Microwaves are non-ionizing electromagnetic waves of frequency between 300 MHz to 300 GHz and positioned between the X- ray and infrared rays in the electromagnetic spectrum (16). In modern day science microwaves serves two major purpose – communication and as energy vectors. The latter application is the direct action of waves on materials that has the ability to convert a part of the absorbed electromagnetic energy to heat energy. Microwaves are made up of two oscillating perpendicular field’s i.e. electric field and magnetic field and the former is responsible for heating. Unlike conventional heating which depends on conduction – convection phenomenon with eventually much of the heat energy being lost to the environment. Whereas in
case of MAE, heating occurs in a targeted and selective manner with practically no heat being lost to the environment as the heating occurs in a closed system. This unique heating mechanism can significantly reduce the extraction time (usually less than 30 min) as compared to Soxhlet. The principle of heating using microwave is based upon its direct impact with polar materials/solvents and is governed by two phenomenon’s: ionic conduction and dipole rotation, which in most cases occurs simultaneously (6,16). Ionic conduction refers to the electrophoretic migration of ions under the influence of the changing electric field. The resistance offered by the solution to the migration of ions generates friction, which eventually heats up the solution. Dipole
rotation means realignment of the dipoles of the molecule with the rapidly changing electric field. Heating is affected only at a frequency of 2450 MHz. The electric component of the wave changes 4.9 × 104 times per second. Every time the solvent molecules tries to align itself with the electric field to keep itself in the same phase, but with the electrical component of the wave changing at such a rapid speed, the molecules fails to realign itself and starts vibrating which
generates heat through frictional force. With frequency greater than 2450 MHz the electrical component even changes at a much higher speed as a result the molecules doest not get sufficient time to even start to align itself with the external field as a result no heating occurs. If the frequency is less than 2450 MHz the electrical component changes at a much lower speed and the molecules get sufficient time to align itself with the electric field, thus there occurs no
heating. The above mechanisms clearly indicate that onlydielectric material or solvents with permanent dipoles only do get heated up under microwave. The efficiency with which different solvents heat up under microwave depends on the dissipation factor (tan[), which is indeed the measure of the ability of the solvent to absorb microwave energy and pass it on as heat to the surrounding molecules. The dissipation factor is given by the equation:
tan [ = ]’’ / ]’,
where ]’’ is the dielectric loss which indicates the efficiency of converting microwave energy into heat. ]’ is the dielectric constant which is the measure of the ability to absorb microwave energy. ethanol and methanol will undergo lesser microwave absorption than water due to their lower ]’
value but the overall heating efficiency for both the solvents will remain higher than water (due to increased tan [ value). Whereas on the other hand hexane and other less polar solvents like chloroform will remain transparent to microwave, thus producing no heat.
