of rapid reactions in multiphase systems using slug flow in capillaries. Lab Chip 2001, 1, 10–15. (7) Kashid, M. N.; Agar, D. W. Hydrodynamics of liquidliquid slug flow capillary microreactor: Flow regimes, slug size and pressure drop. Chem. Eng. J. 2007, 131, 1–13. (8) Pohar, A.; Plazl, I. Laminar to turbulent transition and heat transfer in a microreactor: mathematical modeling and experiments. Ind. Eng. Chem. Res. 2008, 47, 7447–7455. (9) Lao, A. I. K.; Lee, T. M. H.; Hsing, I.-M.; Ip, N. Y. Precise temperature control of microfluidic chamber for gas and liquid phase reactions. Sensor. Actuat. A, Phys. 2000, 84, 11–17. (10) Seike, Y; Kamio, E; Ono, T; Yoshizawa, H. Extraction of ethyl ester of polyunsaturated fatty acids by utilizing slug flow prepared by microreactor. J. Chem. Eng. Jpn. 2007, 40, 1076–1084. (11) Kamio, E.; Seike, Y.; Yoshizawa, H.; Ono, T. Modeling of extraction behavior of docosahexaenoic acid ethyl ester by utilizing slug flow prepared by microreactor. AIChE J. 2010, 56, 2163–2172. (12) Colombo, J.; Kannass, K., N.; Shaddy, D. J.; Kundurthi, S.; Maikranz, J. M.; Anderson, C. J.; Blaga, O. M.; Carlson, S. E. Maternal DHA and the development of attention in infancy and toddlerhood. Child Dev. 2004, 75, 1254–1267. (13) Horrocks, L. A.; Yeo, Y. K. Health benefits of docosahexaenoic acid (DHA). Pharmacol. Res. 1999, 40, 211–225. (14) Teramoto, M.; Matsuyama, H.; Ohnishi, N.; Uwagawa, S.; Nakai, K. Extraction of ethyl and methyl esters of polyunsaturated fatty acids with aqueous silver nitrate solutions. Ind. Eng. Chem. Res. 1994, 33, 341–345. (15) Kralj, J. G.; Schmidt, M. A.; Jensen, K. F. Surfactant-enhanced liquidliquid extraction in microfluidic channels with inline electricfield enhanced coalescence. Lab Chip 2005, 5, 531–535. (16) Nakano, K.; Kato, S.; Noritomi, H.; Nagahama, K. Extraction of polyunsaturated fatty acid ethyl esters from sardine oil using Agþ-containing o/w/o emulsion liquid membranes. J. Membr. Sci. 1996, 110, 219–227. (17) Nakano, K.; Kato, S.; Noritomi, H.; Nagahama, K. Extraction of eicosapentaenoic acid ethyl ester from a model media using Ag(I)contaning o/w/o-type emulsion liquid membranes. J. Membr. Sci. 1997, 136, 127–139. (18) Kumemura, M.; Korenaga, T. Quantitative extraction using flowing nanoliter droplet in microfluidic system. Anal. Chim. Acta 2006, 558, 75–79. 6923
dx.doi.org/10.1021/ie102207c |Ind. Eng. Chem. Res. 2011, 50, 6915–6924
Industrial & Engineering Chemistry Research
ARTICLE
(19) Mary, P.; Studer, V.; Tabeling, P. Microfluidic droplet-based liquidliquid extraction. Anal. Chem. 2008, 80, 2680–2687. (20) Kashid, M. N.; Gerlach, I.; Goetz, S.; Franzke, J.; Acker, J. F.; Platte, F.; Agar, D. W.; Turek, S. Internal circulation within the liquid slugs of a liquidliquid slug-flow capillary microreactor. Ind. Eng. Chem. Res. 2005, 44, 5003–5010. (21) Baret, J. C.; Kleinschmidt, F.; El Harrak, A.; Griffiths, A. D. Kinetic aspects of emulsion stabilization by surfactants: A microfluidic analysis. Langmuir 2009, 25, 6088–6093. (22) Peltonen, L.; Hirvonen, J.; Yliruusi, J. The behavior of sorbitan surfactants at the wateroil interface: Straight-chained hydrocarbons from pentane to dodecane as an oil phase. J. Colloid Interface Sci. 2001, 240, 272–276. (23) Kashid, M. N.; Agar, D. W.; Turek, S. CFD modelling of mass transfer with and without chemical reaction in the liquidliquid slug flow microreactor. Chem. Eng. Sci. 2007, 62, 5102–5109. (24) Fries, D. M.; Voitl, T.; von Rohr, P. R. Liquid extraction of vanillin in rectangular microreactors. Chem. Eng. Technol. 2008, 31, 1182–1187. (25) Sotowa, K.; Katsuragi, H.; Kusakabe, K. Simulation and behaviors of internal flow in a droplet formed with a microchannel device. Kagaku Kogaku Ronbunshu 2004, 30, 173–176. (26) Wlike, C. R.; Chang, P. Correlation of diffusion coefficients in dilute solutions. AIChE J. 1955, 1, 264–270. (27) Bico, J.; Quere, D. Liquid trains in a tube. Europhys. Lett. 2000, 51, 546–550. (28) Kashid, M. N.; Renken, A.; Kiwi-Minsker, L. CFD modelling of liquidliquid multiphase microstructured reactor: Slug flow generation. Chem. Eng. Res. Des. 2010, 88, 362–368. (29) Kralj, J. G.; Sahoo, H. R.; Jensen, K. F. Integrated continuous microfluidic liquidliquid extraction. Lab Chip 2007, 7, 256–263. (30) Ríos, H. E.; Gonzalez-Navarrete, J.; Pe~na, M. L.; Sagredo, C. D.; Urzua, M. D. Adsorption of poly-2-(dimethylamino) ethylmethacrylateN-alkyl quaternized at the water/chloroform interface. Colloid Surf. A 2010, 364, 61–66. (31) Fidalgo, L. M.; Whyte, G.; Bratton, D.; F. Kaminski, C.; Abell, C.; Huck, T. S. W. From microdroplets to microfluidics: Selective emulsion separation in microfluidic devices. Angew. Chem., Int. Ed. 2008, 47, 2042–2045.
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