**12. Microscopic behavior of ethanol and MBBA molecules under microwave irradiation revealed by MD simulation**

The microscopic behavior of ethanol in an ethanol-hexane mixed solution and MBBA molecules under microwave irradiation was further investigated using MD simulation. The results of ethanol-hexane and MBBA systems are shown in **Figure 8A(a–e)** and **B(f–h)**, respectively. Eight main simulations of the ethanolhexane system were performed at 303, 313, 323, and 333 K in the presence and absence of an oscillating electric field. The net dipole moment induced by the oscillating electric field of 2.45 GHz is shown in **Figure 8(a)**. **Figure 8(b)** shows an ensemble of electric dipole moments along the x-direction (*p*x) as a function of time at 303 K; *p*x oscillated as a function of time. The phase of *p*x was delayed from the electric field oscillation by a delay of around 36 ps, which was close to the experimentally measured average dielectric relaxation time of the ethanol-hexane mixture averaged around 30 ps [58]. This result is related to the dielectric constant and dielectric loss of the ethanol-hexane mixed solution, which induces the thermal microwave effect. Similar behavior was also observed in previous simulations of ice, water, and saline solutions [5]. In contrast, *p*x fluctuated around zero in the absence of an electric field because of the random thermal fluctuation of the molecules in the solution.

The polar OH groups of ethanol molecules could affect the dipole moment of the system. Therefore, the orientation behavior of the OH groups was evaluated. The net direction of OH groups along the x-axis was calculated during all simulations, which is the sum of the length of all OH groups ( *<sup>x</sup>* = − ∑ ( *xi xi* , , ) *<sup>i</sup> OH H O* ).

**Figure 8(c)** shows the net x-direction of OH groups during the simulations at 303 K in the presence (black) and absence (red) of the electric field E. The net direction oscillated as a function of time in the presence of an electric field, unlike the random fluctuation without an electric field. This indicates that microwave irradiation suppresses the random movement of the polar groups and causes their coherent alignment [59]. This alignment decreases the entropy of the system compare with

**181**

that the 1

**Figure 8.**

*respectively.*

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

that of the random orientations. **Figure 8(d)** and **(e)** show microscopic pictures of the ethanol molecules at 303 K in the presence of positive (+E) and negative (−E) electric fields at 0.45 and 0.24 ns, respectively. It is important to note that the orientation behavior of OH bonds along the electric field direction can be observed in these figures. In contrast, the directions of the OH groups were mostly oriented in random directions in the absence of an electric field [34]. Similar results were observed in other simulations at 313, 323, and 333 K (data are not shown). A time delay between the oscillation phases of the electric field and of the net OH direction was observed, which may have perturbed the ordered state of the polar groups and caused an increase in entropy due to the dielectric loss. The energy can consequently be dissipated into the system to increase the temperature via a thermal microwave effect. Furthermore, the OH groups formed a higher number of hydrogen bonds under the electric field compared to the groups without an electric field at higher temperatures [34]. This result explains the experimental data showing

*A. Ethanol-hexane (1:1) system. (a) Applied electric field as a function of time. (b) the dipole moment of the simulations in the presence (black) and absence (red) of an electric field along the x-axis as a function of time at 303 K. (c) Net direction of OH groups as a function of time. (e) and (d) show examples of hydrogen bond formation in snapshots of the simulation at 0.24 and 0.45 ns under negative and positive electric field directions at 303 K, respectively. The hexane molecules were omitted for clarity. The dashed lines represent the hydrogen bonds between ethanol molecules, and the black arrows indicate the electric field directions in (d) and (e). Adapted with permission from [34]. Copyright (2020) American Chemical Society. B. MBBA system. (f) Applied electric field (dashed black) and the dipole moments of MBBA along the x-axis as a function of time. Net direction of C-O (g) and C=N (h) bonds of MBBA as a function of time. Black and green lines indicate the oscillation of these values as a function of time under the applied electric field at 293 and 315 K, respectively. Red and blue lines indicate the random fluctuations of the values without the electric field at 293 and 315 K,* 

H resonance of the OH group has a lower field shift and thus a lower CSC-temperature than the bulk temperature, as well as the lower field chemical shift increases at a higher temperature. It should be noted that this type of chemical shift induced by the electron polarization in the presence of an electric field under microwave irradiation may not appear under conventional thermal heating.

MD simulations of MBBA at 293 K (<Tc) and 315 (>Tc) K with and without the external electric field were also performed. **Figure 8(f )** shows the applied electric field (dashed black line) and the dipole moments of MBBA in these simulations

*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 8.**

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

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

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.

**12. Microscopic behavior of ethanol and MBBA molecules under** 

MBBA molecules under microwave irradiation was further investigated using MD simulation. The results of ethanol-hexane and MBBA systems are shown in **Figure 8A(a–e)** and **B(f–h)**, respectively. Eight main simulations of the ethanolhexane system were performed at 303, 313, 323, and 333 K in the presence and absence of an oscillating electric field. The net dipole moment induced by the oscillating electric field of 2.45 GHz is shown in **Figure 8(a)**. **Figure 8(b)** shows an ensemble of electric dipole moments along the x-direction (*p*x) as a function of time at 303 K; *p*x oscillated as a function of time. The phase of *p*x was delayed from the electric field oscillation by a delay of around 36 ps, which was close to the experimentally measured average dielectric relaxation time of the ethanol-hexane mixture averaged around 30 ps [58]. This result is related to the dielectric constant and dielectric loss of the ethanol-hexane mixed solution, which induces the thermal microwave effect. Similar behavior was also observed in previous simulations of ice, water, and saline solutions [5]. In contrast, *p*x fluctuated around zero in the absence of an electric field because of the random thermal fluctuation of the molecules in

The microscopic behavior of ethanol in an ethanol-hexane mixed solution and

The polar OH groups of ethanol molecules could affect the dipole moment of the system. Therefore, the orientation behavior of the OH groups was evaluated. The net direction of OH groups along the x-axis was calculated during all simulations,

**Figure 8(c)** shows the net x-direction of OH groups during the simulations at 303 K in the presence (black) and absence (red) of the electric field E. The net direction oscillated as a function of time in the presence of an electric field, unlike the random fluctuation without an electric field. This indicates that microwave irradiation suppresses the random movement of the polar groups and causes their coherent alignment [59]. This alignment decreases the entropy of the system compare with

which is the sum of the length of all OH groups ( *<sup>x</sup>* = − ∑ ( *xi xi* , , ) *<sup>i</sup> OH H O* ).

**microwave irradiation revealed by MD simulation**

in the presence of an electric field, so that the electron density of OH protons

H chemical shift is therefore expected to shift to the lower

**180**

the solution.

O− H+

may be reduced and the 1

*A. Ethanol-hexane (1:1) system. (a) Applied electric field as a function of time. (b) the dipole moment of the simulations in the presence (black) and absence (red) of an electric field along the x-axis as a function of time at 303 K. (c) Net direction of OH groups as a function of time. (e) and (d) show examples of hydrogen bond formation in snapshots of the simulation at 0.24 and 0.45 ns under negative and positive electric field directions at 303 K, respectively. The hexane molecules were omitted for clarity. The dashed lines represent the hydrogen bonds between ethanol molecules, and the black arrows indicate the electric field directions in (d) and (e). Adapted with permission from [34]. Copyright (2020) American Chemical Society. B. MBBA system. (f) Applied electric field (dashed black) and the dipole moments of MBBA along the x-axis as a function of time. Net direction of C-O (g) and C=N (h) bonds of MBBA as a function of time. Black and green lines indicate the oscillation of these values as a function of time under the applied electric field at 293 and 315 K, respectively. Red and blue lines indicate the random fluctuations of the values without the electric field at 293 and 315 K, respectively.*

that of the random orientations. **Figure 8(d)** and **(e)** show microscopic pictures of the ethanol molecules at 303 K in the presence of positive (+E) and negative (−E) electric fields at 0.45 and 0.24 ns, respectively. It is important to note that the orientation behavior of OH bonds along the electric field direction can be observed in these figures. In contrast, the directions of the OH groups were mostly oriented in random directions in the absence of an electric field [34]. Similar results were observed in other simulations at 313, 323, and 333 K (data are not shown). A time delay between the oscillation phases of the electric field and of the net OH direction was observed, which may have perturbed the ordered state of the polar groups and caused an increase in entropy due to the dielectric loss. The energy can consequently be dissipated into the system to increase the temperature via a thermal microwave effect. Furthermore, the OH groups formed a higher number of hydrogen bonds under the electric field compared to the groups without an electric field at higher temperatures [34]. This result explains the experimental data showing that the 1 H resonance of the OH group has a lower field shift and thus a lower CSC-temperature than the bulk temperature, as well as the lower field chemical shift increases at a higher temperature. It should be noted that this type of chemical shift induced by the electron polarization in the presence of an electric field under microwave irradiation may not appear under conventional thermal heating.

MD simulations of MBBA at 293 K (<Tc) and 315 (>Tc) K with and without the external electric field were also performed. **Figure 8(f )** shows the applied electric field (dashed black line) and the dipole moments of MBBA in these simulations

along the x-axis as a function of time. The black and green lines show the oscillation of the dipole moments along the x-axis under the applied electric field at 293 and 315 K, respectively. Due to the applied electric field, the dipole moment (black and green) at 293 and 315 K were delayed by around 32.5 and 18 ps, respectively. The values are close to the experimentally measured dielectric relaxation time of MBBA at 28 and 23.7 ps at 20 and 50°C, respectively [60]. In contrast, there is no oscillation of the dipole moment without the electric field at 293 K or 315 K, which indicates that the applied electric field affected the polar groups of the MBBA molecules.

The net direction of C-O and C=N groups of MBBA during these simulations were also analyzed, and are shown in **Figure 8(g)** and **(h)**, respectively. The α' and 7′ protons respectively bind to the C-O and C=N groups in the MBBA molecule, as shown in **Figure 6A**. **Figure 8(g)** and **(h)** show the net directions of the C-O and C=N groups as a function of time. The black and green lines show the net direction of C-O or C=N groups under the electric field at 293 and 315 K, respectively. The red and blue lines display the net directions in the simulations at 293 and 315 K without an electric field. The oscillation phase of the C-O groups is consistent with the phase of the dipole moments, as shown in **Figure 8(f )**, and their amplitudes are higher than those of the C=N groups shown in **Figure 8(h)**. This indicates that microwave heating has a more significant influence on the C-O groups of MBBA molecules.

These simulations showed the ordering of the polar molecules in the presence of an electric field as a non-thermal microwave effect. This polarization can only appear under microwave irradiation. Furthermore, this type of ordered state is also considered a low entropy state and is a distinct state occurring under microwave irradiation.
