Microwave heating of water–ethanol mixtures

Alberto Schiraldi,Alberto Paoli

doi:10.1007/s10973-021-11074-w

Abstract

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, XEtOH = 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·(H2O)m ring clusters that change the hydrogen bond network and/or favor intermolecular hydrophobic hydration. The decamer, (EtOH)9·H2O, could be responsible for the peculiar MW susceptibility of the azeotropic mixture.

Highlights

This paper is a critical reedition of a 25 year old congress communication [1]
The liquid water–ethanol binary seemed a simple model system, since ethanol tends to flash off, as do many aromas from food products, during the MW heating, and can exemplify the competition between different compounds that are susceptible to 2450-MHz MW
The heating rate, Ṫ, produced in water–ethanol mixtures by a 2450 MHz MW irradiation depends on the composition, and attains the largest value for the azeotropic composition, at temperatures below the respective boiling point

Summary

Introduction

This paper is a critical reedition (with data and figures updated to the current standards and including not published results) of a 25 year old congress communication [1] (that remained someway hidden to most chemists and physicists). The work, originally aimed to provide some fundament to the common practice of comparing the heating rate in a given product, and in a sample of pure water during 2450MHz MW thawing/cooking food products, and/or preparing concentrated essential oils [2, 3]. The present work reexamines the old data and provides some tentative interpretation taking into account the studies appeared in the 25 year . The simple check of the raising temperature produced by a MW irradiation leads to conclusions in agreement with spectroscopy and molecular dynamics studies. This finding suggests that this approach may be of help in preliminary investigations on new materials, including novel food products, allowing a tentative ranking of their MW susceptibility

Methods

Results

Conclusion