Variation of dielectric properties of aqueous solutions of ethanol and acids at various temperatures with low acid concentration levels

Microwave-assisted extraction is gaining in popularity for the production of plant extracts and the dielectric properties determine the behaviour of different materials in a microwave environment; hence the measurement of the dielectric properties of different solvents used for analytical purposes and the interpretation of their heating rate can help in minimising the time and solvent consumption. In this study, the dielectric constant and dielectric loss of different combinations (generated using central composite design (CCD)) of ethanol concentrations and acid (HCl and citric acid) at different temperature levels were measured at frequencies varying from 0.5 GHz to 6 GHz, and the dissipation factor and depth of penetration were calculated for the respective cases, which were analysed for the interpretation of heating rates of the different solvent combinations. Overall, for all the responses (observed and calculated), ethanol concentration levels and temperature levels were found to be the most significant factors; it was interpreted and observed that the heating rate increases with an increase in ethanol concentration and decreases with increasing temperature. In general, the dissipation factor was interpreted to increase with increase in acid concentration (observed from screening analysis); however, citric acid concentration was found to have an insignificant effect on the dissipation factor (as interpreted from CCD with response surface methodology (CCD-RSM)), and hence was found to have insignificant contribution to the heating rate at lower concentrations. In case of CCD-RSM analysis of the values of dissipation factor for the combinations of ethanol with HCl, the effect of acid concentration on the dissipation factor was found to be significant; however, it was still found to be negligible when the solvent combinations were heated in the microwave reactor.