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High-Speed Imaging Of The ph Drop In Aqueous solutions In Contact With Supercritical Co2 Segments

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This is the submitted version of a paper presented at MicroTAS - The 20th International Conference on Miniaturized Systems for Chemistry and Life Sciences, 9-13 October 2016 - Dublin, IRELAND.

Citation for the original published paper: Andersson, M., Hjort, K., Klintberg, L. (2016)

High-Speed Imaging Of The ph Drop In Aqueous solutions In Contact With Supercritical Co2 Segments.

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Topic No. 1.02 Droplets & Multiphase Systems Abstract Reference No: 0163 Oral

HIGH-SPEED IMAGING OF THE pH DROP IN AQUEOUS SOLUTIONS IN CONTACT WITH

SUPERCRITICAL CO2 SEGMENTS

Martin Andersson, Lena Klintberg and Klas Hjort Uppsala University, Sweden

A high-speed imaging system based on light absorption of bromophenol blue (BPB) pH sensitive dye in a glass high-pressure microchip is used to study the instantaneous dynamics of a pH drop in an aqueous phase in contact with segments of subcritical (liquid) and supercritical CO2. The dynamics of the pH-drop has been studied and

visualized, demonstrating acidification rates of up to 3.5 pH/s. INTRODUCTION

Multiphase fluid systems with supercritical CO2 have found interest for chemical processing, such as in particle

formation, and in extraction for food and pharmaceutical applications [1-2]. In the field of CO2 capture, properties

such as the equilibrated pH of brine in contact with supercritical CO2 have been investigated [3]. To study

dynamic processes of high-density CO2 in multiphase systems, high-pressure microfluidics can be used [4]. With

transparent high-pressure tolerant glass chips, analytical absorption spectroscopy be can be used to study supercritical fluids. The measuring of chemical concentration gradients by optical fibers or lenses at site specific locations on chip offer great potential and have been demonstrated for solvent extraction [5]. High-speed imaging offers advantages in determining fluid behavior and being able to add chemical information to such data would give valuable information. In this paper, a high-pressure system is presented that study high-speed fluid dynamics with added chemical information. By this method, the acidification of an aqueous solution in contact with sub- and supercritical CO2 is demonstrated.

EXPRIMENTAL

The high-pressure microfluidic system used consists of a borosilicate glass chip, figure 1, mounted between a 430 nm high-brightness LED light array and a stereoscope with a high-speed camera, enabling high-speed

transmission light imaging over a large view of field. The microfluidic chip consists of a T-junction and a 200 µm wide and 90 µm deep meander channel with a total length of 35 mm. Fabrication of the chips is done using wet etching, direct bonding and a thermal treatment, and details on the fabrication can be found elsewhere [6]. The experimental setup consists of two different high-pressure pumps, leading to two inlets on the chip, feeding liquid CO2, and water, respectively. A sample loop is connected to the water inlet making it possible to load the

chip with aqueous sample solutions. Using a backpressure of 80 bar, the temperature of the chip was either 25 or 47 °C, allowing the CO2 to go from subcritical to supercritical state. The sample solution has a pH of 7.5 and

contains 5 mM BPB. To calibrate the system, citric buffers, also containing 5 mM BPB were prepared at 4 pH levels, namely 2.76, 2.93, 3.19 and 4.39. Variable additions of NaCl to all solutions kept the ionic strength constant at 15 mM. The pH of calibration solutions and sample solution was verified with a pH-meter.

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By the use of the sample loop and water pump, the calibration solutions were injected into the flow system as the chips was held at a backpressure of 80 bar. While operating the LED at a constant power, and setting the high-speed camera to a fixed exposure of 410 µs with a frame rate of 2400 fps, the pixel-wise light intensity over each channel position was recorded as images for each of the calibration buffers s, as well as for a blank containing only water. Using Matlab, these calibration images where first overlapped and centered using Hough line recognition on the channel geometry, thereby removing possible errors resulting from movement and shaking of the image setup. Transmittance was calculated as the pixel intensity ratio between sample or calibration solution, and the blank. From this, the absorbance of BPB in the channel was extracted pixel-wise. Along the channels, cross-channel absorption averages were used to linearly calibrate the response. This is used to calculate the corresponding drop in pH of aqueous segments as they move along the channel. By measuring the speed of the CO2 segments, length coordinates of the channel are correlated with the elapsed time since the first contact

between the phases. The CO2 segments are filtered out by use of a peak identification tool and by averaging

several images together, a complete pH drop profile, over the channel could be was found. RESULTS

The monochromatic light leaves the aqueous solution transparent before contacting CO2. In figure 2a, when a CO2

segment enters the channel, no initial absorption is seen.

Figure 2. CO2 segments are formed at the T-junction, and no response is initially seen, a. As the CO2 segment grows, a dark triangular region associated with a pH change is seen to the right of the segment, b. The arrow shows the flow direction.

When the segment develops in figure 2b, a dark region in front of the segment is seen. As the protonated form of BTB absorbs light at 436 nm, the pixel intensity can be associated with a pH drop. Segments continue in the channel and become surrounded by an acid front and a less acid back, figure 3. The average pH drop rate over the length of the channel is 1.2 and 3.5 pH/s at 25 and 47 °C, respectively, figure 4.

Figure 3: Instantaneous pH around a CO2 segment taken 82 ms after contact with the aqueous phase at 81 mm/s.

Time since contact (s)

0 0.1 0.2 0.3 0.4 0.5 pH 3 3.2 3.4 3.6 3.8 4 4.2 subcritical CO2 25 °C supercritical CO2 47 °C

Figure 4: The pH drop profile over the channel as a function of time since contact between phases.

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The pH drop takes less than 0.5 s and is faster at the higher temperature. Figure 5 shows a visualization of the pH drop that this is reached while flowing at 11 mm/s and 60 mm/s.

Figure 5. Visualization of a pH drop as CO2 segments are introduced. Flow speed is 60 mm/s (left) and 11 mm/s (right). .

DISCUSSION

The observed pH drop reaches levels close to a pH of 3, the pH at saturated conditions at these pressures [3]. CO2

and water constitute the carbonate buffer system, where CO2 can dissolve in the water and form small amounts of

carbonic acid, H2CO3, which then deprotonates into hydrogen and carbonate ions. The total solubility of CO2 in

water is low, about 1100 ppm at 90 bar [7], and only a small portion form H2CO3, which can deprotonate and

acidify the water. The local, instantaneous, variations, figure 3, seen around CO2 segments close to the T-junction

indicate that a significant amount of CO2 enters the aqueous phase as the fluids meet. The kinetics of the

deprotonation reactions are several orders of magnitude faster than the formation of H2CO3 [8]. The observed

dynamics in the chip are therefore likely a result of the rate limiting formation of H2CO3, as well as the required

mass transfer. The necessary transfer of CO2 into the aqueous phase requires both mixing and diffusion over the

phase boundaries. References

1. Micronization processes with supercritical fluids: Fundamentals and mechanisms, A. Martín, M.J. Cocero, Adv. Drug Deliv. Rev., 60, 21-33, (2008)

2. Supercritical fluids: technology and application to food processing, G. Brunner, J. Food Eng., 67, 21-33, (2005)

3. In situ spectrophotometric determination of pH under geologic CO2 sequestration conditions: Method development and application, H. Shao, C. J. Thompson, O. Qafoku, and K. J. Cantrell, Environ. Sci. Technol., 47, 63–70, (2013)

4. Supercritical microfluidics: Opportunities in flow-through chemistry and materials science, S. Marre, Y. Roig, C. Aymonier, J. Supercri. Fluids, 66, 251 – 264, (2012)

5. An Integrated UV-Vis Microfluidic Device for the Study of Concentrated Solvent Extraction Reactions, Ciceri D, Nishi K, Stevens G W, Perera J M 2013 Solvent Extr. Res. Dev., Jpn. 20, 197 – 203, (2013)

6. Fracture strength of glass chips for high-pressure microfluidics, M, Andersson, K. Hjort, L. Klintberg J. Micromech. Microeng. (2016), accepted

7. Water Solubility in CO2 Mixtures: Experimental and Modelling Investigation, M. Ahmad, S. Gersen, Energy Procedia, 63, 2402-2411, (2014)

8. Stopped-flow studies of carbon dioxide hydration and bicarbonate dehydration in H2O and D2O Acid-Base and Metal Ion Catalysis, Y. Pocker, D. W. Bjorkquist, J. Am. Chem. Soc., 99, 6537–6543, (1977)

Acknowledment

Funding from the Swedish Research Council (contract no. 2011-5037) is acknowledged as well as the funding for the laboratory facilities by Knut and Alice Wallenberg Foundation.

References

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