Dielectric heating interactions with molecular dipoles

There are two mechanisms by which microwaves interact with reaction mixtures [7]. Polarization of dielectric material arises when the distribution of an electron cloud is distorted or physical rotation of molecular dipoles occurs. For generation of heat on irradiation with microwaves, at least one component of a reaction mixture must have a dipole moment. Compounds with high dipole moments also have large dielectric constants, e. The selectivity of microwave irradiation is clear when comparing the heating of water and hexane. Water, a polar solvent, has a high dielectric constant and therefore heats rapidly on microwave irradiation whereas hexane, a nonpolar solvent, heats very slowly. 

The choice of solvent is of particular importance. First, it has to be chosen such that the solubility is in a suitable range for the selected type of crystallization experiment (reasonably high solubility for cooling experiments, very low solubility in solvents used for precipitation, etc.). Second, it is important to use solvents with diverse physical properties in order to explore the whole parameter space of possible environments. In addition to molecular solvent-solute interactions, bulk properties of solvents such as viscosity may play a role. Gu et al. [19] examined 96 solvents in terms of 8 relevant solvent properties hydrogen bond acceptor propensity, hydrogen bond donor propensity, polarity/dipolarity, dipole moment, dielectric constant, viscosity, surface tension, and cohesive energy density (calculated from the heat of vaporization). Based on all 8 properties, the 96 solvents were sorted into 15 groups... 

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