Ionic conduction, microwave heating

The principle of FMW involves the heating of both the solvent and the matrix by wave/matter interactions. The microwave energy is converted into heat by two mechanisms dipole rotation and ionic conductance. The heating is, therefore, selective with only polar or moderately polar compounds susceptible. Due to the use of low microwave energy the structure of target molecules remains intact. 

Ionic liquids interact very efficiently with microwaves through the ionic conduction mechanism (see Section 2.2) and are rapidly heated at rates easily exceeding 10 °C s"1 without any significant pressure build-up [52]. Therefore, safety problems arising from over-pressurization of heated sealed reaction vessels are minimized. 

Differences in sample size, shape and composition can also affect heating rates. The last case particularly applies when ionic conduction becomes possible through the addition or formation of salts. For compounds of low molecular weight, the dielectric loss contributed by dipole rotation decreases with rising temperature, but that due to ionic conduction increases. Therefore as an ionic sample is microwave-irradiated, the heating results predominantly from dielectric loss by dipole rotation initially, but the contribution from ionic conduction becomes more significant with temperature rise. 

Ions in a food oscillate transversely under the influence of the microwave electric field, colliding with their neighboring atoms or molecules. These collisions impart molecular motion which is defined as heat. Materials with mobile ions are conductive. The more available ions in a food, the higher the electrical conductivity. Microwave absorption in a food thus increases with its ionic content. The portion of microwave absorption due to ionic conduction can be described as a portion of the dielectric loss factor, ec. Geyer (1990) recently discussed this concept in his publication. 

Due to the particular effects of the microwaves on matter (namely dipole rotation and ionic conductance), heating of the section, including its core, occurs instantaneously, resulting in rapid breakdown of protein crosslinkages. Furthermore, the extraction and recovery of a solute from a solid matrix with microwave heating is routinely obtained in the field of analytical chemistry (Camel, 2001). However, a definite, full explanation of the effects of microwave heating on the molecular aspect of antigen retrieval is awaited. 

MAOS is mainly based on the efficient heating of materials by the microwave dielectric heating effect [15] mediated by dipolar polarization and ionic conduction. When irradiated at microwave frequencies, the dipoles (e.g., the polar solvent... 

The second microwave heating mechanism arises from the migration of ions in the electric field. The resulting current from the oscillating ions gives rise to heat in the familiar way, following the i2r law, where / is the current and r reflects the resistance or impedence to ionic movement through collisions with other ions and molecules present in the medium. Ionic conduction is important in situations where the ions are free to move to some extent. 

Microwave energy is lost to the sample via two mechanisms ionic conduction and dipole rotation. Both occur simultaneously in many practical applications of microwave heating. 

Temperature dictates to a great extent the relative contribution of each of the two energy conversion mechanisms (dipole rotation and ionic conduction). With small molecules such as water and other solvents, the dielectric loss to a sample due to the contribution of dipole rotation decreases as the sample temperature increases. By contrast, the dielectric loss due to ionic conduction increases as the sample temperature increases. Therefore, as an ionic sample is heated by microwave energy, the dielectric loss to the sample is initially dominated by the contribution of dipole rotation. As the temperature increases, the dielectric loss is dominated by ionic conduction. 

The relative contribution of these two heating mechanisms depends on the mobility and concentration of the sample ions and on the relaxation time of the sample. If the ion mobility and concentration of the sample ions are low, then sample heating will be entirely dominated by dipole rotation. On the other hand, as the mobility and concentration of the sample ions increase, microwave heating will be dominated by ionic conduction and the heating time will be independent of the relaxation time of the solution. As the ionic concentration increases, the dissipation factor will increase and the heating time decrease. The heating time depends not only on the dielectric absorptivity of the sample but also on the microwave system design and the sample size. 

The assistance of microwave heating has been also proposed to accelerate RCM reactions using classic ruthenium-based catalysts. The reaction can be rapidly conducted in either ionic liquids, such as [bmim][BF4], or in a micro-wave transparent solvent such as dichloromethane. 

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