Tube Radial Distribution Phenomenon with a Two-Phase Separation

ANALYTICAL SCIENCES JUNE 2014, VOL. 30 687 2014 © The Japan Society for Analytical Chemistry

Notes

Tube Radial Distribution Phenomenon with a Two-phase Separation Solution of a Fluorocarbon and Hydrocarbon Organic Solvent Mixture in a Capillary Tube and Metal Compounds Separation

Koichi KITAGUCHI,* Naoya HANAMURA,* Masaharu MURATA,** Masahiko HASHIMOTO,* and Kazuhiko TSUKAGOSHI*,***†

*Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe 610–0321, Japan **Department of Advanced Medical Initiatives, Kyushu University, Fukuoka 812–8582, Japan ***Tube Radial Distribution Phenomenon Recearch Center, Doshisha University, Kyotanabe 610–0321, Japan

A fluorocarbon and hydrocarbon organic solvent mixture is known as a temperature-induced phase-separation solution. When a mixed solution of tetradecafluorohexane as a fluorocarbon organic solvent and hexane as a hydrocarbon organic solvent (e.g., 71:29 volume ratio) was delivered in a capillary tube that was controlled at 10°C, the tube radial distribution phenomenon (TRDP) of the solvents was clearly observed through fluorescence images of the dye, perylene, dissolved in the mixed solution. The homogeneous mixed solution (single phase) changed to a heterogeneous solution (two phases) with inner tetradecafluorohexane and outer hexane phases in the tube under laminar flow conditions, generating the dynamic liquid–liquid interface. We also tried to apply TRDP to a separation technique for metal compounds. A model analyte mixture, copper(II) and hematin, was separated through the capillary tube, and detected with a chemiluminescence detector in this order within 4 min.

Keywords Tube radial distribution phenomenon (TRDP), fluorocarbon and hydrocarbon organic solvent mixture, metal compound separation, capillary tube, chemiluminescence detection

(Received March 10, 2014; Accepted April 17, 2014; Published June 10, 2014)

inner and outer phases in the microspace, generating a specific Introduction dynamic liquid–liquid interface.11 The interface forms radially with a specific linear velocity in a capillary tube. We have An investigation of fluorocarbon organic solvent solutions, or previously investigated TRDP in chromatography, extraction, fluorous chemistry, has been reported since seminal work done chemical reaction, and mixing processes.11 The TRDP with the by Horváth and Rábai in 1994.1 Two-phase separation in other two-phase separation aqueous solution systems, such as fluorocarbon and hydrocarbon organic solvent mixed solutions water–surfactant and water–ionic liquid mixed solutions, were is known in the field of analytical chemistry and separation also reported in our previous paper.11 However, these science.2,3 Mixed solutions of fluorocarbon–hydrocarbon experimental data seemed to be just initial data in the field of organic solvents separate into two distinct phases in a batch the TRDP and related research, because TRDP involves a quite vessel when cooled below a certain temperature; that is, a novel microfluidic behavior that has never been investigated. temperature-induced phase separation occurs.4 The lower phase We have to continue examining the fundamental facts about the is an almost a fluorocarbon organic solvent phase, while the TRDP as much as we can. In this study, a fluorocarbon– upper phase is a hydrocarbon organic solvent phase. Based on hydrocarbon organic solvent mixed solution, not an aqueous this two-phase system, liquid–liquid and liquid–solid extractions solution, which showed two-phase separation in a batch vessel, have been reported using different fluorocarbon organic was fed into a capillary tube to explore the TRDP, for the first solvents.5–7 time. The finding here is that it is important to know the TRDP When ternary mixed solvents of water–hydrophilic/ capacity, and expand to the related technology. hydrophobic organic solvents (e.g., water–acetonitrile–ethyl acetate mixtures) are fed into a microspace, such as glass microchannels or capillary tubes (fused-silica, polyethylene, and Experimental PTFE for example)8–11 under laminar-flow conditions, the solvent molecules are radially distributed in the microspace. Reagents and materials This “tube radial distribution phenomenon” (TRDP) creates Water was purified with an Elix 3 UV system (Millipore Co., Billerica, MA). All reagents used were obtained commercially, † To whom correspondence should be addressed. and were of analytical grade. Tetradecafluorohexane (C6F14; Mw E-mail: [email protected] 338.04 and density 1.67 g cm–3) as a model fluorocarbon 688 ANALYTICAL SCIENCES JUNE 2014, VOL. 30

Fig. 1 Schematic diagram of the separation system of metal compounds with CL detection.

organic solvent and hexane (C6H14; Mw 86.18 and density 0.66 g cm–3) as a model hydrocarbon organic solvent were Fig. 2 Phase diagram of a tetradecafluorohexane–hexane mixed purchased from Wako Pure Chemical Industries, Ltd. (Osaka, solution that was examined in a batch vessel. Japan) and Nacalai Tesque, Inc. (Kyoto, Japan), respectively. The fluorescence dye perylene was purchased from Wako Pure Chemical Industries, Ltd. Also, cobalt(II) sulfate (Co(II)), copper(II) sulfate (Cu(II)), and hematin were purchased from Nacalai Tesque, Inc., Wako Pure Chemical Industries, Ltd., and temperature is under ca. 30°C.4 The phase diagram shown in Sigma-Aldrich Japan (Tokyo, Japan), respectively. A fused- Fig. 2, was constructed for varying the tetradecafluorohexane silica capillary tube (75 μm inner diameter and 150 μm outer mole fractions. The curve in the diagram marks the boundary diameter) was purchased from GL Sciences (Tokyo, Japan). between homogeneous (single phase) and heterogeneous (two phases) solutions. The tetradecafluorohexane–hexane mixed Fluorescence microscope-CCD camera system homogeneous solutions Nos. 1 – 9 in Fig. 2 were investigated. A fluorescence microscope-CCD camera with a fused-silica The heterogeneous solution includes two phases: the lower capillary tube (100 cm total length) was set up. Capillary tube tetradecafluorohexane and upper hexane phases in a batch vessel. observations were monitored at a length of 80 cm from the inlet For example, the 0.36 mole fraction tetradecafluorohexane– using a fluorescence microscope (BX51; Olympus, Tokyo, hexane solvent mixed solution (No. 4) was cooled from 30 to Japan) equipped with an Hg lamp, a filter (U-MWU2, ex. 330 – 10°C. Phase separation occurred at 10°C, generating an upper 385 nm, em. > 420 nm), and a CCD camera (JK-TU53H). Blue hexane phase (blue solution with perylene) and a lower fluorescence photographs were obtained because perylene emits tetradecafluorohexane phase. The lower-to-upper phase volume light at 470 nm. The capillary tube used for observation was ratio was estimated to be 8:9. Similarly, the volume ratios of maintained at a temperature of 10°C with a thermo-controller the tetradecafluorohexane to hexane phases in the heterogeneous (MATS-555RO; Tokai Hit Co., Shizuoka, Japan). solutions were (No. 1) 15:65; (No. 2) 29:71, (No. 3) 42:58, (No. 4) 47:53, (No. 5) 50:50, (No. 6) 63:37, (No. 7) 71:29, (No. 8) Separation system of metal compounds using an open-tubular 78:22, and (No. 9) 80:20. capillary tube We also examined homogeneous mixtures of octafluorotoluene– The microspace separation system of metal compounds was toluene and trifluorotoluene–toluene with fluorocarbon organic developed using an open-tubular capillary tube (100 cm length), solvent mole fractions of 0.3, 0.5, and 0.7 in the temperature a microsyringe pump (MF-9090; Bioanalytical Systems, Inc., range of 0 – 20°C. Under these conditions, no two-phase system West Lafayette, IN), and a batch-type chemiluminescence (CL) was observed. Consequently, in the present work we examined detection cell, as shown in Fig. 1. The mixed solution of the TRDP in the tetradecafluorohexane–hexane mixture in detail. tetradecafluorohexane and hexane (71:29 volume ratio) containing 50 μM luminol as a carrier solution was delivered TRDP creation into the tube at 1.8 μL min–1. The oxidant solution (40 mM The homogeneous tetradecafluorohexane–hexane solvent hydrogen peroxide dissolved in 10 mM carbonate buffer mixed solutions Nos. 1 – 9 were delivered into a capillary tube solution, pH 10.8) was put in a CL detection cell equipped with at a temperature of 10°C and at flow rates of 0.5 – 20 μL min–1. an optical fiber.12 The analyte solutions were prepared with the No tube radial distribution of the solvents was observed for carbonate buffer solution-acetonitrile (1:2 volume ratio) and solutions Nos. 1 – 6; the solvents were distributed in the axial introduced directly into the capillary inlet side by the gravity direction, as shown in Fig. 3a). On the other hand, for solutions method (from 20 cm height and for 20 s). The analytes, metal Nos. 7 – 9 the tube radial distribution of the solvents was clearly compounds, having catalytic activity for the luminol–hydrogen observed in the capillary tube at all flow rates. The fluorescence peroxide CL reaction, were mixed with the CL reagents at the images shown in Fig. 3b) indicate that the TRDP creates an tip of the capillary outlet in the cell to generate CL light. The inner major tetradecafluorohexane and outer minor hexane CL was detected with the photomultiplier tube (PMT) (H5783- phases; perylene (blue) was dissolved in hexane, but little 20, Hamamatsu Photonics K.K., Shizuoka, Japan). dissolved in tetradecafluorohexane. Fluorocarbon organic solvent does not mix with hydrocarbon organic solvent or water at a lower temperature. Then, the volume ratios of the two Results and Discussion phases in a batch-vessel and a capillary tube at a lower temperature agreed with the mixing ratios of the solvents. Also, Phase diagram the TRDP creation was confirmed in the microchannel (100 μm Tetradecafluorohexane–hexane mixture is a fluorocarbon– width and 45 μm depth) on a microchip made of glass. hydrocarbon organic solvent mixed solution; its phase-separation Our TRDP for solutions Nos. 7 – 9 followed the same trend as ANALYTICAL SCIENCES JUNE 2014, VOL. 30 689

Fig. 3 Fluorescence images of the microfluidic behavior of a tetradecafluorohexane–hexane mixed solution. a) Non-TRDP (No. 2 in Fig. 2, tetradecafluorohexane mole fraction 0.2) and b) TRDP (No. 7 in Fig. 2, tetradecafluorohexane mole fraction 0.6). Flow rate 5.0 μL min–1.

the TRDP from a ternary mixed solvent solution of the water– solvent solution of tetradecafluorohexane–hexane as a carrier. acetonitrile–ethyl acetate previously reported;11 major solvents The analyte solution of each metal compound was fed through were distributed around the middle of the tube, whereas minor the capillary tube and mixed with the reagents at the tip of the solvents were distributed near the inner wall, irrespective of capillary outlet to generate CL. The retention times of Cu(II), whether the carrier solutions were organic solvent-rich or Co(II), and hematin were 2.2, 2.2, and 2.7 min, respectively. water-rich phases. The linear velocity in the radial face section The retention times of Cu(II) and Co(II) were the same; they of the capillary tube under laminar flow conditions is circular, were delivered with almost the average linear velocity, while, indicating that the inside area of the tube has a lower velocity- that of hematin was delayed; it was fed with a lower velocity change gradient than the outside area; however, the real velocity than the average linear velocity. The analytes (Cu(II), Co(II), curve in an aqueous–organic solvent mixed solution may deviate and hematin) were dissolved in the aqueous and acetonitrile from an ideal velocity curve. Fluidic stabilities based on linear mixture to prepare the analyte solutions. The Cu(II) and Co(II) velocity gradients in the radial profiles under laminar flow were not dissolved in both tetradecafluorohexane and hexane, conditions infer that the major solvents must occupy the inside and then they were not distributed to the carrier solution in the area instead of the outside area in the capillary tube. capillary tube. The hematin was dissolved in hexane, but not Because fluorocarbon organic solvents have high viscosities, dissolved in the tetradecafluorohexane. It was then distributed stiff structures, and low surface tensions compared with to the hexane outer phase in the capillary tube. In the TRDP hydrocarbon organic solvents, these specific properties might with the tetradecafluorohexane and hexane mixed solvent, the support TRDP, thus allowing for major inner fluorocarbon hexane phase as an outer phase moved with much smaller organic solvent phase formation. The fluidic behavior of the velocity than the average linear velocity which was easily TRDP was also in accordance with the viscous-dissipation estimated as being a parabolic curve in the tube under laminar principle, which postulates that the degree of viscous dissipation flow conditions. Consequently, Cu(II) and Co(II) were eluted is smaller for a given flow rate. Thus, the high-viscosity fluid is with the average linear velocity, followed by the elution of located at the core of the tube, away from the inner wall.13–16 hematin with a lower velocity than the average linear velocity. However, why an axial distribution of the solvents occurs for Tentatively, the mixed analyte solution of Cu(II) and hematin solutions Nos. 1 – 6 has not yet been clarified. was subjected to the present separation system. The obtained CL separation profile is shown in Fig. 4. They were completely Microspace separation system for metal compounds separated; Cu(II) was first detected, and next hematin was The retention times of Cu(II), Co(II), and hematin as model detected. The separation in the capillary tube was performed metal compounds were examined with the microspace separation without applying any high-voltage like capillary electrophoresis system using the fused-silica capillary tube and the mixed and using specific columns, such as monolithic and packed 690 ANALYTICAL SCIENCES JUNE 2014, VOL. 30

reaction, using a fluorocarbon–hydrocarbon organic solvent mixed solution in a microspace.

Acknowledgements

This work was supported by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. It was also supported by “Advanced Study for Integrated Particle Science and Technology”, Strategic Development of Research Infrastructure for Private Universities, the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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