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Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012, pp. 215-228. A Publication of the International Microwave Power Institute Unusual Effect of the Magnetic Field Component of the Microwave Radiation on Aqueous Electrolyte Solutions Satoshi Horikoshi, Takuya Sumi Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioicho, Chiyodaku, Tokyo 102-8554, Japan Nick Serpone Gruppo Fotochimico, Dipartimento di Chimica, Universita di Pavia, via Taramelli 10, Pavia 27100, Italy Received: November 8, 2012 Accepted: November 28, 2012 ABSTRACT The heating characteristics of aqueous electrolyte solutions (NaCl, KCl, CaCl2, NaBF4, and NaBr) of varying concentrations in ultrapure water by 2.45 GHz microwave radiation from a single-mode resonance microwave device and a semiconductor microwave generator were examined under conditions where the electric field (E-field) was dominant and where the magnetic field (H-field) dominated. Although magnetic field heating is not generally used in microwave chemistry, the electrolyte solutions were heated almost entirely by the microwaves’ H-field. The heating rates under H-field irradiation at the higher concentrations of electrolytes (0.125 M to 0.50 M) exceeded the rates under E-field irradiation. This inversion phenomenon in heating is described in terms of the penetration depth of the microwaves. On the other hand, the action of the microwave radiation on ethylene glycol containing an electrolyte differed from that observed for water under E-field and H-field conditions. KEYWORDS: Microwave, Magnetic field, Electric field, Dielectric heating. INTRODUCTION As a heating source, microwave radiation has become one of the most attractive heating methods in chemical syntheses, contrary to conventional heating methods that provide heat externally through conduction and convection at the interface between the reactor walls and the heat bath (e.g. an oil bath) [Stadler and Kappe, 2000; De la Hoz et al., 2004]. Microwave heating typically occurs in the bulk of a reaction sample through penetration of the microwaves. The microwave method presents several advantages over conventional methods in thermally-driven chemical reactions: e.g. shorter reaction times, uniform temperature distribution, energy saving, and high product yields [Kappe, 2004]. In this regard, Metaxas and Meredith have described heat and mass transfer in microwave-assisted processes [Metaxas and Meredith, 1983]. A number of other analyses of microwave heat mass transfer have also appeared in the recent literature [Clemens and Saltiel, 1996]. However, research of microwave heating in liquids has been relatively sparse compared to solids. For instance, Lidström et al. [2001] reported heating distilled water and tap water in a single-mode microwave irradiation International Microwave Power Institute 215 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... device and found that tap water was heated more efficiently than distilled water, which suggested that extraneous impurities present in tap water may have had an impact on the heating efficiency. By extrapolation, it is likely that electrolytes contained in water may also influence the microwave heating phenomenon. The role of water as a solvent in microwave-assisted organic syntheses has risen dramatically because of the interest in ecofriendly processes germane to Green Chemistry as recently described by Polshettiwar and Varma [2010]. With regard to the microwave heating of solids, Cheng et al. [2002a] reported significant differences in the heating behavior of solids by the microwaves’ electric field (E-field) and magnetic field (H-field). In fact, in that article and later in a patent [Cheng et al., 2002b] the authors showed that the E (electrical) and H (magnetic) fields in a 2.45 GHz microwave reactor interact very differently with matter. They demonstrated for the first time that the microwave field itself, independent of temperature, profoundly affected the thermodynamics of any system where electrons have unpaired spins. This important effect was realized with single mode microwave radiation where the point of maximal E field and H field were spatially separated. Moreover, they showed that using a 2.45 GHz waveguide cavity, in single-mode TE103 excitation, they could physically position compacted 5 mm pellets of samples separately at the H (magnetic) node (where the E field is nearly zero), or at the E (electric) node (where H field is nearly zero) [Cheng et al., ]. It must be emphasized that the authors were able to separate physically the maximal density of the E-field and the H-field [Cheng et al., , 2002b]. They further noted that for the general theory of energy loss in various materials, when these are placed in a microwave field, the effect of the magnetic field component could no longer be ignored, particularly for 216 conductor and semiconductor materials. They also suggested that contributions to the magnetic loss mechanism could be hysteresis, eddy currents, magnetic resonance, and domain wall oscillations. Their empirical data re-opened the matter of microwavematerial interaction to incorporate more detailed consideration for the effects of the microwaves’ magnetic field. The H-field was effective in the heating of iron, but failed to heat a metal oxide such as ZnO. To the best of our knowledge, a comparative behavior of the microwaves’ Efield and H-field in heating a liquid/solution has hitherto not been investigated in great details. In the present study we report the heating behavior of aqueous electrolyte solutions subjected to microwave electric field (E-field) and magnetic field (H-field) irradiation using a semiconductor microwave generator and a single-mode resonance applicator. Some heating mechanisms are also proposed. Toward this purpose, the rates of the microwave-induced heating of ultrapure water in the presence of such electrolytes as NaCl, KCl, CaCl2, NaBF4, and NaBr at various concentrations have been evaluated on the basis of dielectric parameters (dielectric constant, dielectric loss and penetration depth) and electrical conductivity. The effects of an added electrolyte in the microwave heating of ethylene glycol have also been examined for comparison. EXPERIMENTAL SECTION Chemical reagents Ultrapure water (TOC: < 100 ppb; Electrical conductivity: 0.57 × 10-5 S m-1) was provided by Nomura Micro Science Co., Ltd.; tap water was the drinking water of Tokyo. High purity grade electrolytes (NaCl, KCl, CaCl2, NaBF4, and NaBr) were provided by the Tokyo Chemical Industry Co. Ltd. Setup of the microwave apparatus The microwave irradiation setup with the single-mode cavity TE103 (transverse Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... electric 103 mode), used to irradiate the reactor contents and illustrated in Figure 1, included a short plunger, an iris, a threestub tuner, a power monitor and an isolator. The continuous microwave radiation was generated from a 2.45 GHz microwave semiconductor generator (Fuji Electronic Industrial Co. Ltd.; GNU-201AA; maximal power, 200 W). The resonance of the microwaves was adjusted with the iris and the plunger at 1.5 cycles. Heating of the sample solution (1.0 mL) was achieved by positioning the quartz tube (diameter: 5.0 mm; internal diameter: 4.0 mm) in the single-mode microwave apparatus of Figure 1a-b, either at positions of maximal electric field density (E-field; position (i)) or at the maximal magnetic field density (H-field; position (ii)) within the waveguide. Temperatures of the solutions were measured at 5 sec intervals with an optical fiber thermometer (FL-2000, Anritsu Meter Co. Ltd.) whose tip was fixed at the center of the sample; unless noted otherwise, initial heating rates (ºC sec–1) were calculated for a 30 sec irradiation period. The wavelength of propagation of the microwaves in the TE103 mode within the waveguide was ca. 14.78 cm, estimated from Equation 1 [Cronin, 1995]: (1) where λ is the wavelength in the waveguide; λo (2.45 GHz) = 12.24 cm is the wavelength in vacuum given by c/f, {c being the speed of light, 2.9979 × 1010 cm s–1, and f being the microwave frequency 2.45×109 s–1, i.e. 2.45 GHz}; and b is the height of the waveguide, 10.92 cm (other dimensions of the apparatus are singled out in Figure 1b). The maximal position of the E-field from the iris was located at 3/4 the wavelength of the standing wave in the waveguide, namely 11.09 cm. [Horikoshi, et al., 2009]. The maximal position of the H-field from the iris was at 1/2 the wavelength of the standing wave in the waveguide, namely 7.39 cm. [Horikoshi, et al., 2009]. Figure 1c and d illustrates the distribution of the E-field and H-field components of the microwave radiation inside the waveguide in singlemode operation under resonant conditions [Pozar, 2004]. The characteristics of heating the samples by both microwaves E-field and H-field irradiation were examined by positioning the sample at positions (i) and (ii) of Figure 1a. The microwave input power was fixed at 50 W, albeit the power monitor registered 40 W indicating a loss of 10 W through the coaxial cable and the waveguide. No reflective wave that accompanied heating was observed. Nonetheless, even if the reflected microwaves were to arise before the iris, the isolator would have eliminated these microwaves and maintain the stability of the microwave generator. The resonance of the microwaves in the waveguide, measured by the Agilent Technologies 8720C Network Analyzer, can change when placing the sample in the waveguide of the microwave semiconductor generator setup. Accordingly, appropriate adjustments were made with the short plunger and the three-stub tuner. In addition, the dielectric parameters of the sample can change with heating. Consequently, an electric field monitor (Fuji Electronic Industrial Co. Ltd.) was used to maintain the sample at the maximal position of the E field, as the reproducibility of the experiments may be diminished considerably when such operations are neglected. Under our conditions we found no significant changes of the electric field upon heating. Unless noted otherwise, the frequency of the microwave radiation was 2.45 GHz. To the extent that the E and H fields are out-of-phase by a 1/4 wave (see Figure 1c and d) [Pozar, 2004] relative to one another judicious positioning of the electrolyte samples in the waveguide where the fields have maximal density (Figure 1a) makes it possible for the microwaves to irradiate the Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute 217 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... a) b) c) d) Figure 1. a) Details of the experimental setup and positioning of the samples in the single-mode microwave resonator; (i) maximal position of the electric field (E-field) density and (ii) maximal position of the magnetic field (H-field) density. b) Photograph of the single-mode microwave resonator and the 2.45 GHz semiconductor microwave generator; photograph also shows the actual position of the sample at the H field maximum; Distribution of the c) E-field and (d) H-field component of the microwave radiation inside a waveguide in single-mode operation, resonant condition. Each image for TEM103 mode was drawn with Equation S1 for the E field and S2/S3 for the H field using the Mathematica 8 Software (Wolfram Research, Inc.). Note that the E field occurs only in the direction of the Y-axis. Ey: E-field vector in the Y direction; Hx and Hz: H-field vector of the x and z direction; a: long side of the waveguide; b: short side of the waveguide; d: movement of the microwave in the waveguide; l: 3 (X of TE10X mode). The images are in agreement with those given by Yoshikawa and coworkers [Pozar, 2004]. samples either by the E-field component or by the H-field component. Hereafter, we shall refer to such irradiation conditions simply as E-field and H-field irradiation. RESULTS AND DISCUSSION Effects of the 2.45-GHz microwave radiation on various aqueous electrolyte solutions The influence of the microwaves’ E-field and H-field irradiation on heating aqueous electrolyte solutions was examined 218 using an ultrapure water sample and aqueous samples that contained various electrolytes (NaCl, KCl, CaCl2, NaBF4, and NaBr) and whose density differed relative to that of ultrapure water. The heating rates of the electrolyte solutions were far more significant than that of ultrapure water. The heating rates tended to plateau at the low electrolyte concentrations under E-field irradiation and remained fairly constant throughout the concentration range examined (Figure 2a). For example, the sample at 0.5 M in NaCl Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... Figure 2. – Heating rates of an ultrapure water sample and aqueous electrolyte solutions of NaCl, KCl, CaCl2, NaBF4, and NaBr obtained for a 30 sec irradiation period in a quartz tube under; a) predominantly electric field (E-field) and; b) predominantly magnetic field (H-field) irradiation conditions using the single-mode method. reached a heating rate of 2.0 ºC s–1 after which the rates remained nearly the same within experimental error (1.9 ± 0.1 ºC s–1). Other electrolyte solutions displayed a similar general behavior. This is a result of the conduction mechanism whereby a solution that contains ions in a hydrogen-bonded water cluster moves through the solution under the influence of the E field causing the collision rate to increase. Evidently, under E-field conditions the Joule heating mechanism (see below) reflects a much stronger interaction than the dipolar mechanism (i.e. dielectric heating) in relation to the heat generating capacity. Contrary to E field irradiation, under H-field irradiation conditions the heating rates showed a near-exponential increase on addition of the electrolytes to the ultrapure water sample reaching a plateau at ca. 4.0 ºC s–1 at a concentration of 3 M in NaCl and KCl. Clearly, the data displayed in Figure 2b demonstrate that heating of aqueous electrolyte solutions was enhanced considerably under H-field conditions and was remarkably different from E-field irradiation conditions with the 2.45-GHz microwaves (Figure 2a). In the case of CaCl2, the heating rate increased up to 4 M and then showed a sharp drop at 6 M in CaCl2. Three questions can then be formulated from these data: (1) why does the heating rate of the CaCl2 solution fall at 6 M?; (2) what causes magnetic field heating of the aqueous electrolyte solutions; and (3) why is the H-field heating efficiency higher than electric field heating? Where the concentration of the electrolytes is zero, comparison of the data of Figures 2a and 2b shows a greater heating rate when the water sample was subjected to E-field irradiation than to H-field irradiation, indicating that dielectric heating was more significant in an ion-free solution. Intuitively the heating of water was not expected to occur under the microwaves’ magnetic field component. However, in this regard we must recall that the H field of an electromagnetic wave is an oscillating magnetic field that can cause charges to move. In the present context, the partially charged ends of the dipole of the water molecule tend to move in opposite direction under the magnetic field resulting in the rotation of the molecule and contribute to the heating effect. Two parameters describe the dielectric properties of aqueous electrolyte solutions: (i) the dielectric constant (ε’), which describes the ability of the water molecule to be polarized by the electric field, and (ii) the dielectric loss factor (ε’’) which quantifies the efficiency with which the electromagnetic energy is converted to heat [Hayes, 2002]. Both parameters were determined at various temperatures at 10 ºC intervals on microwave heating ultrapure Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute 219 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... water and the various electrolyte solutions using an Agilent Technologies HP-85070B Network Analyzer and an Agilent dielectric high temperature probe (up to ~200 ºC). In this case, the volume of the aqueous samples was 100 mL in a Pyrex reactor. The temperatures of the solutions were measured with an optical fiber thermometer. Preliminary experiments showed that the thermometer had no effect on the dielectric data. Measurements were also repeated using a slim form probe to verify reproducibility. The dielectric constant of water decreased with increase in temperature to 90ºC, and also decreased with increase in the concentration of the electrolytes. For instance, for the NaCl solution (0.125 M) the dielectric constant ε’ = 75.2 at 30 ºC decreased to ε’ = 61.1 at 90ºC. For the 6.0 M aqueous NaCl solution, ε’ = 30.2 at 30 ºC decreased to ε’ = 9.7 at 90 ºC. Such tendencies are in line with those reported by Ratanadecho et al. [2002] Interestingly, variations in the dielectric constants with electrolyte concentration differed with those observed for the heating rates with increase in concentration (see Figure 2). The behaviors of the dielectric loss at various concentration of electrolyte and at various temperatures are illustrated in Figure 3. For ultrapure water, the dielectric loss factor (ε’’) decreased somewhat with increase in temperature: from ε’’ = 8.0 at 30 ºC to ε’’ = 2.8 at 90 ºC in parallel with the dielectric constant. By contrast, the presence of electrolytes in ultrapure water caused the dielectric loss factor to increase with increase in temperature. Thus, addition of NaCl at a concentration of 0.125 M led the dielectric loss factor ε’’ (= 19.2) at 30ºC to increase to 25.3 at 90ºC (Figure 3a). In addition, increasing the concentrations of the electrolytes caused the dielectric loss factors to be greater and to increase with temperature. Most of the dielectric loss factors of the electrolyte solutions increased with concentration in electrolyte, except for CaCl2 which displayed a decrease in ε’’ at concentrations greater than 3 M, 220 and was particularly significant when the concentration was 6 M in CaCl2 (Figure 3c). These observations accord with the results of the behavior of the heating rates of CaCl2 under H-field irradiation reported in Figure 2b. In microwave heating the dielectric loss (ε”) consists of the sum of two terms (see Equation 2): (i) dielectric heating (first term) and (ii) Joule heating (second term) [Stuerga and Delmotte, 2006; Gabriel et al., 1998]: (2) where εs is the relative permittivity (dielectric constant) at low frequencies and ε∞ is the relative permittivity at high frequencies, ω is the angular frequency of the electromagnetic radiation, τ is the relaxation time {a measure of the time required for water to rotate (τ = 4πηr3/κT) where r is the molecular radius, κ is Boltzmann’s constant, T is temperature, and η is the viscosity}, also considered as the delay of the molecules (or particles) to respond to the field change; and σ is the ionic conductivity of the electrolyte solution. Accordingly, enhancement of the conductivity σ of the aqueous electrolyte solution on addition of ions to water should further enhance the heating efficiency ( 2) to a point. The influence of an electrolyte in microwave heating may then be expected to cause increased efficiency through the Joule heating mechanism. Changes in the electrical conductivity of the aqueous electrolyte solutions examined at 30 ºC with changes in concentrations are reported in Figure 4. The curves bear a strong resemblance to the overall H-field heating behavior reported in Figure 2b. For instance, the electrical conductivity drop resembles the drop of the heating rate at the high concentration of CaCl2. The existence of a correlation between the concentration dependence of the electrical conductivity and the concentration dependence of Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... a) b) c) d) e) Figure 3. Temperature and concentration profiles of the changes in dielectric loss (ε’’) for ultrapure water and electrolyte solutions subjected to 2.45 GHz microwave irradiation: a) NaCl; b) KCl; c) CaCl2; d) NaBF4; and e) NaBr. the dielectric loss at 30 ºC and 90 ºC is particularly evident on comparing Figure 4 with Figure 5. As such then, H-field heating can be taken to take place through Joule heating. As to a reply to the first question formulated earlier for the case of an aqueous CaCl2 solution, Joule heating is likely controlled by some specific heating of the CaCl2 electrolyte solution by the microwaves’ magnetic field, no doubt related to the structure-making of water by the Ca2+ ions in competition with the Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute 221 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... Microwave heating mechanism Three types of microwave heating phenomena are relevant to solutions: dielectric heating, Joule heating, and magnetic heating of molecules that may be impacted differently by the E and H fields of the microwave radiation [Horikoshi and Serpone, 2009]. Dielectric heating mechanism: dielectric materials (i.e. nonconductive) are heated by the microwaves’ E field owing to the presence of electric dipoles in polar molecules as in, for example, the microwave dielectric heating of water through dipolar polarization (i.e. dipole rotation). Joule heating mechanism: the electrical resistance of water decreases on addition of electrolytes such as NaCl, KCl, CaCl2, NaBF4, NaBr and thus Joule heating occurs through increased conductivity of the aqueous solution. Magnetic loss heating mechanism: magnetic losses typically occur in the microwave region for such metal oxides as ferrites and other magnetic materials [Cheng et al., 2002a; Cheng et al. 2002b]. Such magnetic losses are different from hysteresis or eddy current losses because they are induced by domain wall and electron-spin resonance. The above data and discussion leads us to propose the mechanism illustrated in the cartoon of Figure 6 as to why the microwaves’ H field presents advantages to heating electrolyte solutions relative to the microwaves’ E field germane to the second question raised earlier. We begin by supposing that the magnetic field does not a) b) Figure 4. Changes in electrical conductivity (S m-1) of aqueous electrolyte solutions as a function of concentration at 30 ºC (NaCl, KCl, CaCl2, NaBF4, and NaBr). structure-breaking of water by the Cl ¯ ions. A discussion on the effect of ions on the structure of water was beyond the scope of the present study [Hribar et al., 2002; Botti et al., 2004; Sloutskin et al., 2007; Y Marcus, 2009; Luo and Roux, 2010]. In summary then, relative to Joule heating promoted through addition of electrolytes, the efficiency of dielectric heating of water tended to decrease. As to a reply to the second question, the heating of electrolyte solutions under H-field irradiation is likely connected with Joule heating. Figure 5. Concentration dependence of the dielectric loss at two different temperatures (a) 30 ºC and (b) 90 ºC of the five electrolytes. 222 Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... Figure 6. Cartoon depicting the heating model for electrolyte/water solutions under predominant magnetic field irradiation. heat an electrolyte solution directly. The alternating magnetic field lines are vertical to the quartz reactor walls and penetrate the entire solution since the solution does not absorb magnetic fields. Induced ring currents are generated perpendicular to the magnetic field lines, and since these currents are generated everywhere in the solution they cause the solutions to be heated. Therefore, heating by H-field irradiation can be regarded as indirect conduction loss heating, or otherwise as induction heating by the alternating magnetic field. Recall that induction heating is the process of heating an electrically conducting system by electromagnetic induction where eddy currents are generated within the system, and resistance leads to Joule heating of the system. The next discussion deals with why electric field heating of an electrolyte solution is not more effective than magnetic field heating. When an electrolyte is added to water, microwave heating is dominated by Joule heating under E-field and H-field irradiations. However, the data show that Efield heating is significantly different from H-field heating. For instance, the heating rate reached saturation at concentrations greater than 0.50 M in NaCl under E-field irradiation (Figure 2a). By contrast, the heating rate was enhanced significantly under H-field irradiation as the concentration of the electrolytes increased (Figure 2b); the mechanism remains nonetheless Joule heating. The contrast between Joule heating by E-field irradiation and H-field irradiation can be considered on the basis of the penetration depth of the microwaves into the aqueous solutions. The penetration depth Dp (in cgs units) is the depth at which microwaves pervade into the material and the power flux has fallen to 1/e (= 0.368%) of its surface value; it can be estimated by Equation 3 [Metaxas and Meredith, 1983]: (3) where λ is the wavelength of the radiation, λ(2.45GHz) = 12.24 cm. That is, it denotes the depth at which the power density of the microwaves is reduced to 1/e of its initial value. The depth at which the 2.45 GHz microwaves can penetrate into the ultrapure water sample in the reactor increased nearly 2.6-fold from ca. 21 mm to 55 mm with increase in temperature from 30 ºC to 90 ºC (Figure 7), whereas the depth of penetration decreased on addition of the electrolyte (NaCl) and with increase in temperature {from 8.8 mm at 30 ºC to 6.1 mm at 90 ºC (0.125 M), i.e. a 31% decrease; from 5.3 mm to 3.6 mm (0.50 M), i.e. a 32% decrease; from 2.7 mm to 1.7 mm (1.0 M), i.e. a 37% decrease and from 1.0 mm to 0.8 mm (6.0 M), i.e. a 20% decrease}. These penetrations depths should be compared to the internal diameter of the quartz reactor: 4.0 mm. As heating of the solution progressed, the penetration depth (8.8–6.1 mm) of the microwaves into the Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute 223 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... Figure 7. Temperature profiles of the changes in penetration depth (mm) of the 2.45 GHz microwaves into an ultrapure water sample containing NaCl at various concentrations: (a) 0 M, (b) 0.125 M, (c) 0.25 M, (d) 0.5 M and (e) 1.0 M. electrolyte solution (0.125 M) was significant compared to the quartz reactor diameter (left scheme in Figure 8). The optimal penetration depth for the aqueous solution with 0.50 M in NaCl was 5.3–3.6 mm (versus 5.0 mm of reactor size) so that the entire solution was bathed with microwaves at this concentration, which is precisely the concentration at which the maximal heating rate was observed under E-field irradiation. The penetration depth of the microwaves under E-field irradiation becomes shallower on increasing the electrolyte concentration. Figure 8. Cartoon illustrating the heating model of aqueous electrolyte solutions NaCl: 0.125 M and 6.0 M under E-field irradiation; internal diameter of reactor = 4 mm. 224 As a result, heating the entire sample does not occur. Only the aqueous solution at the interface between the quartz walls and the solution (1.0–0.8 mm) is heated at 6.0 M concentration of electrolyte (NaCl) – see the right hand scheme in Figure 8. Therefore, the overall heating rate is expected to be smaller because the inner bulk solution is not heated. Such a phenomenon is reminiscent of the skin effect displayed by microwave heating of a metal [Sun et al., 2005]. The rates of Joule heating of aqueous electrolyte solutions under H-field irradiation evolved with increase of the electrolyte concentration, as the Joule heating induced by alternating magnetic fields occurred in the inner bulk of the solutions owing to the establishment of ring currents (Figure 6). Accordingly, magnetic field heating is nearly independent of penetration depth, contrary to dielectric heating that occurs via E-field irradiation. Heat distribution under E-field irradiation The heat distribution in the electrolyte solution depends on the penetration depth of the irradiating microwaves. To obtain the distribution, we measured the temperature of the solution using an optical fiber thermometer (FL-2000, Anritsu Meter Co. Ltd.) located at the center of the solution (see Figure 9a), whereas the temperature at the wall of the reactor was monitored with an infrared thermometer (R-100, Anritsu Meter Co. Ltd.). The heating rates in both cases are reported in Figure 9b. In this experiment, a quartz reactor with an internal diameter of 10 mm was used to accommodate the spot size of the infrared thermometer. The reactor was positioned where the electric field density was maximal. However, since the reactor size was larger than that shown in Figure 8 the microwave radiation also had an H-field component mixed in with the E-field component. To the extent that the temperature measured near the reactor wall was cooled by the surrounding atmosphere, Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... a) b) Figure 9. (a) Method of measuring the temperature in the samples; (b) heating rates as a function of the concentration of NaCl and as a function of the location where the temperatures were measured experimentally. not unexpectedly the measured temperature was lower than the temperature measured at the center of the solution [Kappe, 2004]. The heating rate determined at the center of the electrolyte solution was maximal at 0.5 M in NaCl and decreased sharply at the higher concentration in electrolyte (2.0 M in NaCl; Figure 9b). The heating rate determined near the reactor surface displayed a smooth decrease with an increase in electrolyte concentration. In the more concentrated electrolyte solution, the microwaves are less likely to reach the center of the solution, in which case the temperature distribution at the center and at the reactor walls is reversed at the higher concentration. Effect of microwave irradiation on addition of an electrolyte to an organic solvent Adding an electrolyte to an organic solvent was examined for ethylene glycol to determine what effect a salt might have on the heating rates, on the dielectric loss parameter, and on the penetration depth of the microwaves. The ethylene glycol solution was saturated with NaCl, the concentration of which reached ca. 0.50 M. Under E-field irradiation conditions, the heating rate of the ethylene glycol was enhanced 3.7fold on addition of 0.125 M NaCl (Figure 10a), whereas under H-field irradiation the addition of NaCl had but a minor effect on the heating rates. Clearly, the phenomenon of H-field heating of the ethylene glycol and its solutions with the electrolyte differed significantly with that observed for aqueous solutions (Figure 2). This raises an additional question about magnetic field heating versus electric field heating between aqueous and non-aqueous media in general. Future studies will address this question further. The dielectric loss factor (ε’’) of ethylene glycol increased on adding the electrolyte (Figure 10b), and decreased with increasing temperature. Note the significant difference with aqueous media (Figure 3). On the other hand, the penetration depth (Figure 10c) of the microwaves into the solutions at all the concentrations of NaCl was such that the whole glycol/electrolyte solution could be heated. The standard microwave frequency commonly used for dielectric heating is 2.45 GHz which corresponds to a relaxation time of 65 ps [Gabriel et al.,1998]. To the extent that the relaxation time of the water Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute 225 Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... a) b) c) Figure 10. (a) Heating rates obtained during a 30-sec period for ethylene glycol on addition of NaCl under E-field irradiation and H-field irradiation conditions. (b) Temperature profiles of the changes in dielectric loss factor (ε’’). (c) Penetration depths (mm) of the 2.45-GHz microwaves into ethylene glycol/NaCl mixture. In all cases, temperatures were measured with the fiber optic thermometer at the center of the solutions. Size of reactor was 5 mm in diameter. molecule at 25 ºC is 2 ps, [Struis et al.,1987] the water molecule can easily follow the footsteps of the 2.45 GHz frequency of the microwave radiation. Our data do not preclude the possibility that the movement of water clusters may be at the origin of microwave dielectric heating in aqueous media. As noted earlier, the presence of ions in water causes the structure of water to be modified, leading the dielectric polarization of the water clusters [Hribar et al., 2002] to change with the microwave frequency. Water behaves as a cluster molecule caused by the hydrogen bond between single water molecules, unlike polar organic solvents where hydrogen bonding is limited. Hence, hydrogen bonding may be an important factor 226 in dielectric heating. Germane to this, Huang et al. [2009] reported that the electrical conductivity of aqueous solutions of NaCl can be changed from their interactions with microwaves, and further suggested that the cluster structure of the electrolyte solution is affected by the microwave radiation. CONCLUDING REMARKS The present study has shed some light into the heating mechanisms of aqueous electrolyte solutions relative to an ultrapure water sample under E-field and H-field irradiation conditions; they were shown to be significantly different. In nearly all the microwave-assisted chemical syntheses, the microwaves’ field most often used to Journal of Microwave Power and Electromagnetic Energy, 46 (4), 2012 International Microwave Power Institute Satoshi Horikoshi et al., Unusual Effect of the Magnetic Field Component of The Microwave Radiation on ... irradiate a reactive system is the E field. Consequently, it is necessary to select an appropriate reactor vessel to ensure full penetration of the microwaves throughout the sample. Contrary to E-field irradiation, however, Joule heating by H-field irradiation can heat the reactive system efficiently as in the latter case penetration of the microwaves is independent of reactor size. The effect of addition of an electrolyte to polar solvents (e.g., water and ethylene glycol) has been shown to be different, such differences being unexpected on the sole basis of the dielectric parameters of the molecules in the systems. Clearly, the action of the microwaves on water containing ionlike salts is fairly significant relative to the action of microwaves on pure water. ACKNOWLEDGMENTS Financial support from the Japan Society for the Promotion of Science (JSPS) to S. H. through a Grant-in-aid for young scientists (No. B-23750247) is gratefully appreciated. One of us (NS) thanks Prof. Albini of the University of Pavia for his hospitality during the many semesters in his Laboratory. We are grateful to the Nomura Micro Science Co. 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