Microwave irradiation is becoming an increasingly popular method of
heating samples in the laboratory. It offers a clean, cheap, and
convenient method of
heating which often results in higher yields and shorter rection times.
Despite this popularity, and an increasing amount of literature on
the subject, microwaves remain an area of mystery and magic for many
people. Myths abound about the capabilities and properties of
microwaves, which unfortunately leads to unwarranted scorn when they
fail to live up to this. Microwaves are not a panacea, but used correctly
and with understanding, they can be a
collosal benefit to the chemist,
saving both time and money.
Chemists are often told that
microwaves are tuned so that water molecules absorb microwaves into
rotational energy levels, and that it is this which causes molecular
motion, and hence heating. This common misunderstanding is wrong, and
comes from a failure to realise that it is gaseous water that has
quantized rotational energy levels in the microwave region.
In the liquid state, for all practical purposes, the quantization of
rotational levels is non-existent. It is this distinction which most
people forget.
The easiest way to visualise the true mechanism is to picture microwaves for what they are - a high frequency oscillating electric and magnetic fields. Anything that is put into this field, if it may be electrically or magnetically polarised at this oscillation frequency, will be affected. Two principal heating methods exist:
Dipolar polarisation, and Conduction mechanisms
A third mechanism - interfacial polarisation - occurs, although this is often of limited importance.
For a molecule in a polar liquid such as water (methanol, ethanol, THF, etc), there are intermolecular forces which give any motion of the molecule some inertia. Under a very high frequency electric field, the polar molecule will attempt to follow the field, but intermolecular inertia stops any significant motion before the field has reversed, and no net motion results. If the frequency of field oscillation is very low, then the molecules wil be polarised uniformly, and no random motion results. In the intermediate case, the frequency of the field will be such that the molecules will be almost, but not quite, able to keep in phase with the field polarity. In this case, the random motion resulting as molecules jostle to attempt in vain to follow the field is the heating we observe in the sample.
It is interesting to note that whist the efficiency of microwave absorbance varies markedly with frequency for any liquid, the frequency of a domestic microwave oven (2.45GHz) is NOT selected so that it is at the maximum absorbancy for water (something like 10GHz). If it were you would find that most of the microwave energy was absorbed by the outer layers of your food, whilst the inside stayed unheated and hence uncooked.
Where the irradiated sample is an electrical conductor, the
charge carriers (electrons, ions, etc) are moved through the material
under the infuence of the electric field, E, resulting in a
polarisation, P. These induced currents will cause
heating in the sample due to any electrical resistance. For a very good
conductor, complete polarisation may be achieved in approximately
10-18 seconds, indicating that under the influence of a 2.45GHz
microwave, the conducting electrons move precisely in phase with the
field.
If the sample is too conducting, such as a metal, most of the microwave energy does not penetrate the surface of the material, but is reflected. However, the colossal surface voltages which may still be induced are responsible for the arcing that is observed from metals under microwave radiation
Thus, if one takes pure water and heats it in a microwave oven, where the polarisation mechanism dominates, we find that the heating rate is significantly less than when one takes the same volume of water and add salt. In the latter case, both mechanisms occur, and contribute to the heating effect.
© Gavin Whittaker, 1997.