Microwave-Assisted Superheating and/or Microwave-Specific Superboiling (Nucleation-Limited Boiling) of Liquids Occurs under Certain Conditions but is Mitigated by Stirring.

Anthony Ferrari,Albert Stiegman,Gregory Dudley,Jacob Hunt

doi:10.3390/molecules201219793

Anthony Ferrari, Albert Stiegman + Show 2 more

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https://doi.org/10.3390/molecules201219793

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Journal: Molecules (Basel, Switzerland)	Publication Date: Dec 4, 2015
Citations: 17	License type: CC BY 4.0

Affiliation: Florida State University

Abstract

Temporary superheating and sustained nucleation-limited “superboiling” of unstirred liquids above the normal atmospheric boiling point have been documented during microwave heating. These phenomena are reliably observed under prescribed conditions, although the duration (of superheating) and magnitude (of superheating and superboiling) vary according to system parameters such as volume of the liquid and the size and shape of the vessel. Both phenomena are mitigated by rapid stirring with an appropriate stir bar and/or with the addition of boiling chips, which provide nucleation sites to support the phase-change from liquid to gas. With proper experimental design and especially proper stirring, the measured temperature of typical organic reaction mixtures heated at reflux will be close to the normal boiling point temperature of the solvent, whether heated using microwave radiation or conventional convective heat transfer. These observations are important to take into consideration when comparing reaction rates under conventional and microwave heating.

Highlights

Accurate temperature measurements are critical to understanding thermochemical processes
Even when accurate bulk temperature is determined, one must recognize and consider that microwave radiation creates heat through mechanisms that are distinct from those of convective heating, which potentially can result in selective heating and inhomogeneous temperature distributions that cannot be detected by bulk temperature measurements
Much of the early microwave chemistry lite7rature has been called into question—sometimes appropriately—due to concerns over irreproducibility and underestimation of bulk solution temperatures

Summary

Introduction

Accurate temperature measurements are critical to understanding thermochemical processes. We immediately ruled out this alternative explanation of the observations recounted in Scheme 1 by directly measuring in situ the reflux temperature of our (stirred) reaction mixture (Figure 1, black line) [2,8].

Results

Conclusion