Chemistry Reference
In-Depth Information
Since the response of various materials to microwave radiation is diverse, they can be broadly classified based on
their response to microwaves: i) materials that are transparent to microwaves (e.g., sulphur), ii) materials that reflect
microwaves (e.g., copper) and iii) materials that absorb microwaves (e.g., water). These latter are called dielectric
materials, comprising permanent or induced dipoles, which allow charge to be stored, but no conductivity is
observed. Thereby, only materials that absorb microwave radiation are relevant for microwave chemistry [15].
The theory of microwave heating has been developed by many workers, amongst them Debye, Frohlich, Daniel,
Cole and Cole, Hill and Hasted [16]. This theory is based on the efficient heating of materials by “microwave
dielectric heating” effect, which drives chemical reactions by taking advantage of the ability of some liquids and
solids to convert electromagnetic radiation into heat [5,14]. As microwave irradiation is a short electromagnetic
wave, which consists of an electric and magnetic field component (Fig.
4
), several materials can be heated by
applying it. However, only the electric field is responsible for transferring energy to heat substances and the
magnetic field normally does not play any role in chemical synthesis [8].
Figure 4:
Electric and magnetic field components in microwaves. Figure adapted from Hayes, 2002.
The origin of the heating effect produced by the high frequency electromagnetic waves arises from the ability of an
electric field to exert a force on polar molecules and ions. If the charge carriers are bound to certain regions they will
move until a counter force balances them and the result is a dielectric polarization. However, if the particles present
in the substance can move freely through it, then a current has been induced [16]. Thus, electric component of an
electromagnetic field causes heating by two fundamental mechanisms: dipolar polarization (polarization of
dielectrics) and ionic conduction. The dipoles in the reaction mixture (the polar solvent molecules) are involved in
the dipolar polarization effect and the charged particles in a sample (ions) are affected by ionic conduction [17].
At the molecular level, the dipoles and ions are sensitive to external electric fields and will attempt to align
themselves or be in phase with the field [4]. As the applied field oscillates, they try to realign themselves with the
alternating electric field which results in a rotation of molecules and an oscillation of ions (Fig.
5
).
The existence of resisting forces (inter particle interaction and electric resistance) restricts the movement of these
particles generating random motion. Therefore, the energy absorbed in this process is converted into kinetic energy,
which is lost as heat through molecular friction and dielectric loss [18].
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-
-
-
(b)
(a)
Electric field
Figure 5:
Dipolar polarization (a) and ionic conduction mechanisms (b). Figure adapted from Lidström
et al.
2001.
