μFlowCal – High‐Resolution Differential Flow Microcalorimeter for the Measurement of Heats of Mixing

Abstract

Invited for this month's cover is the group of Prof. Luís M. N. B. F. Santos at the University of Porto (Portugal). The cover picture shows a scheme of the μFlowCAL working principle. This flow type microcalorimeter is able to measure the heat of mixing/reaction of two fluids injected in a glass micromixer micro reactor of 200 μL. A second micromixer/reactor is used as a calorimetric reference. This calorimeter allows for an accurate measurement of heats of mixing or dilution using very small samples. Read the full text of their Research Article at 10.1002/cmtd.202100099.

What prompted you to investigate this topic/problem?

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Amongst other research topics, the group has been studying the solvation of alcohols in ionic liquids at infinite dilution using isothermal titration calorimetry (ITC). As the project advanced, there was the need to explore higher concentration ranges. Despite of the indisputable versatility and sensitivity of the ITC used to perform batch experiments at high dilutions, the measurement of the complete composition range would never be possible in a unique experiment and would always involve several measurements and a very careful experimental design to overcome some drawbacks as for example, the presence of a gas phase. The recent advances in the mili- and micro flow reactor technology allowed the miniaturization of conventional calorimeters. Nevertheless, until this moment, no miniaturized flow calorimeter is commercially available in the market.

What aspects of this project do you find most exciting?

There is an unquestionable scientific effort and advance in the miniaturization of the calorimeters up to the chip-calorimetry technology and in its utilization to the investigation of fast processes and small samples. These advances are notorious in the commercialization of fast-scanning calorimeters wish can be now be used for example to study the fusion of samples that before would decompose. However, far more modest advances have been made to the high-level miniaturization of calorimeters to explore slow and low energy processes. In this work, a new differential heat conduction flow microcalorimeter was designed to study novel promising solvents and their mixtures. In contrast to most of the miniaturized calorimeters, μFlowCal was developed sacrificing the ability to follow instantaneously the dynamics of the process under analysis. Instead, it was constructed to have long baseline stability with minimum noise level (± 150 nV over 15 h), along with high sensitivity (116 μV/mW), allowing to measure accurately the heats of mixing (minimum detectable power of 170 nW) of relatively viscous samples.

What future opportunities do you see (in the light of the results presented in this paper)?

The future opportunities can be seen at two levels:

To our research group, this calorimeter allowed the expansion of the experimental methodologies available, filling the need for a microcalorimeter capable of measuring the heat of mixture of viscous samples, in a large composition range, in the absence of gas phase and using low amounts of sample. This will allow the group to proceed with its ongoing research projects and to explore some new research lines associated to more complex mixtures.

For the academic community, a new calorimetric methodology has been totally described in this publication instead of protected by a patent. As such, the calorimeter can now be reproduced in other labs opening the door to new and innovative areas of research related with solvents of recent academic and industrial interest as ionic liquids and deep eutectic solvents. It is also foreseen that this technique might as well, motivate and inspire future advances in isothermal flow calorimetry. [IMAGE OMITTED. SEE PDF.]