Non-contact concentration measurement of aqueous ethylene glycol solutions

doi:10.21203/rs.3.rs-4448573/v2

Download PDF

Research Article

Non-contact concentration measurement of aqueous ethylene glycol solutions

https://doi.org/10.21203/rs.3.rs-4448573/v2

This work is licensed under a CC BY 4.0 License

Version 2

posted

You are reading this latest preprint version

To evaluate the concentration dependence of THz spectra, THz time-domain spectroscopy (THz-TDS) was used to measure the transmittance of aqueous solution samples of ethylene glycol in the frequency region below 0.4 THz at room temperature, and the spectrum was analyzed in the region below 0.2 THz, where water absorption is small. The results showed that the interference spectral shape based on the dielectric constant change changed when the liquid concentration of the ethylene glycol sample was varied. Peak separation was performed for the peaks in the obtained spectra, and a comparison of the change in peak position with concentration showed a shift in the peak at 0.17 THz. This result indicates the possibility of non-contact concentration measurement using terahertz waves.

Terahertz spectra

Non-contact

Concentration measurement

Intermolecular interactions

There are two methods for measuring the concentration of liquids: refractometers, which calculate from the difference in refractive index of visible light, and specific gravity meters, which calculate from the difference in specific gravity due to concentration. Both are contact measurement methods in which a small amount of liquid is sampled and measured. On the other hand, the technology to measure concentration in a non-contact manner is still in its infancy. In recent years, there have been active research reports on fiber optic mutation sensors for opaque dye solutions [1], microwave sensing technology for real-time monitoring of liquid concentration [2,3], and RF sensors for microfluidics [4], but there are still few contactless concentration measurement methods that can be used generally. In light of this situation, we investigated the possibility of non-contact concentration measurement using terahertz waves, which have a higher frequency than microwaves. We performed terahertz wave transmission measurements on aqueous ethylene glycol solution samples and analyzed changes in spectral characteristics with changes in concentration. The advantage of the terahertz wave transmission over the visible region is that terahertz waves can be applied to opaque liquids, whereas visible light is scattered by solid impurities in the liquid and cannot be used for opaque liquids. Non-contact measurement of solvent concentration is also very important for the evaluation of electrochemical properties.

Terahertz wave properties of solid samples have been studied extensively and are expected to have a wide range of applications, such as nondestructive testing of cracks and reinforcing bars in building structures [5] and sorting of waste plastics [6]. Recently, the physical properties of liquid samples have also been elucidated [7, 8], and studies have been conducted on the measurement of water by terahertz spectroscopy and the measurement of broadband low-frequency spectra of water and glycine solutions by total reflection (ATR), a new non-contact measurement method for liquids [9 - 11]. Previous studies directly related to this work include Far-infrared dielectric properties of polar liquids by femtosecond terahertz pulse spectroscopy (J.T. Kindt, and C.A. Schmuttenmaer, J. Phys. Chem., 1996, 100, 10373-10379), Comparative dielectric study of monohydric alcohols with terahertz time-domain spectroscopy (Y. Yomogida, Sato, R. Nozaki, T. Mishina, and J. Nakahara, J. Mol. Struct. 981 (2010) 173-178), and Probing hydrogen-bonding in binary liquid mixtures with terahertz time-domain spectroscopy: a comparison of Debye and absorption analysis (N. Y. Tan, R. Li, P. Bräuer, C. D'Agostino, L. F. Gladdena, and J. Axel Zeitler, Phys. Chem. Chem. Phys. 17 (2015) 5999-6008.) exists. In a study by Kindt and coworkers, CH₃OH, C₂H₅OH, and C₃H₇OH were measured as molecules with different percentages of hydroxy groups (OH groups), and the influence of OH groups on the terahertz band properties was discussed [12]. In contrast, this study did not focus on the terahertz absorption property itself based on the OH group, although it was measured for an aqueous solution of ethylene glycol (C₂H₆O₂) containing the OH group. A quantitative discussion was made to estimate the concentration of the liquid sample indirectly by capturing the change in viscosity with the change in concentration as a change in the terahertz spectrum including interference. The novelty of this study lies in the fact that the THz spectra of high-viscosity samples were obtained and compared with those of samples with similar structure but lower viscosity, with the aim of indirectly estimating the concentration of liquid samples by capturing the change in viscosity with concentration as a change in THz interference spectrum. Yomogida and coworkers measured 14 monovalent alcohols in the frequency band of 0.2 to 2.5 THz using THz-TDS, and found no absorption peaks for each substance in the low frequency band, indicating a difference in absorption trends [13]. In addition, Tan and co-workers have also measured liquid samples in the frequency band of 0.2 to 2.5 THz using THz-TDS and reported that they obtained absorption spectra of diol-water, aprotic liquid-water and aprotic liquid-aprotic liquid mixtures [14]. While these studies were measured in the range of 0.2~2.5 THz, the present study focuses on the low frequency region below 0.2 THz. This band was chosen because the absorption of water in the terahertz band decreases rapidly below 0.2 THz while the absorption increases linearly above 0.2 THz [15]. In this study, transmission measurements were performed in the frequency band below 0.2 THz, where the absorption of water is relatively small, to obtain spectra of aqueous solutions containing interference fringes. Furthermore, we found that the shape of the transmission spectrum including interference fringes changed with changes in the viscosity of the aqueous solution. The idea of using terahertz waves as a non-contact concentration measurement method in this study was based on the fact that the dielectric constant of terahertz waves is affected by intermolecular interactions. Since differences in the viscosity of liquids are due to the magnitude of intermolecular interactions, if the viscosity of a liquid changes with a change in concentration, the intermolecular interactions of the liquid also change. We considered that the change in intermolecular interactions caused by the change in concentration of aqueous solution could be captured as a change in the interference spectrum of terahertz waves.

Aqueous ethylene glycol solutions were used at 60 %, 80 %, and 100 % mass percent concentrations with known viscosity at room temperature [16]. Water was used as the solvent because it is a polar molecule in which ethylene glycol is easily soluble, inexpensive, and easy to handle. Transmission measurements of these samples were performed at room temperature using THz-TDS, a spectroscopic method that measures the time waveforms of electromagnetic waves transmitted through and not transmitted through a sample, respectively, and obtains the absorption spectrum of the sample from the ratio of their Fourier transform spectra. For the measurements, THz-TDS used a Tera Prospector (Nippo Precision Co., Ltd.). The photoconductive antenna of the THz-TDS is of the dipole type and has a dynamic range signal-to-noise ratio of about 35 dB. The liquid cell was made of 0.2 mm thick z-cut quartz crystal, which is highly transparent to terahertz waves. The liquid cell was an assembled type and was checked to ensure that there were no air bubbles inside. Measurements were taken a total of eight times, alternating between sample and reference spectra, to confirm reproducibility. The obtained spectra were averaged and transmittance and absorbance were calculated.

Figure 1 shows the transmission spectrum of aqueous ethylene glycol solution: in the region above 0.2 THz, the absorption of water is large and the solution is difficult to transmit. On the other hand, below 0.2 THz, the absorption of water becomes small and the solution is easily transmitted. Therefore, it is possible to measure the transmittance regardless of the percentage of water in the aqueous solution, and a large difference in transmittance was observed for both of the two solutions. Figure 2 shows the absorbance spectrum of ethylene glycol. The absorbance was calculated from the transmittance based on Lambert–Beer law. The plots of the Yomogida et al. measurements are at about 0.05 THz intervals [13], whereas the present data were measured at about 0.005 THz intervals. The 10-fold difference in resolution results in very fine absorption peaks. These spectra contain interference fringes.

Based on the shape of the absorption spectrum including the interference component, we assumed that there were three absorption peaks and performed peak separation for the waveform located at 0.16~0.19 THz. In this study, we focused on the peak shift around 0.17 THz, which is the peak that shows the same trend of peak shift with concentration change in the three peaks obtained by peak separation. Figure 3 plots the peak shift around 0.17 THz for ethylene glycol. Peak separation results are also shown at the same time. As the reagent concentration increased, the peak position shifted toward the lower frequency side. This is a new finding in this paper. This means that changes in the spectral shape including interference fringes can capture changes in the intermolecular forces of aqueous solution, i.e., changes in viscosity.

Since this change in viscosity is associated with a change in the concentration of the aqueous solution, this finding has potential for terahertz wave applications in aqueous solution concentration measurement applications. The reason why the peak shifts to the lower frequency side as the amount of reagent increases is thought to be that the number of OH groups in the solution increases as the amount of reagent increases, and the intermolecular interaction increases, weakening the vibration. Conversely, as the amount of reagent decreases and the concentration decreases, the reagent molecules become isolated by water molecules. Therefore, the reagent molecules become more mobile (due to the decrease in viscosity) and the absorption peak shifts to the high frequency side. This result can be assumed to indicate that the absorption peak near 0.17 THz is correlated with the ease of movement of the molecules. In this experiment, there is a correlation between concentration and the ease of movement of the reagent molecules because the more the percentage of water molecules in the aqueous solution increases, the easier it is for the reagent molecules to move. Therefore, the possibility that the concentration of the aqueous solution can be estimated using the peak shift around 0.17 THz, which correlates with the ease of molecular motion, is demonstrated.

In this study, terahertz time-domain spectroscopy (THz-TDS) transmission measurements were performed on aqueous solution samples of ethylene glycol in the region below 0.4 THz to obtain aqueous solution spectra containing interference fringes. In order to capture slight changes in the spectral shape including the interference, peak separation was performed for the region below 0.2 THz, where the absorption of water is low. As a result, it was confirmed that the peak around 0.17 THz shifts to the lower frequency side as the concentration of the reagent in the ethylene glycol solution increases. This indicates that there is a correlation between the intermolecular interaction, i.e., the viscosity of the aqueous solution and the peak shift, and at the same time, the concentration of the aqueous solution can be captured from the position of the absorption peak near 0.17 THz. From this, we conclude that terahertz waves can be used for non-contact concentration measurement of highly viscous liquids.

The authors have no competing interests to declare that are relevant to the content of this article.

・Ethics approval and consent to participate

Approve of the ethical responsibilities of authors and consents to participate.

・Consent for publication

All authors agree to the content, all expressly agree to the submission, and consent has been obtained from the responsible authorities of the institute/organization where the work was performed to publication of the work.

・Data availability

Data and material are available on request.

・Competing interests

・The authors have no relevant financial or non-financial interests to disclose.

・The authors have no competing interests to declare that are relevant to the content of this article.

・All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

・The authors have no financial or proprietary interests in any material discussed in this article.

・Funding

This work was supported by Japan Science and Technology Agency, CREST (Grant number JPMJCR21L6).

・Author Contributions Statement

Kazuma Iwasaki, Sho Fujii and Tadao Tanabe contributed the analysis. Tsuyoshi Kimura and Masaya Yamamoto contributed the discussion. All authors read and approved the final manuscript.

・Acknowledgements

We thank Mr. Hiromasa Yamashita of Shibaura Institute of Technology and Mr. Toshiyuki Iwamoto of Nippo Precision Co., Ltd. for their technical assistance with the experiments.

[1] Krishnan G, Aziz MSA, Abdullah M, and Harun W (2021) Concentration measurement of opaque dye solution using a non-contact fiber displacement sensor. Optical Fiber Technology 65: 102624.　 https://doi.org/10.1016/j.yofte.2021.102624

[2] Hosseini N, Baghelani M (2021) Selective real-time non-contact multi-variable water-alcohol-sugar concentration analysis during fermentation process using microwave split-ring resonator based sensor. Sensors and Actuators A: Physical 325: 112695.

https://doi.org/10.1016/j.sna.2021.112695

[3] Baghelani M, Hosseini N, and Daneshmand M (2021) Non-contact real-time water and brine concentration monitoring in crude oil based on multi-variable analysis of microwave resonators. Measurement 177: 109286.　https://doi.org/10.1016/j.measurement.2021.109286

[4] Abbasi Z, Baghelani M, Nosrati M, Sanati-N A, and Daneshmand M (2020) Real-Time Non-Contact Integrated Chipless RF Sensor for Disposable Microfluidic Applications. IEEE Journal of Electromagnetics, RF and Microwaves in Medicine and Biology 4: 3. 171-178.

https://doi.org./10.1109/JERM.2019.2954219

[5] Nishiwaki T, Shimizu K, Tanabe T, Gardner D, and Maddalena R (2023) Terahertz (THz) Wave Imaging in Civil Engineering to Assess Self-Healing of Fiber-Reinforced Cementitious Composites (FRCC). JACT 21: 58-75.

https://doi.org./10.3151/jact.21.58

[6] Yu J, Liu X, Manago G, Tanabe T, Osanai S, and Okubo K (2022) New Terahertz Wave Sorting Technology to Improve Plastic Containers and Packaging Waste Recycling in Japan. Recycling 7: 5. 66.

https://doi.org/10.3390/recycling7050066

[7] Querry MR, Wieliczka DM, and Segelstein DJ (1991) “Water (H2O),” in Handbook of Optical Constants of Solids II, E. D. Palik, ed. Academic, San Diego, California, pp. 1059–1077.

[8] Shiraga K, Fujii Y, Koreeda A, Tanaka K, Arikawa T, and Ogawa Y (2021) Dynamical Collectivity and Nuclear Quantum Effects on the Intermolecular Stretching Mode of Liquid Water. J. Phys. Chem. B 125: 1632-1639.

https://doi.org/10.1021/acs.jpcb.0c10154

[9] Ogawa Y, Cheng L, Hayashi S, and Fukunaga K (2009) Attenuated total reflection spectra of aqueous glycine in the terahertz region. IEICE Electronics Express 6: 2. 117-121.

https://doi.org/10.1587/elex.6.117

[10] Hirori H, Yamashita K, Nagai M, and Tanaka K (2004) Attenuated Total Reflection Spectroscopy in Time Domain Using Terahertz Coherent Pulses. J. Appl. Phys 43: 10A. L1287.

https://doi.org/10.1143/JJAP.43.L1287

[11] Shiraga K, Suzuki T, Kondo N, Tanaka K, and Ogawa Y (2015) Hydration state inside HeLa cell monolayer investigated with terahertz spectroscopy. Appl. Phys. Lett. 106: 253701.

https://doi.org/10.1063/1.4922918

[12] Kindt JT, Schmuttenmaer CA (1996) Far-Infrared Dielectric Properties of Polar Liquids Probed by Femtosecond Terahertz Pulse Spectroscopy. J. Phys. Chem. 100: 10373-10379.

https://doi.org/10.1021/jp960141g

[13] Yomogida Y, Sato Y, Nozaki R, Mishina T, and Nakahara J (2010) Comparative dielectric study of monohydric alcohols with terahertz time-domain spectroscopy. J. Mol. Struct. 981: 173-178.

https://doi.org/10.1016/j.molstruc.2010.08.002

[14] Tan NY, Li R, Bräuer P, D'Agostino C, Gladden LF, and Zeitler JA (2015) Probing hydrogen-bonding in binary liquid mixtures with terahertz time-domain spectroscopy: a comparison of Debye and absorption analysis. Phys. Chem. Chem. Phys. 17. 5999.

https://doi.org/10.1039/C4CP04477K

[15] Pickwell E, Cole BE, Fitzgerald AJ, Wallace VP, and Pepper M (2004) Simulation of terahertz pulse propagation in biological systems. Appl. Phys. Lett. 84. 2190-2192.

[16] Jerome FS, Tseng JT, Fan LT (1968) Viscosities of aqueous glycol solutions. J. Chem. Eng. Data 13: 4. 496.

https://doi.org/10.1021/je60039a010

The authors declare no competing interests.

Supportingdata.pptx

Download PDF

Version 2

posted

You are reading this latest preprint version

Non-contact concentration measurement of aqueous ethylene glycol solutions

Status:

Version 2

Abstract

Figures

1. Introduction

2. Experiment

3. Results & discussion

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 2