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Experimental demonstration of a microwave non-thermal effect in DMSO-NaCl aqueous solution
Chemical Physics 528 (2020) 110523
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Experimental demonstration of a microwave non-thermal effect in DMSONaCl aqueous solution
T
Wenyan Tiana, , Zemin Lia, Li Wub ⁎
a b
College of Electronics and Information Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China
ARTICLE INFO
ABSTRACT
Keywords: Microwave non-thermal effect Electrical conductivity Resistance Design of experimental system
In this paper, a special experimental system was elaborately designed to probe the existence of microwave nonthermal effect. Based on the experimental system, the resistance of solution was precisely measured during microwave irradiation. In order to make a comparison with the results of microwave irradiation, resistance and electrical conductivity were respectively measured with the same experimental system and electrical conductivity meter under conventional water bath heating. Results showed that the resistances were all decreasing with temperature rising during conventional heating process, which was contrary to the results of microwave irradiation method. It proves the existence of a microwave non-thermal effect.
1. Introduction Since Gedye and Gigurer published the first article on the microwave irradiation to accelerate organic chemical reaction in 1986 [1–2], the use of microwave energy as an alternative to conventional heating had presented a great prospect in chemistry and chemical industry. More and more researchers began to study the microwave application of various chemical processes, such as organic synthesis [3], material sciences [4], nanotechnology [5], medicine [6] and food chemical engineering [7]. Over the past years, microwave irradiation has been demonstrated being able to dramatically reduce the processing time, enhance the product yield and purities compared to the conventional method [8,9]. However, there has been much controversy about the mechanism of the interaction between microwave and chemical reactants, and the exact reasons why and how the microwave was able to accelerate the chemical reactions have not been well understood. At present, two models of the chemical mechanism of microwave interaction with the irradiated material have been proposed to interpret the observations, namely thermal effects and non-thermal effects [10,11]. As for the thermal effects, it is generally accepted by all researchers that the purely thermal effect is the sole factor to accelerate chemical reactions, in other words, nothing but the consequence of rapid rise reaction temperature in a microwave field [12,13]. In contrast, the non-thermal effects which are resulting from a direct stabilizing interaction with specific molecules in the reaction medium that is not caused by a macroscopic temperature effect has led to a
⁎
controversial debate in the scientific community [14,15]. Loupy and others believe that microwave non-thermal effects do indeed exist, though they cannot yet be comprehensively explained [16]. Tellez and others have investigated the synthesis of poly obtained from 6FDA and BAPHF monomers in a low microwave absorbing p-dioxane solvent and proved that a microwave non-thermal effect is presented in the system [17]. Ahirwar and Tanwar have demonstrated that the microwavemediated ELISA which occurs in less than 5 min is due to microwave non-thermal effect and not an experimental flaw [10]. Some studies, however, have argued that there is no evidence to support the existence of microwave non-thermal effects. Shazman and Stuerga have theoretically analyzed the interaction between microwave fields and heated chemicals and finally denied the existence of non-thermal effect because the microwave energy (0.3–300 GHz, 10−6–10−3 eV) is too low to break hydrogen bonds [18,19]. Besides, Stuerga and Gaillard predicted that a measurable microwave non-thermal effect existed only when the electric field intensity is up to 107 V/m [19]. Herrero experimentally investigated microwave assisted four synthetic transformations and excluded the existence of the non-thermal effects [14]. Unfortunately, until now, the existence of non-thermal effects is still in dispute because of the difficulty in proving the phenomena [20]. It is generally believed that experiment is an effective way to demonstrate the non-thermal effects. A range of technical difficulties indicate that it is necessary to design special experimental set-up, system and method for the research of the non-thermal effects. The main reasons that make it difficult to obtain satisfactory
Corresponding author. E-mail address: [email protected] (W. Tian).
https://doi.org/10.1016/j.chemphys.2019.110523 Received 10 July 2019; Received in revised form 6 September 2019; Accepted 7 September 2019 Available online 09 September 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 528 (2020) 110523
W. Tian, et al.
experimental results in the current studies on microwave non-thermal effects are as follows: (1) Many experimental studies of non-thermal effects were carried out in household microwave oven or the modified ones. The experimental results always failed to give credible results because the frequency spectrum of the household microwave ovens changed randomly and the distributions of electric field was non-uniform. (2) The experimental method was sometimes unreasonable and there was difficulty in removing the thermal energy in the experimental process to maintain a constant temperature. (3) The temperatures were sometimes measured with an infrared radiation (IR) pyrometer that only measured the surface temperature of the sample rather than the inside ones [21]. In this paper, a special experimental transmission system for proving the existence of a microwave non-thermal effect on DMSONaCl aqueous solution have been carefully designed based on our previous ridged waveguide [22]. The distribution of electric field was uniform and the electric field intensity in solution was about 104 V/m, while the electrodes coupled negligibly small amount of unnecessary microwave energy. In order to keep a constant temperature in the process of experiment, a high-speed fluid solution was applied to exclude the possibility of microwave thermal effects caused by temperature rise. Furthermore, the temperature were measured by a UMI-8 optical fiber thermometer. The resistance as an indicator of microcosmic changes of solution was precisely measured. Dimethyl sulfoxide (DMSO) which is usually called “super solvent” [23,24] has been widely used in organic synthesis [25], petroleum processing [26], pharmaceutical production [27] and many other industries. DMSO is able to accept protons to form hydrogen bonds although it is a non-hydrogen bonding polar aprotic solvent, while the sodium chloride (NaCl) aqueous solution can release protons [28,29]. Accordingly, strong hydrogen bonds will be formed and the molecular network structure will be strengthened when the two solutions are mixed together [30,31]. Conventional hydrogen bonds play a crucial role in various chemical and biological processes such as stabilizing biomolecule structures, modulating specificity and speed of enzymatic reactions, and constructing supramolecular structures [32,33]. Therefore, the DMSO-NaCl aqueous solution is used in the experimental study in this paper.
energy. The Wheatstone bridge circuit including a galvanometer with the unit of μA as shown in Fig. 3 is used to accurately measure the resistance R variation resulted from the slight electrical conductivity change of the solution. In Fig. 3, resistances R1 = 10 KΩ and R2 = 100 Ω are fixed values. R is the resistance of DMSO-NaCl aqueous solution and RM is a variable resistor with a range of 0–1 KΩ. The resistance R = (R1/R2) × RM is determined by adjusting the RM so that the Wheatstone bridge attains balance and the pointer of galvanometer is at the position of zero. Besides, as given in Fig. 4, an electrical conductivity meter is used to measure the electrical conductivity of the solution, which can reflect the variation of the resistance R as it is inverse to the resistance. The experiments were carried out with 25 W microwave power at 5.8 GHz and the inlet flow rate of the glass pipeline is 15 m/s. The microwave power was on-off-modulated irregularly during the process of microwave irradiation. DMSO-NaCl aqueous solution was prepared with 20 g sodium chloride, 0.68 L de-ionized water and 0.894 L DMSO. The distributions of electric field intensity in the solution and around the electrodes at the cross-section of z = 0 mm of the ridged waveguide were calculated by multi-physics calculation method used in our previous article [22] and the results were given in Fig. 5. It can be seen that the electric field in the solution is uniform and the maximum value is 2.322 × 104 V/m. Nevertheless, the maximum value of electric field intensity around electrodes is only 7.9 V/m, meaning that the electrodes coupled negligibly small amount of unnecessary microwave energy and caused little effect to the measurement. The maximum temperature rise 0.037 °C is obtained by the multiphysics calculation. 3. Results and discussion The resistance R0 and temperature T0 at the initial moment and resistance R0′ and temperature T0′ after microwave irradiation were measured by the Wheatstone bridge circuit and the optic fiber thermometer when input power is 25 W and inlet speed is 15 m/s, and the results were given in Table 1. It is shown in Table 1 that the resistances R of the solution are all increased (ΔR = R0′ − R0 > 0) in every measurement, and the temperatures T increased (ΔT = T0′ − T0 > 0) in the process of microwave irradiation. Nevertheless, the temperature is almost invariable and the maximum value is only ΔTmax = 0.05 °C, which proves that the method of solution flowing with high-speed can availably remove the thermal energy rapidly from the experiment system. It is contrary to the generally accepted fact that the resistance of solution heated by conventional method reduces with temperature, indicating that the increased resistance may be induced by microwave non-thermal effect in the process of microwave irradiation. For determining whether the increase of the resistance R was caused by the pure rise of the temperature or the microwave non-thermal effect, it is necessary to measure the resistance R in the same experimental system but under the condition of heating by conventional water bath without microwave irradiation. The resistances R at different temperatures were measured by the Wheatstone bridge circuit and the results were plotted in Fig. 6. What’s more, the electrical conductivities σ of the solution were measured by an electrical conductivity meter when the solution was heated by a conventional water bath without microwave irradiation, and the experimental results were given in Fig. 7. It can be clearly seen from Fig. 6 that the resistance R is declining with the temperature increasing. From Fig. 7, it can be also concluded that the resistance R is reduced with the increase of temperature too, as the electrical conductivity σ that is opposite to resistance gets larger with temperature. It is worth noting that the variation rule of the resistance R in Fig. 7 is in accordance with the rule in Fig. 6. The experimental results obtained by the two methods with conventional water bath heating were all contrary to the rise rule of the resistance R
2. Experiment and methods The experimental system is shown in Fig. 1. Microwave with T10 mode is generated using a magnetron at a frequency of 5.8 GHz. A circulator connected with water load is employed to protect the source from the reflection power. With a dual directional coupler and an AV2433 microwave power meter as power monitoring, microwave power is finally fed into the ridged waveguide which is connected with a water load. A quartz glass pipeline (inner diameter 3 mm and length 60 mm) is placed in the hole which is designed in the ridged waveguide along y-direction as shown in Fig. 2. In order to eliminate the temperature rise in the process of microwave irradiation experiment, a selfpriming micro-diaphragm pump DP80 with a maximum flow rate of 5.5 L/min, namely 15 m/s at the inlet of glass tube, is used to pump the DMSO-NaCl aqueous solution into the quartz glass tube during the process of experiment. Meanwhile, two optical fibers placed at the inlet and outlet of the glass pipeline are used to precisely measure the temperatures by a UMI-8 optic fiber thermometer with an accuracy of 0.05 °C before and after microwave irradiation, by which the temperature rise can be easily obtained. Furthermore, because electric conductivity as an indicator of microcosmic change of solution is opposite to the resistance, resistance R is chosen as a macroscopic measurable parameter to study the microwave non-thermal effect. A pair of Pt electrodes (outside dimension 0.8 × 0.2 mm2) connected to the Wheatstone bridge circuit is inserted into the quartz glass pipeline to observe the resistance variations. The electrodes should not be too close to the ridge to prevent coupling too much unnecessary microwave 2
Chemical Physics 528 (2020) 110523
W. Tian, et al.
Fig. 1. The experimental system used for microwave heating. (a) schematic diagram; (b) photograph of actual experimental system.
of the solution under microwave irradiation obtained in Table 1. If the resistance R in Table 1 increases resulted from the purely the temperature rise, the resistance R must be decreased as those obtained by conventional heating method in Figs. 6 and 7, but this is not the case.
From the above analysis of the resistance variations, it is suggested that the increase of resistance of DMSO-NaCl aqueous solution under microwave irradiation is not caused by the increase of solution temperature, but the microwave non-thermal effect. Therefore, it is reasonable
Fig. 2. The ridged waveguide used in the experiment. 3
Chemical Physics 528 (2020) 110523
W. Tian, et al.
Fig. 3. The Wheatstone bridge circuit used to measure the resistance.
Fig. 5. The distribution of electric field when input power is 25 W and the inlet speed is 15 m/s. (a) electric field in the solution; (b) electric field around electrodes. Table 1 The resistance and temperature in DMSO-NaCl aqueous solution before and after microwave irradiation with input power 25 W and inlet speed 15 m/s.
Fig. 4. The electrical conductivity meter used to measure electrical conductivity.
T0 (°C)
R0 (KΩ)
T0′ (°C)
R0′ (KΩ)
ΔT (°C)
ΔR (KΩ)
17.25 17.05 16.85 16.75 16.65 15.95 15.80
7.42 6.92 6.79 6.74 6.70 6.54 6.50
17.30 17.10 16.90 16.75 16.70 15.95 15.90
7.58 7.12 6.89 6.82 6.76 6.59 6.54
0.05 0.05 0.05 0.00 0.05 0.00 0.05
0.16 0.20 0.10 0.08 0.06 0.05 0.04
to believe that a microwave non-thermal effect is present in our system. that the resistance was increased and the maximum temperature rise was restricted to be lower than 0.05 °C. In order to determine whether the increase in resistance was caused by the temperature rising, the resistance was measured with the same experimental system when the solution was heated by conventional water bath without microwave irradiation. Furthermore, the electrical conductivity which is in inverse proportion to the resistance was measured by electrical conductivity meter under the conventional water bath heating. Results showed that the resistances were all reduced with the increase of temperature when the solution was heated by conventional water bath. It was demonstrated that the microwave non-thermal effect rather than the thermal effect should be responsible for the increase in the resistance under microwave irradiation. A microwave non-thermal effect indeed exists in the DMSO-NaCl aqueous solution under microwave irradiation.
4. Conclusion The present study examined the possibility of microwave nonthermal effect due to microwave radiation on the DMSO-NaCl aqueous solution. For this purpose, a special experimental system was designed carefully. The resistance and temperature of the solution were precisely measured by the Wheatstone bridge circuit and optical fiber thermometer, respectively. The experiment was performed with 25 W microwave power at 5.8 GHz, and the solution flowing at a high-speed of 15 m/s removed the thermal energy from the process of microwave irradiation. Based on this experimental system, the resistance as an indicator of microstructure change of the solution was precisely measured by the Wheatstone bridge circuit in the process of low-lever microwave irradiation at the order of 104 V/m, and the results showed 4
Chemical Physics 528 (2020) 110523
W. Tian, et al.
[2] R.J. Giguere, T.L. Bray, S.M. Duncan, G. Majetich, Application of commercial microwave ovens to organic synthesis, Tetrahedron Lett. 27 (1986) 4945–4948. [3] L.W. Jiang, J. Zhou, X.Z. Yang, X.N. Peng, X.F. Yu, Microwave-assisted synthesis of surface-passivated doped ZnSe quantum dots with enhanced fluorescence, Chem. Phys. Lett. 510 (2011) 135–138. [4] C. Singhal, Q. Murtaza, Parvej, Microwave sintering of advanced composites materials: a review, Mater. Today: Proc. 5 (2018) 24287–24298. [5] P. Vahdatkhah, H.R.M. Hosseini, A. Khodaei, A.R. Montazerabadi, H. Delavari, Rapid microwave-assisted synthesis of PVP-coated ultrasmall gadolinium oxide nanoparticles for magnetic resonance imaging, Chem. Phys. 453–454 (2015) 35–41. [6] S. Seo, I. Han, H.S. Lee, J.J. Choi, E.H. Choi, K. Kim, G. Park, K. Kim, Antibacterial activity and effect on gingival cells of microwave-pulsed non-thermal atmospheric pressure plasma in artificial saliva, Sci. Rep. 7 (2017) 1–11. [7] E. Malinowska-Pańczyk, K. Królik, K. Skorupska, M. Puta, B. Kiełbratowska, Microwave heat treatment application to pasteurization of human milk, Innov. Food Sci. Emerg. Technol. 52 (2019) 42–48. [8] J.L. Klinger, T.L. Westover, R.M. Emerson, C.L. Williams, S. Hernandez, G.D. Monson, J.C. Ryan, Effect of biomass type, heating rate, and sample size on microwave-enhanced fast pyrolysis product yields and qualities, Appl. Energy 228 (2018) 535–545. [9] A.J. Buttress, J.M. Rodriguez, A.U.R.S. Ferrari, C. Dodds, S.W. Kingman, Production of high purity silica by microfluidic-inclusion fracture using microwave pre-treatment, Miner. Eng. 131 (2019) 407–419. [10] R. Ahirwar, S. Tanwar, U. Bora, P. Nahar, Microwave non-thermal effect reduces ELISA timing to less than 5 minutes, RSC Adv. 6 (2016) 20850–20857. [11] X. Zhang, D.O. Hayward, D.M.P. Mingos, Effects of microwave dielectric heating on heterogeneous catalysis, Catal. Lett. 88 (2003) 33–38. [12] C.O. Kappe, A. Stadler, Microwaves in Organic and Medicinal Chemistry, first ed., Wiley-VCH Verlag, Weinheim, Germany, 2005. [13] C.O. Kappe, D. Dallinger, Controlled microwave heating in modern organic synthesis: highlights from the 2004–2008 literature, Mol. Divers. 13 (2009) 71–193. [14] M.A. Herrero, J.M. Kremsner, C.O. Kappe, Nonthermal microwave effects revisited: on the importance of internal temperature monitoring and agitation in microwave chemistry, J. Org. Chem. 73 (2008) 36–47. [15] C. Zhai, N. Teng, B. Pan, J. Chen, F. Liu, J. Zhu, H. Na, Revealing the importance of non-thermal effect to strengthen hydrolysis of cellulose by synchronous cooling assisted microwave driving, Carbohydr. Polym. 197 (2018) 414–421. [16] A. Loupy, Microwaves in Organic Synthesis, Wiley-VCH Verlag, Weinheim, Germany, 2006. [17] H.M. Tellez, J.P. Alquisira, C.R. Alonso, J.G.L. Cortés, C.A. Toledano, Comparative kinetic study and microwaves non-thermal effects on the formation of poly(amic acid) 4,4′-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA) and 4, 4′(Hexafluoroisopropylidene) bis(p-phenyleneoxy) dianiline (BAPHF). Reaction activated by microwave, ultrasound and conventional heating, Int. J. Mol. Sci. 12 (2011) 6703–6721. [18] A. Shazman, S. Mizrahi, U. Cogan, E. Shimoni, Examining for possible non-thermal effects during heating in a microwave oven, Food Chem. 103 (2007) 444–453. [19] D.A.C. Stuerga, P.J. Gaillard, Microwave athermal effects in chemistry: a myth’s autopsy part I: historical background and fundamentals of water matter interaction, J. Microwave Power EE 31 (1996) 87–100. [20] Z. Chen, Y. Li, L. Wang, S. Liu, K. Wang, J. Sun, B. Xu, Evaluation of the possible non-thermal effect of microwave radiation on the inactivation of wheat germ lipase, J. Food Process Eng. 40 (2017) 1–11. [21] C. Eskicioglu, N. Terzian, K.J. Kennedy, R.L. Droste, M. Hamoda, Athermal microwave effects for enhancing digestibility of waste activated sludge, Water Res. 41 (2007) 2457–2466. [22] W.Y. Tian, K.M. Huang, An improved experimental set-up based on ridged waveguide for microwave non-thermal effects, J. Instrum. 6 (2011) 1–13. [23] X.Q. Yang, L.J. Yang, K.M. Huang, W.Y. Tian, H. Shang, Experimental and the theoretical studies of the dielectric properties of DMSO-H2O mixtures, J. Solut. Chem. 39 (2010) 849–856. [24] T. Guo, C.G. Lim, M. Noshin, J.P. Ringel, J.P. Fisher, 3D printing bioactive PLGA scaffolds using DMSO as a removable solvent, Bioprinting 10 (2018) 1–8. [25] D. Schleicher, H. Leopold, T. Strassner, Ruthenium (II) DMSO complexes with C^C* cyclometalated phenylimidazol NHC ligands, J. Organomet. Chem. 829 (2017) 101–107. [26] S.K. Yadav, A. Basaiahgari, R.L. Gardas, Understanding the solvation behavior of SO3H functionalized Brønsted acidic ionic liquids in water and DMSO: Volumetric and acoustic approach, J. Mol. Liq. 266 (2018) 269–278. [27] S.M. Rosolina, J.Q. Chambers, C.W. Lee, Z. Xue, Direct determination of cadmium and lead in pharmaceutical ingredients using anodic stripping voltammetry in aqueous and DMSO/water solutions, Anal. Chim. Acta 893 (2015) 25–33. [28] M. Chalaris, S. Marinakisa, D. Dellis, Temperature effects on the structure and dynamics of liquid dimethyl sulfoxide: a molecular dynamics study, Fluid Phase Equilib. 267 (2008) 47–60. [29] W.L. Reynolds, Dimethyl sulfoxide in inorganic chemistry. Progress in Inorganic Chemistry, first ed., John Wiley & Sons Inc, New York, USA, 1970. [30] A.M. Huang, C.L. Liu, L. Ma, Z.F. Tong, R.S. Lin, Effects of clustering structure on volumetric properties of amino acids in DMSO plus water mixtures, J. Chem. Thermodyn. 49 (2012) 95–103. [31] N.S. Venkataramanan, A. Suvitha, Nature of bonding and cooperativity in linear DMSO clusters: A DFT, AIM and NCI analysis, J. Mol. Graphics Modell. 81 (2018) 50–59. [32] H. Wang, M. Wang, E. Liu, M. Xin, C. Yang, DFT/TDDFT study on the excited-state hydrogen bonding dynamics of hydrogen-bonded complex formed by methyl cyanide and methanol, Comput. Theor. Chem. 964 (2011) 243–247. [33] Q. Li, N. Wang, Z. Yu, Solvent effect on the role of methyl groups in formation of O⋯HO hydrogen bond in dimethyl ether–methanol complex, J. Mol. Struct. Theochem 862 (2008) 74–79.
Fig. 6. Change of resistance of solution measured by Wheatstone bridge circuit as a function of temperature heating by conventional water bath.
Fig. 7. Change of electrical conductivities of solution measured by electrical conductivity meter as a function of temperature heating by conventional water bath.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi [OIT]; the National Natural Science Foundation of China [No. 61401298]; Scientific and Technological Innovation Team of Shanxi Province [201705D131025] and the Key Innovation Team of Shanxi 1331 Project. References [1] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, The use of microwave ovens for rapid organic synthesis, Tetrahedron Lett. 27 (1986) 279–282.
5
Contents lists available at ScienceDirect
Chemical Physics journal homepage: www.elsevier.com/locate/chemphys
Experimental demonstration of a microwave non-thermal effect in DMSONaCl aqueous solution
T
Wenyan Tiana, , Zemin Lia, Li Wub ⁎
a b
College of Electronics and Information Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China College of Electronics and Information Engineering, Sichuan University, Chengdu 610064, China
ARTICLE INFO
ABSTRACT
Keywords: Microwave non-thermal effect Electrical conductivity Resistance Design of experimental system
In this paper, a special experimental system was elaborately designed to probe the existence of microwave nonthermal effect. Based on the experimental system, the resistance of solution was precisely measured during microwave irradiation. In order to make a comparison with the results of microwave irradiation, resistance and electrical conductivity were respectively measured with the same experimental system and electrical conductivity meter under conventional water bath heating. Results showed that the resistances were all decreasing with temperature rising during conventional heating process, which was contrary to the results of microwave irradiation method. It proves the existence of a microwave non-thermal effect.
1. Introduction Since Gedye and Gigurer published the first article on the microwave irradiation to accelerate organic chemical reaction in 1986 [1–2], the use of microwave energy as an alternative to conventional heating had presented a great prospect in chemistry and chemical industry. More and more researchers began to study the microwave application of various chemical processes, such as organic synthesis [3], material sciences [4], nanotechnology [5], medicine [6] and food chemical engineering [7]. Over the past years, microwave irradiation has been demonstrated being able to dramatically reduce the processing time, enhance the product yield and purities compared to the conventional method [8,9]. However, there has been much controversy about the mechanism of the interaction between microwave and chemical reactants, and the exact reasons why and how the microwave was able to accelerate the chemical reactions have not been well understood. At present, two models of the chemical mechanism of microwave interaction with the irradiated material have been proposed to interpret the observations, namely thermal effects and non-thermal effects [10,11]. As for the thermal effects, it is generally accepted by all researchers that the purely thermal effect is the sole factor to accelerate chemical reactions, in other words, nothing but the consequence of rapid rise reaction temperature in a microwave field [12,13]. In contrast, the non-thermal effects which are resulting from a direct stabilizing interaction with specific molecules in the reaction medium that is not caused by a macroscopic temperature effect has led to a
⁎
controversial debate in the scientific community [14,15]. Loupy and others believe that microwave non-thermal effects do indeed exist, though they cannot yet be comprehensively explained [16]. Tellez and others have investigated the synthesis of poly obtained from 6FDA and BAPHF monomers in a low microwave absorbing p-dioxane solvent and proved that a microwave non-thermal effect is presented in the system [17]. Ahirwar and Tanwar have demonstrated that the microwavemediated ELISA which occurs in less than 5 min is due to microwave non-thermal effect and not an experimental flaw [10]. Some studies, however, have argued that there is no evidence to support the existence of microwave non-thermal effects. Shazman and Stuerga have theoretically analyzed the interaction between microwave fields and heated chemicals and finally denied the existence of non-thermal effect because the microwave energy (0.3–300 GHz, 10−6–10−3 eV) is too low to break hydrogen bonds [18,19]. Besides, Stuerga and Gaillard predicted that a measurable microwave non-thermal effect existed only when the electric field intensity is up to 107 V/m [19]. Herrero experimentally investigated microwave assisted four synthetic transformations and excluded the existence of the non-thermal effects [14]. Unfortunately, until now, the existence of non-thermal effects is still in dispute because of the difficulty in proving the phenomena [20]. It is generally believed that experiment is an effective way to demonstrate the non-thermal effects. A range of technical difficulties indicate that it is necessary to design special experimental set-up, system and method for the research of the non-thermal effects. The main reasons that make it difficult to obtain satisfactory
Corresponding author. E-mail address: [email protected] (W. Tian).
https://doi.org/10.1016/j.chemphys.2019.110523 Received 10 July 2019; Received in revised form 6 September 2019; Accepted 7 September 2019 Available online 09 September 2019 0301-0104/ © 2019 Elsevier B.V. All rights reserved.
Chemical Physics 528 (2020) 110523
W. Tian, et al.
experimental results in the current studies on microwave non-thermal effects are as follows: (1) Many experimental studies of non-thermal effects were carried out in household microwave oven or the modified ones. The experimental results always failed to give credible results because the frequency spectrum of the household microwave ovens changed randomly and the distributions of electric field was non-uniform. (2) The experimental method was sometimes unreasonable and there was difficulty in removing the thermal energy in the experimental process to maintain a constant temperature. (3) The temperatures were sometimes measured with an infrared radiation (IR) pyrometer that only measured the surface temperature of the sample rather than the inside ones [21]. In this paper, a special experimental transmission system for proving the existence of a microwave non-thermal effect on DMSONaCl aqueous solution have been carefully designed based on our previous ridged waveguide [22]. The distribution of electric field was uniform and the electric field intensity in solution was about 104 V/m, while the electrodes coupled negligibly small amount of unnecessary microwave energy. In order to keep a constant temperature in the process of experiment, a high-speed fluid solution was applied to exclude the possibility of microwave thermal effects caused by temperature rise. Furthermore, the temperature were measured by a UMI-8 optical fiber thermometer. The resistance as an indicator of microcosmic changes of solution was precisely measured. Dimethyl sulfoxide (DMSO) which is usually called “super solvent” [23,24] has been widely used in organic synthesis [25], petroleum processing [26], pharmaceutical production [27] and many other industries. DMSO is able to accept protons to form hydrogen bonds although it is a non-hydrogen bonding polar aprotic solvent, while the sodium chloride (NaCl) aqueous solution can release protons [28,29]. Accordingly, strong hydrogen bonds will be formed and the molecular network structure will be strengthened when the two solutions are mixed together [30,31]. Conventional hydrogen bonds play a crucial role in various chemical and biological processes such as stabilizing biomolecule structures, modulating specificity and speed of enzymatic reactions, and constructing supramolecular structures [32,33]. Therefore, the DMSO-NaCl aqueous solution is used in the experimental study in this paper.
energy. The Wheatstone bridge circuit including a galvanometer with the unit of μA as shown in Fig. 3 is used to accurately measure the resistance R variation resulted from the slight electrical conductivity change of the solution. In Fig. 3, resistances R1 = 10 KΩ and R2 = 100 Ω are fixed values. R is the resistance of DMSO-NaCl aqueous solution and RM is a variable resistor with a range of 0–1 KΩ. The resistance R = (R1/R2) × RM is determined by adjusting the RM so that the Wheatstone bridge attains balance and the pointer of galvanometer is at the position of zero. Besides, as given in Fig. 4, an electrical conductivity meter is used to measure the electrical conductivity of the solution, which can reflect the variation of the resistance R as it is inverse to the resistance. The experiments were carried out with 25 W microwave power at 5.8 GHz and the inlet flow rate of the glass pipeline is 15 m/s. The microwave power was on-off-modulated irregularly during the process of microwave irradiation. DMSO-NaCl aqueous solution was prepared with 20 g sodium chloride, 0.68 L de-ionized water and 0.894 L DMSO. The distributions of electric field intensity in the solution and around the electrodes at the cross-section of z = 0 mm of the ridged waveguide were calculated by multi-physics calculation method used in our previous article [22] and the results were given in Fig. 5. It can be seen that the electric field in the solution is uniform and the maximum value is 2.322 × 104 V/m. Nevertheless, the maximum value of electric field intensity around electrodes is only 7.9 V/m, meaning that the electrodes coupled negligibly small amount of unnecessary microwave energy and caused little effect to the measurement. The maximum temperature rise 0.037 °C is obtained by the multiphysics calculation. 3. Results and discussion The resistance R0 and temperature T0 at the initial moment and resistance R0′ and temperature T0′ after microwave irradiation were measured by the Wheatstone bridge circuit and the optic fiber thermometer when input power is 25 W and inlet speed is 15 m/s, and the results were given in Table 1. It is shown in Table 1 that the resistances R of the solution are all increased (ΔR = R0′ − R0 > 0) in every measurement, and the temperatures T increased (ΔT = T0′ − T0 > 0) in the process of microwave irradiation. Nevertheless, the temperature is almost invariable and the maximum value is only ΔTmax = 0.05 °C, which proves that the method of solution flowing with high-speed can availably remove the thermal energy rapidly from the experiment system. It is contrary to the generally accepted fact that the resistance of solution heated by conventional method reduces with temperature, indicating that the increased resistance may be induced by microwave non-thermal effect in the process of microwave irradiation. For determining whether the increase of the resistance R was caused by the pure rise of the temperature or the microwave non-thermal effect, it is necessary to measure the resistance R in the same experimental system but under the condition of heating by conventional water bath without microwave irradiation. The resistances R at different temperatures were measured by the Wheatstone bridge circuit and the results were plotted in Fig. 6. What’s more, the electrical conductivities σ of the solution were measured by an electrical conductivity meter when the solution was heated by a conventional water bath without microwave irradiation, and the experimental results were given in Fig. 7. It can be clearly seen from Fig. 6 that the resistance R is declining with the temperature increasing. From Fig. 7, it can be also concluded that the resistance R is reduced with the increase of temperature too, as the electrical conductivity σ that is opposite to resistance gets larger with temperature. It is worth noting that the variation rule of the resistance R in Fig. 7 is in accordance with the rule in Fig. 6. The experimental results obtained by the two methods with conventional water bath heating were all contrary to the rise rule of the resistance R
2. Experiment and methods The experimental system is shown in Fig. 1. Microwave with T10 mode is generated using a magnetron at a frequency of 5.8 GHz. A circulator connected with water load is employed to protect the source from the reflection power. With a dual directional coupler and an AV2433 microwave power meter as power monitoring, microwave power is finally fed into the ridged waveguide which is connected with a water load. A quartz glass pipeline (inner diameter 3 mm and length 60 mm) is placed in the hole which is designed in the ridged waveguide along y-direction as shown in Fig. 2. In order to eliminate the temperature rise in the process of microwave irradiation experiment, a selfpriming micro-diaphragm pump DP80 with a maximum flow rate of 5.5 L/min, namely 15 m/s at the inlet of glass tube, is used to pump the DMSO-NaCl aqueous solution into the quartz glass tube during the process of experiment. Meanwhile, two optical fibers placed at the inlet and outlet of the glass pipeline are used to precisely measure the temperatures by a UMI-8 optic fiber thermometer with an accuracy of 0.05 °C before and after microwave irradiation, by which the temperature rise can be easily obtained. Furthermore, because electric conductivity as an indicator of microcosmic change of solution is opposite to the resistance, resistance R is chosen as a macroscopic measurable parameter to study the microwave non-thermal effect. A pair of Pt electrodes (outside dimension 0.8 × 0.2 mm2) connected to the Wheatstone bridge circuit is inserted into the quartz glass pipeline to observe the resistance variations. The electrodes should not be too close to the ridge to prevent coupling too much unnecessary microwave 2
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Fig. 1. The experimental system used for microwave heating. (a) schematic diagram; (b) photograph of actual experimental system.
of the solution under microwave irradiation obtained in Table 1. If the resistance R in Table 1 increases resulted from the purely the temperature rise, the resistance R must be decreased as those obtained by conventional heating method in Figs. 6 and 7, but this is not the case.
From the above analysis of the resistance variations, it is suggested that the increase of resistance of DMSO-NaCl aqueous solution under microwave irradiation is not caused by the increase of solution temperature, but the microwave non-thermal effect. Therefore, it is reasonable
Fig. 2. The ridged waveguide used in the experiment. 3
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Fig. 3. The Wheatstone bridge circuit used to measure the resistance.
Fig. 5. The distribution of electric field when input power is 25 W and the inlet speed is 15 m/s. (a) electric field in the solution; (b) electric field around electrodes. Table 1 The resistance and temperature in DMSO-NaCl aqueous solution before and after microwave irradiation with input power 25 W and inlet speed 15 m/s.
Fig. 4. The electrical conductivity meter used to measure electrical conductivity.
T0 (°C)
R0 (KΩ)
T0′ (°C)
R0′ (KΩ)
ΔT (°C)
ΔR (KΩ)
17.25 17.05 16.85 16.75 16.65 15.95 15.80
7.42 6.92 6.79 6.74 6.70 6.54 6.50
17.30 17.10 16.90 16.75 16.70 15.95 15.90
7.58 7.12 6.89 6.82 6.76 6.59 6.54
0.05 0.05 0.05 0.00 0.05 0.00 0.05
0.16 0.20 0.10 0.08 0.06 0.05 0.04
to believe that a microwave non-thermal effect is present in our system. that the resistance was increased and the maximum temperature rise was restricted to be lower than 0.05 °C. In order to determine whether the increase in resistance was caused by the temperature rising, the resistance was measured with the same experimental system when the solution was heated by conventional water bath without microwave irradiation. Furthermore, the electrical conductivity which is in inverse proportion to the resistance was measured by electrical conductivity meter under the conventional water bath heating. Results showed that the resistances were all reduced with the increase of temperature when the solution was heated by conventional water bath. It was demonstrated that the microwave non-thermal effect rather than the thermal effect should be responsible for the increase in the resistance under microwave irradiation. A microwave non-thermal effect indeed exists in the DMSO-NaCl aqueous solution under microwave irradiation.
4. Conclusion The present study examined the possibility of microwave nonthermal effect due to microwave radiation on the DMSO-NaCl aqueous solution. For this purpose, a special experimental system was designed carefully. The resistance and temperature of the solution were precisely measured by the Wheatstone bridge circuit and optical fiber thermometer, respectively. The experiment was performed with 25 W microwave power at 5.8 GHz, and the solution flowing at a high-speed of 15 m/s removed the thermal energy from the process of microwave irradiation. Based on this experimental system, the resistance as an indicator of microstructure change of the solution was precisely measured by the Wheatstone bridge circuit in the process of low-lever microwave irradiation at the order of 104 V/m, and the results showed 4
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Fig. 6. Change of resistance of solution measured by Wheatstone bridge circuit as a function of temperature heating by conventional water bath.
Fig. 7. Change of electrical conductivities of solution measured by electrical conductivity meter as a function of temperature heating by conventional water bath.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi [OIT]; the National Natural Science Foundation of China [No. 61401298]; Scientific and Technological Innovation Team of Shanxi Province [201705D131025] and the Key Innovation Team of Shanxi 1331 Project. References [1] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, The use of microwave ovens for rapid organic synthesis, Tetrahedron Lett. 27 (1986) 279–282.
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