Microwave heating of water-ethanol mixtures

A simple measure of the susceptibility of a substance to microwaves (MW) is the resulting heating rate that depends on its heat capacity, density, starting temperature, MW extinction coefficient at the used MW frequency and distance from the irradiated surface. Water, that is ubiquitous in many products, currently treated with MW, shows a large susceptibility at 2450 MHz MW. This is why water is a suitable reference to rank the MW susceptibility of other compounds. Aqueous solutions are the simplest systems to investigate how the presence of extra compounds can modify (normally, reduce) this property. The present work provides a very simple evidence of a peculiar MW susceptibility of the water-ethanol mixture with azeotropic composition, X-EtOH = 0.90 mol fraction, at temperatures rather below the respective boiling point at ambient pressure. The available literature reports a number of experimental and theoretical investigations that suggest the formation of (EtOH)(n)center dot(H2O)(m) ring clusters that change the hydrogen bond network and/or favor intermolecular hydrophobic hydration. The decamer, (EtOH)(9)center dot H2O, could be responsible for the peculiar MW susceptibility of the azeotropic mixture.