**11. Characterization of non-thermal microwave effect of ethanol-hexane mixed solution and MBBA isotropic state**

As shown in **Figure 3**, the CSC-temperature of the OH group of ethanol under microwave irradiation was considerably lower than those of the CH2 and CH3 protons. OH protons have a different CSC-temperature from the other protons. A temperature increase of the OH protons induces a higher field shift due to a reduction of hydrogen bonding under thermal heating [40]. Similar higher field shifts have been observed in H2O protons [56]. In contrast, the CSC-temperatures of the OH protons are lower than the bulk temperature under microwave irradiation; therefore, the lower field shift of OH protons indicates the presence of a nonthermal microwave effect. On the other hand, the CSC-temperature of 7′ and α' protons of MBBA in the isotropic state showed much higher CSC-temperature than the other non-polar proton groups.

It is important to consider microwave heating from a physicochemical (thermodynamical) point of view to explain the energy flow mechanism. When energy is supplied to the solution under constant pressure (P) and temperature (T), the change of the Gibbs free energy (dG) is described as the difference of the changes in the enthalpy term dH = dU + d(PV), where dU is the change in the total internal energy which consist of dQ(heat) and dW(work), and entropy (TdS) terms. In the case of a solution state, the change of volume (V) is very small and the d(PV) term can be neglected, so that the Gibbs free energy is given by dG = dH – TdS = dQ + dW - TdS. In the case of a conventional thermal heating process, the change of the Gibbs free energy (dG) mainly increases the heat energy (dQ ) term, which causes a temperature increase.

**Figure 7** shows the energy flow processes from a physicochemical point of view. In the thermal heating process, the directions of polar molecules fluctuate

**179**

**Figure 7.**

CH3-O (α') groups in MBBA.

*Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features…*

randomly, so that no net dipolar moment is induced (**Figure 7(a)**). In contrast, the microwave heating process induces order of the polar molecules to the electric field and a decrease of the entropy term (dS1 < 0), therefore, the Gibbs free energy is increased to absorb microwave energy in the system as a first step, dG = dQ1 + dW1 – TdS1, as shown in **Figure 7(b)**(1). The electric field oscillates with a frequency of 2.45 GHz; therefore, the polar molecular order is simultaneously reduced by an increase of the entropy term (dS2 > 0) by conserving the Gibbs free energy, 0 = dQ2 + dW2 – TdS2, as a second step (**Figure 7(b)**(2)). Energy (TdS2) thus dissipates as heat and work terms, dQ2 + dW2 to the system. By considering the sum of these two steps, dG = d(Q1 + Q2) + d(W1 + W2) – Td(S1 + S2), (**Figure 7(b)**(3)), the heat term d(Q1 + Q2) increases the temperature as thermal microwave effect and the work term d(W1 + W2) may change the CSC-temperature for the OH group as a non-thermal microwave effect involving work terms such as molecular ordering and hydrogen bond formation. We interpret how the dW term changes the CSC-temperature of the OH groups in ethanol and H-C=N (7′) and

*Energy flow pathway under (a) conventional thermal heating and (b) microwave heating. Blue arrows indicate electric dipolar moment vectors and orange arrows indicate electric field vectors. Adapted with* 

*permission from [34]. Copyright (2020) American Chemical Society.*

It is important to note that polar molecules follow the oscillating electric field in a coherent manner. In this case, coherently aligned polar molecules are able to

*DOI: http://dx.doi.org/10.5772/intechopen.97356*

*Microwave Heating of Liquid Crystals and Ethanol-Hexane Mixed Solution and Its Features… DOI: http://dx.doi.org/10.5772/intechopen.97356*

#### **Figure 7.**

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

**10. Microwave heating process of MBBA in the isotropic phase**

under continuous wave (CW) microwave irradiation. The CSC-temperature increased from 50 to 70°C within 2 min under the application of 65 w CW microwave irradiation, based on the majority of protons. After microwave irradiation for 5 min, the CSC-temperature increase plateaued. However, there were significant variations in the CSC-temperatures among the protons; the 7′ and α' protons indicated 110 and 80°C, respectively. When microwave irradiation was applied at 130 W, the CSC-temperature was increased to 140°C for the majority of the protons, although values of 210 and 330°C were reached for the α' and 7′ protons, respectively. A further 8 min irradiation was required to obtain a saturated temperature. It is noted that the CSC-temperature of 7′ and α' protons showed higher CSC-temperature than the other protons. After microwave irradiation at 195 W, the CSC-temperature increased to 160°C within 5 min, although the α' and 7′ protons were discrepant from the other protons, with 220 and 350°C, respectively. Thus, α' and 7′ protons show much higher CSC-temperature than the other protons due to a

non-thermal microwave effect as in the case of the OH protons of ethanol.

**mixed solution and MBBA isotropic state**

the other non-polar proton groups.

which causes a temperature increase.

**11. Characterization of non-thermal microwave effect of ethanol-hexane** 

As shown in **Figure 3**, the CSC-temperature of the OH group of ethanol under

It is important to consider microwave heating from a physicochemical (thermodynamical) point of view to explain the energy flow mechanism. When energy is supplied to the solution under constant pressure (P) and temperature (T), the change of the Gibbs free energy (dG) is described as the difference of the changes in the enthalpy term dH = dU + d(PV), where dU is the change in the total internal energy which consist of dQ(heat) and dW(work), and entropy (TdS) terms. In the case of a solution state, the change of volume (V) is very small and the d(PV) term can be neglected, so that the Gibbs free energy is given by dG = dH – TdS = dQ + dW - TdS. In the case of a conventional thermal heating process, the change of the Gibbs free energy (dG) mainly increases the heat energy (dQ ) term,

**Figure 7** shows the energy flow processes from a physicochemical point of view. In the thermal heating process, the directions of polar molecules fluctuate

microwave irradiation was considerably lower than those of the CH2 and CH3 protons. OH protons have a different CSC-temperature from the other protons. A temperature increase of the OH protons induces a higher field shift due to a reduction of hydrogen bonding under thermal heating [40]. Similar higher field shifts have been observed in H2O protons [56]. In contrast, the CSC-temperatures of the OH protons are lower than the bulk temperature under microwave irradiation; therefore, the lower field shift of OH protons indicates the presence of a nonthermal microwave effect. On the other hand, the CSC-temperature of 7′ and α' protons of MBBA in the isotropic state showed much higher CSC-temperature than

that the chemical shifts of different protons showed very different temperature variations. However, the chemical shift did exhibit a linear change as a function of the temperature for each different proton; therefore, it was possible to estimate the CSC-temperature of MBBA in the isotropic phase under microwave irradiation.

**Figure 6B** presents an increase of the CSC-temperature for the MBBA sample

**178**

*Energy flow pathway under (a) conventional thermal heating and (b) microwave heating. Blue arrows indicate electric dipolar moment vectors and orange arrows indicate electric field vectors. Adapted with permission from [34]. Copyright (2020) American Chemical Society.*

randomly, so that no net dipolar moment is induced (**Figure 7(a)**). In contrast, the microwave heating process induces order of the polar molecules to the electric field and a decrease of the entropy term (dS1 < 0), therefore, the Gibbs free energy is increased to absorb microwave energy in the system as a first step, dG = dQ1 + dW1 – TdS1, as shown in **Figure 7(b)**(1). The electric field oscillates with a frequency of 2.45 GHz; therefore, the polar molecular order is simultaneously reduced by an increase of the entropy term (dS2 > 0) by conserving the Gibbs free energy, 0 = dQ2 + dW2 – TdS2, as a second step (**Figure 7(b)**(2)). Energy (TdS2) thus dissipates as heat and work terms, dQ2 + dW2 to the system. By considering the sum of these two steps, dG = d(Q1 + Q2) + d(W1 + W2) – Td(S1 + S2), (**Figure 7(b)**(3)), the heat term d(Q1 + Q2) increases the temperature as thermal microwave effect and the work term d(W1 + W2) may change the CSC-temperature for the OH group as a non-thermal microwave effect involving work terms such as molecular ordering and hydrogen bond formation. We interpret how the dW term changes the CSC-temperature of the OH groups in ethanol and H-C=N (7′) and CH3-O (α') groups in MBBA.

It is important to note that polar molecules follow the oscillating electric field in a coherent manner. In this case, coherently aligned polar molecules are able to

interact with each other; therefore, there is an electrostatic interaction between the molecules that may specifically change the electric polarization in the polar group (OH, H-C=N, and CH3-O groups) and thus cause a change of the electron density in these group, thereby inducing a chemical shift change. OH groups are polarized to O− H+ in the presence of an electric field, so that the electron density of OH protons may be reduced and the 1 H chemical shift is therefore expected to shift to the lower field under microwave irradiation. Since this process does not change the thermal heat energy of the system, the non-thermal microwave effect of dW is evident. In the case of ethanol, molecular order may increase the number of hydrogen bonds between the OH groups because ethanol molecules form clusters in a non-polar solvent [57], which induces a lower field chemical shift due to a microwave nonthermal effect that is in the direction opposite to conventional thermal heating. In summary, the entropy term is decreased to supply the microwave energy to the system and the entropy term is then subsequently increased because of dielectric loss by the change of electric field to dissipate the (dQ + dW) energy to the system (**Figure 7(b)**). As a result, the temperature is increased by the dQ term due to the thermal microwave effect, and the CSC-temperature of OH groups is further changed by the dW term due to the non-thermal microwave effect. MD simulation was further performed to characterize thermal and non-thermal microwave effects from a microscopic point of view in the following section.
