Microwaves reduce water refractive index

Abstract

Microwaves, long used as a convenient household appliance, have been increasingly used in industrial processes such as organic synthesis and oil processing. It has been proposed that microwaves can enhance these chemical processes via a non-thermal effect. Here we report the instantaneous effect of microwaves on the permittivity and phase velocity of light in water through the in-situ measurement of changes in refractive index. Microwave irradiation was found to reduce the water refractive index (RI) sharply. The reduction increased as a function of microwave power to a far greater extent than expected from the change in temperature. The phase velocity of light in water increases up to ~ 5% (RI of 1.27) during microwave irradiation. Upon stopping irradiation, the return to the equilibrium RI was delayed by up to 30 min. Our measurement shows that microwaves have a profound non-thermal and long-lasting effect on the properties of water. Further investigation is planned to verify if the observed RI reduction is restricted to the region near the surface or deep inside water bulk. The observation suggests a relationship between microwave-induced and the enhanced aqueous reactions.

Influence of microwave on aqueous solutions

“…9 After stopping irradiation, refractive index pro les for temperature behave similarly at each power. The water/ethanol mixtures behave similarly to pure water, 7 with a more substantial reduction. Figure 2a shows the minimum RI at different power and ethanol content.…”

“…On the other hand, the previous result showed up to 1 hour for water. 7 In this case, NaCl prevents voids' formation during MW, and the reduced RI was due to the temporary molecular re-orientation.…”

“…This refractive index system is introduced in a microwave reactor. 7 When the refractive index of surroundings, n 2 is in uenced by the rotation of polar molecules during microwave irradiation, the intensity of leaked evanescent light is changed.…”

“…Recently, we have found that microwave has a non-thermal effect on the refractive index (RI). 7 During microwave irradiation, the refractive index of water was shown to be much lower than thermal-driven reduction. Microwaved water has up to a 5% RI reduction, while hot water has a 1% RI reduction even at a close boiling point.…”