Multiphase Oscillatory Flow Strategy for in Situ Measurement and Screening of Partition Coefficients

Transcription

1 Supporting Information Multiphase Oscillatory Flow Strategy for in Situ Measurement and Screening of Partition Coefficients Milad Abolhasani, Connor W. Coley, and Klavs F. Jensen * Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, , Cambridge, Massachusetts 02139, United States Contents Section S1. Details of the Experimental Setup Figure S1. Photo of oscillatory flow setup Figure S2. Algorithm of the LabVIEW program for slug preparation and oscillation Figure S3. Algorithm of the LabVIEW program for spectral acquisition Section S2. Automated Serial Dilution-Caffeine Figure S4. In-situ obtained caffeine absorbance in DI water Section S3. Batch Scale Partition Coefficient Measurement Figure S5. Room temperature calibration curves for caffeine in (a) DI water and (b) 1-octanol Section S4. Acetaminophen Partition Coefficient Measurement Using the Oscillatory Flow Strategy Figure S6. In-situ time-evolution of absorption spectra of (a) acetaminophen and (b) caffeine S-1

2 S1. Details of the Experimental Setup The automated three-phase oscillatory flow strategy, shown in Figure S1, consists of a 12 cm long Teflon tube ( inch inner diameter, Fluorinated ethylene propylene, FEP) embedded within a custommachined aluminum chuck, a fiber-coupled LED (405 nm, Thorlabs Inc.) and photodetector (Si switchable gain detector, Thorlabs Inc.) as well as a fiber-coupled UV-Vis light source (DH-2000-BAL, Ocean Optics Inc.) and a miniature spectrometer (HR 2000+, Ocean Optics Inc.). A computer-controlled temperature controller (CN 9300, Omega Engineering Inc.) in combination with four cartridge heaters (Omega Engineering Inc.) embedded within the aluminum chuck (two on each side) and a thermocouple (STC-K type, Omega Engineering Inc.) sandwiched between the FEP tube and the top piece of the aluminum chuck are used for heating the reactor. A computer-controlled liquid handler (Gilson GX-241) is used to prepare the aqueous-phase slug, while a computer-controlled syringe pump (Harvard Apparatus) is used to inject the organic phase. The prepared bi-phasic slug is then moved into the oscillation zone of the experimental setup and oscillated back and forth between the two integrated fibers located at two ends of the FEP reactor for the pre-defined time and a set flow velocity. The change in the measured voltage of the photodetector is used as a threshold criterion to automatically switch the flow direction of syringe 1 (LabVIEW) when the slug is detected at the inlet, and the RMS absorbance using a gas reference spectrum is used as a threshold criterion to detect when the slug is at the outlet. The constant oscillatory motion of the bi-phasic slug ensures rapid solute exchange between the two phases. Figure S1. (a) The automated oscillatory flow strategy setup. (b) Enlarged view of the custom-machined aluminum chuck with the integrated fibers as well as the injection valve and sample loop. S-2

3 Slug motion and data acquisition were controlled by two LabVIEW scripts to eliminate the need for manual control and to improve reproducibility. The photodetector and spectrometer at the inlet and outlet of the reactor, respectively, provided a reliable trigger for automated reversal of carrier gas flow direction; in addition, by allowing for drifting baseline voltage and intensity, virtually infinite residence times were enabled independent of reactor volume and flow rate. The fiber-coupled UV-Vis spectrometer at the outlet recorded slug absorbance in both the aqueous and organic phases in addition to serving as a trigger for flow reversal. Spectra were obtained without interfering with slug motion (e.g., mixing properties) or physical sampling (i.e., changing volume). Aqueous slugs are prepared by first withdrawing 20 µl of gas into the liquid handler needle, followed by a 30 µl combination of samples with volumes specified in an Excel spreadsheet (input parameters), followed by an additional 20 µl gas buffer. The needle is connected to a syringe pump equipped with a 100 µl gas-tight glass syringe via a transfer line filled with DI water; the initial gas buffer serves to prevent dilution of the 30 µl slug. The aqueous preparation is mixed by a series of 5 refill/infuse cycles (20 µl each) to increase homogeneity and eliminate effects of sampling order. This slug is injected into a 15 µl sample loop, which is upstream of the aluminum chuck and downstream of the carrier syringe pump. Solvent-only wash slugs precede all experimental slugs to clean all tubing in addition to the injection valve. The preparation volume of 30 µl was chosen to be sufficiently large to fill all dead volume in the injection valve without excessive sample waste. The aqueous slug is then moved towards the T-junction placed after the aluminum chuck. An additional computer-controlled syringe pump equipped with a 250 µl gas-tight glass syringe was used for injection of organic solvent (5-15 µl). A computer-controlled syringe pump was equipped with an 8 ml stainless steel gas-tight syringe (Hamilton) to flow nitrogen (10-15 psig) as the continuous carrier phase. Clear FEP tubing (1/8 O.D., 1/16 I.D., McMaster Carr) was used in the experimental setup, while smaller tubing (1/16 O.D., 1/32 I.D.) was used everywhere else. The outlet tube was connected to a pressurized vessel maintained at a constant pressure (10-15 psig). A fiber-coupled LED (405 nm, 3.7 mw output, Thorlabs Inc.) was connected to the inlet of the aluminum chuck by standard SMA-threaded 400 µm core fibers (Ocean Optics). Opposite this LED was a fiber-coupled high-speed photodetector (Si switchable gain detector, Thorlabs Inc.). Photodetector voltage was read through a National Instruments data acquisition toolbox (USB-6009) directly into LabVIEW at a raw sampling rate of 48 khz smoothed down to 48 Hz. One fiber-coupled UV-Vis light source (DH-2000-BAL, Ocean Optics Inc.) was integrated into the right end of the aluminum chuck in an identical manner. Opposite this light source was a fiber-coupled S-3

4 miniature spectrometer (HR 2000+, Ocean Optics Inc.). Raw light intensity was read directly by LabVIEW at a sampling rate dependent on integration time (20 ms) and the flow velocity of the bi-phasic slug. The algorithm followed by LabVIEW is shown in Figure S2. Constant parameters including pump COM ports, syringe diameters, and flow rates were defined in an external spreadsheet file and parsed by a Matlab script within LabVIEW. Experimental conditions were defined directly in LabVIEW by specifying the volume of each aqueous slug to withdraw in the liquid handler (i.e., stock solution and pure solvent to form a suitable dilution). The slugs passed through the FEP tube, recorded an initial spectrum in the process, and were stopped at the T-junction for addition of the organic phase. Following the addition of the organic phase into the aqueous slug, the flow direction of the carrier syringe was revered (i.e., switched from infuse to withdraw). The carrier syringe pushed the bi-phasic slug back to the oscillation zone of the Teflon tube at a specified flow rate ( µl/min) until the slug was detected at the outlet, where the carrier flow rate was increased to an oscillatory flow rate ( µl/min); provided the slug did not break, higher flow rates were preferred due to increased mixing and decreased equilibration time. Residence times were defined by the number of full periods, although incomplete passes could be used to overcome this quantization. Detection of the bi-phasic slug at the inlet was based on a deviation threshold (20-40 mv) in photodetector voltage using a gas-phase baseline. This method was reliable independent of slug optical properties, because the linear flow velocity and photodetector sampling rate were such that reflection from the parabolic liquid-gas interface always produced step changes in excess of the threshold. Moreover, a dynamic baseline update was implemented to enable virtually infinite processing times. While the initial baseline was recorded immediately after parsing the parameter definition file, baseline voltages at the inlet and outlet were updated every cycle when the slug was detected at the outlet and inlet, respectively. Detection of the bi-phasic slug at the outlet was combined with data acquisition and was based on a previously-developed algorithm using RMS absorbance thresholds. As previously described, the bi-phasic slug before entering the spectral measurement point (placed at the right end of the aluminum chuck) was completely separated such that the aqueous phase was in front of the organic phase. The overall algorithm followed by LabVIEW is shown in Figure S2, with a detailed view of the outlet detection loop in Figure S3. All parameters were defined directly on the front panel, including the folder to write new spectra to, the reference spectra (dark and solvents), the integration time, interface delay, double detection delay, number of samples to average, and delay between samples. The wavelength range relevant to the experiment ( nm) was specified as well to allow for data truncation during processing. Absorbance was calculated using raw intensities by the following relationship: S-4

5 Sample( λ) Dark( λ) Absrobance( λ) = log Reference( λ ) Dark( λ ) (S1). Prior to beginning experimentation at each temperature, a dark reference spectrum and solvent (e.g., DI water and 1-octanol, co-saturated) spectra were obtained at a specific integration time (20 ms) using a separate custom-developed LabVIEW program. Figure S2. Overall algorithm of the LabVIEW program used for the preparation of the bi-phasic slug and the oscillatory motion of the prepared slug within the oscillation zone. During carrier gas infusion, a slug detection loop measured intensity semi-continuously (10 ms delay between spectra in addition to the integration time). This raw spectrum was processed against the gas reference to obtain an RMS absorbance value across the relevant wavelength range. Because the RMS absorbance fluctuated around zero (± 0.01) when gas was present, but increased significantly both at the liquid-gas interface and at the liquid itself, a threshold (0.08) was used as a trigger to indicate the presence of the bi-phasic slug. After a brief delay ( ms) to avoid measurement at the gas-liquid interface, a number of spectra (1-20) were averaged to reduce noise and processed against the aqueous reference. Spectra and corresponding residence times were saved in consecutively numbered data files for postprocessing. An inner slug detection loop immediately followed measurement of the aqueous phase, which was similarly constructed but used the newly-acquired aqueous spectrum as the reference for absorbance. When the aqueous phase was positioned in front of the spectrometer, the RMS absorbance remained close to zero, so an identical threshold was used to monitor when the aqueous-organic interface passed the detector. Recording of the organic absorbance spectra followed the same delay and smoothing logic. S-5

6 Figure S3. Algorithm of the LabVIEW program used for detection of the aqueous and the organic phases for in-situ UV absorption spectroscopy. S2. Automated Serial Dilution-Caffeine To ensure a linear relationship between the absorbance values and the concentration of the organic substance within the stock solutions (i.e., Beer-Lambert law), an automated serial dilution of the aqueous and the organic stock solutions were conducted to calculate the calibration constant, j, for each phase. Figure S4 shows the calibration curve obtained for caffeine in DI water at room temperature. Figure S4. In-situ obtained caffeine absorbance (A 273 A 290) in DI water at T=23 C for a systematic serial dilution of a stock solution with the initial concentration of C a=507 µm, slug volume of 15 µl, and an integration time of 20 ms averaged over 40 spectra. S-6

7 S3. Batch Scale Partition Coefficient Measurement Measurement of the partition coefficient for caffeine was repeated using the traditional shakeflask method for validation of the microfluidic method beyond comparison to literature values. The solvents and solute used were identical to those described in the manuscript. From the initial stock solutions (C a=101 µμ, C o=141 µm), a series of dilutions were prepared by weight; it was assumed that the volume change due to the presence of caffeine was negligible. Disposable UV-transparent cuvettes were used with a fiber-coupled cuvette holder (CUV-ALL-UV, Ocean Optics, Inc.) to eliminate any carry-over between samples. The light source and spectrometer were the same as in the oscillatory flow strategy to avoid any variability due to hardware choice. Linearity was observed over the relevant concentration range as shown in Figure S5. The regression coefficients, in units of µm -1, were found to be 9.86e-3 and 2.91e-3 for the aqueous phase, and 9.56e-3 and 2.94e-3 for the organic phase at 273 nm and 290 nm respectively. Figure S5. Room temperature calibration curves ( A 273 and A 290) obtained for caffeine in (a) DI water and (b) 1- octanol with an integration time of 1 ms, averaged over 100 spectra. Bi-phasic mixtures of aqueous stock solution and caffeine-free 1-octanol were prepared in 50 ml glass screw-top vials. Five volume ratios (V A=10, 20, 25, 30, 40 ml, V O=balance) were tested to ensure that there was no statistical dependence on volume ratio. Micro-emulsions were formed by using a Vortex mixer for 10 seconds, then samples were left sealed overnight to separate. Approximately 700 µl was sampled from each phase for absorption measurement. The room temperature (nominally 23 C) partition coefficient was found to be 0.77± S4. Acetaminophen Partition Coefficient Measurement Using the Oscillatory Flow Strategy Figure S6a shows the time-evolution of the in-situ obtained UV absorption spectra of acetaminophen within the aqueous and the organic phases for four different aqueous-to-organic volume ratios, R AO. Employing a custom-developed MATLAB script, the absorbance of acetaminophen within the S-7

8 aqueous and the organic phases at a defined wavelength was automatically extracted from each acquired absorption spectrum shown in Figure S6a and plotted in Figure S6b for different oscillation cycles (i.e., times). As shown in Figure S6b, similar to the data shown for caffeine (Figure 6a of the manuscript) by decreasing the volume of the organic phase, the equilibrium absorbance values of acetaminophen in the aqueous and the organic phases increase (i.e., the concentration of caffeine in both phases increase), while maintaining the total number of moles of acetaminophen the same within all bi-phasic slugs. Figure S6. (a) In-situ time-evolution of absorption spectra of acetaminophen in ( ) the aqueous and ( ) the organic phase for different aqueous-to-organic volume ratios. (b) In-situ obtained caffeine absorbance at A 265 A 290 within the aqueous, A, and the organic, O, phases for different aqueous-to-organic volume ratios: (A, O ) R AO=3, (A, O ) R AO=2, (A, O ) R AO=1.5, and (A, O ) R AO=1.2; C a (t 0)=602 µm, C o (t 0)=535 µm; V A=15 µl, Q osc=600 µl/min, t I=20 ms, N=10 and T=23 C. S-8