A Basic Introduction to Microwave Chemistry

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 reaction 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 colossal benefit to the chemist, saving both time and money.

The microwave Heating Mechanism

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 quantised rotational energy levels in the microwave region. In the liquid state, for all practical purposes, the quantisation 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.

Dipolar Polarisation

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 will 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.

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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. Note, though, that this is a very simplistic model of microwave heating in hydrogen-bonded systems, which actually involves a complex mechanism that does not simply result from molecular rotations.

Conduction Mechanisms

Where the irradiated sample is an electrical conductor, the charge carriers (electrons, ions, etc.) are moved through the material under the influence 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.

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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 a variant of 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.

Interfacial Polarisation

This mechanism is important for systems comprised of conducting inclusions in a second, non-conducting material. An example would be a dispersion of metal particles in, say, sulphur. Sulphur is microwave transparent and metals reflect microwaves yet, curiously, the combination forms an extremely good microwave absorbing material (So good, in fact, that interfacial polarisation effects are reputed to be the basis of 'Stealth' radar absorbant materials).
Interfacial polarisation is an effect which is very difficult to treat in a simple manner, and is most easily viewed as a combination of the conduction and dipolar polarisation effects.
For a (non-superconducting) metal, there will always be a very thin surface layer in which some of the incident microwaves are attenuated, and in which induced currents will give rise to heating. For a bulk metal this heating effect is so small as to be irrelevant, but in powders this surface layer makes up a large proportion of the material. However, the polarisation induced in the metal is also subject to the properties of the surrounding medium - in simple terms, it induces a 'drag' on the polarisation of the metallic inclusions - making it less effective than it might otherwise be. Under these circumstances, the polarisation of the metallic particles does not take place instantaneously, but lags behind the induced field, as for the polar molecule in the dipolar polarisation mechanism. Hence, the frequency dependence of the sample's heating properties is similar to that of the dipolar polarisation mechanism, despite being due to a conduction mechanism.