Experimental study of dielectric property changes in DMSO–primary alcohol mixtures under low-intensity microwaves

Abstract

By applying a low-intensity microwave to DMSO–primary alcohol mixtures, distinct dielectric property changes have been observed. The results indicated that the intermolecular interaction in these mixtures could be affected by the applied electric field with the order of 10⁵ V m⁻¹.

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Microwave nonthermal effects are postulated to result from a proposed direct interaction of the electric field with polar molecules in the reaction medium that is not related to a bulk temperature effect. Over the past decades, nonthermal effects have been extensively studied by a number of experimental techniques and theoretical methods, but many of their aspects remain controversial. In 1996, Stuerga and Gaillard predicted that an electric field intensity up to 10⁷ V m⁻¹ is required to produce a measurable microwave nonthermal effect.^1,2 Some experiments also suggested that microwave fields could disrupt the weak intermolecular bonds in proteins as well as in polar solution.^3,4 Additionally, investigating the changes of dielectric property induced by external electric fields (and therefore of the microwave power) is an effective approach to interpret the microwave nonthermal mechanisms. Therefore, a significant amount of published works have been devoted to this area. Theoretical works were largely conducted in a wide range of high electric fields from 10⁹ V m⁻¹ to 10¹¹ V m⁻¹,^5–22 most of which focused on the high electric field effects on the dielectric constant change by virtue of quantum chemical calculations and molecular dynamics (MD) simulations.^9–12 However, measuring the dielectric property of solution in such high electric fields is technically difficult, and few experimental results are available in the literature. Substantial experimental progress has been made below 10⁹ V m⁻¹. In 1920, Herweg measured the relative electric permittivity (Δε) of diethyl ether, which was found to be quadratically dependent on the electric field intensity.²³ This motivated another wave of experimental studies on the relationship between the electric field intensity and the dielectric properties of the material.^24–30 For example, the refractive index of water was found to change at 474 THz under the 4 × 10⁸ V m⁻¹ electric field.²⁶ A similar change was further confirmed in the water permittivity (ε = ε′ + jε′′) with more precise experiments based on a silicon microstrip device, where it was established that ε′ and ε′′ changed up to 70% and 50%, respectively, under a 1 × 10⁸ V m⁻¹ electric field.²⁷ When the electric field strength decreased to 10⁷ V m⁻¹, Richert and co-workers investigated the nonlinear dielectric effects of propylene glycol, and it was observed that the dielectric constant decreased while the dielectric loss increased with the electric field.^28,29 Moreover, by employing high voltage frequency-domain impedance technology, an increase of more than 6% in the dielectric loss of glycerol was obtained when varying the external electric field from 1.4 × 10⁶ to 2.8 × 10⁷ V m⁻¹.^30,31 Nevertheless, little attention has been paid to the solution’s dielectric property changes in the presence of a 10⁵ V m⁻¹ electric field, though 10⁵ V m⁻¹ is the most commonly used electric field intensity in a microwave chemistry reactor. Furthermore, an anomaly in the imaginary part of the permittivity was observed in our previous experimental report for DMSO–H₂O mixtures.³² MD simulations also suggested that the hydrogen bonds in DMSO–primary alcohol mixtures were weaker than in the sole substance of methanol and ethanol liquid.^33,34 Therefore, DMSO–primary alcohol mixtures were chosen as the subjects of the experiment.

In this paper, we designed an experimental system to measure the dielectric behavior of DMSO–methanol/ethanol mixtures under low-intensity microwave fields (10⁵ V m⁻¹ electric field amplitude). A specially designed microstrip sensor with high sensitivity to dielectric changes was employed to generate a uniform electric field. Then, we introduced this highly sensitive sensor into a microwave system to measure the dielectric property changes of mixtures which flowed through the designed sensor under microwave irradiation. The temperature effects caused by microwave heating were excluded by the well-designed device. By measurement, slight dielectric property changes with microwave power at 2.45 GHz were explored and observed. It is reasonable to infer that the intermolecular interaction in mixtures can be significantly affected by the external electric field in the order of 10⁵ V m⁻¹.

In the precisely designed experimental apparatus, we first proposed and demonstrated a high-sensitivity radio frequency (RF) sensor to detect the slight dielectric property changes of solution in a flowing channel. Interference is used to cancel background probing signals to improve the sensor’s sensitivity. The designed center frequency is 2.45 GHz, and a picture of the obtained sensor is shown in Fig. 1. Supposing that the solutions’ dielectric properties in the two channels remain unchanged, the two signals will cancel each other. Otherwise, their difference will be measured, if the dielectric properties of the solution in the material under the test (MUT) channel change.

Fig. 1 Photograph of the high-sensitivity sensor.

The designed sensor was then introduced to the following high power experimental setup, which is shown in Fig. 2. A continuous microwave source with a fixed frequency of 2.45 GHz was used to generate a stable output power. The microwaves were transmitted along the circulator, followed by the directional coupler, and eventually fed into the microstrip sensor. The load located at the end of the microstrip sensor ensured that only a minimal amount of microwave power was reflected.

Fig. 2 The schematic of the actual experimental system.

The precisely designed experiments include the following four techniques: (1) two via holes in the microstrip rings were designed in two channels to produce a uniformly distributed electric field around the solution; when the input power was up to 66 W, the electric field intensity in the MUT channel reached 1.17 × 10⁵ V m⁻¹, which was two orders of magnitude larger than that in the reference material (REF) channel. (2) Two equal parts of the measured solution were continuously pumped to the quartz glass pipelines (the diameter is 3 mm) at 8.3 m s⁻¹. (3) The temperature of the solution under testing was precisely controlled using a HWCL-3 thermostatic magnetic blender (±0.5 °C). (4) By applying the UMI-8 optical fiber probe temperature monitoring device, an accurate evaluation of the temperature difference between the output ports of the two pipelines located in the channels was performed; the maximum temperature difference between them was less than 0.05 °C, which guarantees that the thermal effects caused by microwave heating were eliminated.

All pure DMSO (99.5%), methanol (99.5%), and ethanol (99.5%) (analytical reagent) solutions used in these experiments were purchased from Chengdu Kelong Chemical Reagent Factory without further purification. Two sets of primary alcohol–DMSO mixtures, methanol–DMSO (methanol mole fractions 1.75, 0.59, 0.35) and ethanol–DMSO (ethanol mole fractions 1.22, 0.41, 0.24), were prepared. In our experiments, a continuous microwave source was employed, and the generator’s power ranged from 20 W to 100 W operating at a frequency of 2.45 GHz. When the input power gradually increased from 20 W to 100 W, the measured ratio values between output and input power were plotted in the Cartesian coordinate system. If the ratio curves were not linear, it meant that the dielectric properties of the solution in the MUT channel had changed. Each binary mixture was measured at a room temperature of 300 K and repeated six times to eliminate systematic errors. The results are shown in Fig. 3 with error bars.

Fig. 3 The relationships of output–input microwave power for solutions with different mole fraction ratios: (a) methanol/DMSO mixtures at methanol mole fractions of 1.75, 0.59, 0.35, (b) ethanol/DMSO mixtures at ethanol mole fractions of 1.22, 0.41, 0.24, (c) pure DMSO (99.5%) and methanol (99.5%) solution, (d) pure DMSO and ethanol (99.5%) solution.

As illustrated in Fig. 3(a) and (b), similar characteristics were found in the curves of methanol–DMSO and ethanol–DMSO mixtures with different mole fractions. The output power versus input power curves were initially linear, and turned out to show nonlinear characteristics. The curve’s slope became smaller at some “critical point”, which can be viewed as evidence for an electric field-induced dielectric property change in these mixtures. The “critical point” was defined as the value of input power corresponding to the point where the nonlinear characteristic first appeared. Note that all these “critical point” values correspond to the electric field amplitude of 10⁵ V m⁻¹ according to the simulated results using Ansoft High Frequency Structure Simulation (HFSS) software.

However, when the same experimental operations were performed on the sole component DMSO, methanol, and ethanol solutions, the power ratio curves were linear with the input power increasing from 20 W to 100 W, indicating that the dielectric properties remained unchanged for the sole substances. The results are shown in Fig. 3(c) and (d). These measured linear results further excluded the trivial effects caused by the high-power microwave (such as electromagnetic crosstalk), and proved the reliability of our experimental system. In summary, by comparing the mixtures’ nonlinearity with the pure substances’ linearity, we can deduce that the low-intensity microwave is able to affect the intermolecular interaction in DMSO–methanol/ethanol mixtures, and this microscopic interaction eventually results in the macroscopic dielectric changes in our measurements under the stimulus of a 10⁵ V m⁻¹ electric field.

In addition, several works have demonstrated that polar mixtures of DMSO with different solvents, methanol and ethanol, also presented properties deviating from ideality, such as density, viscosity, and relative permittivity.^35–38 Besides, DMSO is an aprotic polar solvent with no ability to form hydrogen bonds with another DMSO molecule.³⁴ Methanol tends to form chain-like hydrogen bonds in a linear structure,³⁹ and a predominantly winding hydrogen bond chain structure is found in liquid ethanol.⁴⁰ Thus, it can be easily figured out that when DMSO solvent is dissolved into the methanol/ethanol solution, the hydrogen bond interaction leads to a structural reorganization compared with their pure components. In other words, when DMSO is added, the long chain-like hydrogen bonds of methanol and ethanol liquid transform into dimers or trimers in mixtures with lower hydrogen bond numbers.⁴¹

As a result, these weak hydrogen bonds are supposed to be altered under a low-intensity microwave in our experiment. And recently experiments on heat-shock proteins (HSPs) have suggested that a low-intensity microwave could disrupt the weak bonds that maintain the active folded forms of proteins.⁴

Conclusions

In summary, we have proposed a sensitive microwave experimental system which can detect the dielectric property changes of DMSO–methanol/ethanol mixtures under low-intensity microwave fields. In our experiments, the corresponding electric field is 10⁵ V m⁻¹. The conclusions are summarized as follows: (1) the measured linear results of the sole components verified the reliability of our experimental system; (2) in the presence of a 10⁵ V m⁻¹ electric field, the measured results indicated that the dielectric properties changed in mixtures while no change was observed in the pure substances; (3) based on the experimental results, it can be deduced that the application of a low-intensity microwave can affect and alter the intermolecular interaction in DMSO–methanol/ethanol systems, thereby leading to macroscopic dielectric property changes in mixtures. The results validate the existence of a nonthermal microwave effect in low-level intensity electric fields. Further theoretical work on the details of the interaction mechanism between microwaves and mixtures is currently underway in our laboratory.

Acknowledgements

This project is supported by Major State Basic Research Development Program (973 Program): Grant Numbers: 2013CB328900 and 2013CB328905.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07914d

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