Chemistry Reference
In-Depth Information
Dipolar polarization should not be confused with conduction, since the latter results from translational motion of the
charges when the electric field is applied [15]. As the dipole re-orientates to align itself with the electric field, the
field is already changing and generates a phase difference between the orientation of the field and that of the dipole.
This phase difference causes energy to be lost from the dipole by molecular friction and collisions yielding dielectric
heating [4].
Ionic conduction in the matrix by the interaction with microwave radiation is a much stronger heat generator than
the corresponding motion of dipoles. Ionic species heat up very quickly when exposed to microwave irradiation.
This property can be used to improve the heating ability of non-polar solvents [13].
It has to be emphasized that the microwave heating effect depends on the frequency as well as the power applied.
The heat generated by this process is directly related to the ability of the matrix to align itself with the frequency of
the field. If the particle does not have enough time to realign (high-frequency irradiation) or reorients too quickly
(low-frequency irradiation) with the applied field, no heating occurs. The allocated frequency of 2.45 GHz used in
all commercial systems lies between these two extremes and gives the particle time to respond (rotate) to the
alternating electric field [17].
IMPORTANT PARAMETERS
One source of microwave dielectric heating lies in the ability of an electric field to polarize charges in a material and
the inability of this polarization to follow quick reversals of an electric field
[16]
. Thus, the heating characteristics of a
particular material (e.g., a solvent) under microwave irradiation are dependent on its dielectric properties [5].
Two parameters define the dielectric properties of materials and are used extensively in microwave chemistry. The
first one is the dielectric constant or relative permittivity (´), which
describes the ability of the molecule to be
polarized or to store electric charge by the electric field. The second one is the dielectric loss or complexed
permittivity (´´), which is the amount of input microwave energy that is lost to the sample dissipated as heat,
reflecting the conductance of the material [8,16].
The ability of a specific substance to convert energy of the electromagnetic radiation into thermal energy at a given
frequency and temperature is determined by the following equation: tan = ´´/ ´. Tangent delta (tan ) or so-called
loss factor is defined as the ratio of the dielectric loss to the dielectric constant. In summary, these three main
dielectric parameters (tangent delta, dielectric constant and dielectric loss) are related to the ability of a solvent to
absorb microwave energy [8].
Considering the importance of solvents in organic chemistry, as well as in microwave chemistry, Table
1
shows the
dielectric parameters differences among some common organic solvents
[8]
. According to the loss factors (tan ),
these solvents can be classified as high (tan > 0.5), medium (tan 0.1-0.5) and low (tan < 0.1) microwave
absorbing. The dielectric loss (´´) value is the most indicative parameter of how quickly a solvent will reach high
temperatures and, in general, the higher this value, the more efficient the solvent converts microwave energy into
thermal heat. Thus, a reaction medium with a high tan value will present efficient absorption and, as a
consequence, quick heating [3,8].
Table 1:
The loss factors (tan ) measured at 20
0
C and 2.45 GHz of different solvents [8].
Solvent
tan
´´
´
Ethylene Glycol
1.350
49.950
37.0
Ethanol
0.941
22.866
24.3
DMSO
0.825
37.125
45.0
2-Propanol
0.799
14.622
18.3
1-Propanol
0.757
15.216
20.1
Formic Acid
0.722
42.237
58.5
Methanol
0.659
21.483
32.6
