Electromagnetic processing of low-loss materials. Microwave spectroscopy of supercooled water confined in W/O microemulsions

The behavior of water-in-oil micro-emulsions is investigated using the Dielectric Spectroscopy approach. The deconvolution of the dielectric relaxation spectra allows the identification of the trapped water, and therefore the evolution of the Dielectric Permittivity with the temperature in the freezing range can be substantiated. In all the water-in-oil systems investigated, the water trapped within the core of the microemulsion does not freeze even at temperatures of about -28 °C. The dielectric response associated to the dispersed water fraction, exhibits a relaxation behavior quite identical to that of water in the supercooled state. The dielectric contribution in the mid-microwave range has a relaxation frequency of about 6.4 GHz at -20°C, comparable to the extrapolated value for water at the same temperature, which is about 6.0 GHz. This indicates that the dispersed droplets in the oil continuous phase contain water that remains liquid even at -20°C. The supercooled water droplets dispersed in oil is used advantageously as microwave energy absorbers to affect positively the efficiency of the microwave processing at freezing temperatures or of materials having a very low microwave absorption.