Melting point depression of tetrabutylammonium halides in supercritical carbon dioxide

Ionic liquids have already been studied as parts of biphasic reaction media or have been demonstrated as an option for capturing carbon dioxide (CO₂). Their phase behaviour in the presence of supercritical CO₂ is often of key importance in the design of high-pressure procedures. The melting behaviour of tetrabutylammonium halides was investigated from 0.1 to 40 MPa pressure. Measurements were taken in parallel using a variable-volume view-cell and an automated device developed by our research group. The new device detects the phase transition based on the combined effect of the dissolution of the medium into the sample and the change in its molar volume. The operation of the newly developed measurement method was compared to the well-established visual approach. Tetrabutylammonium iodide exhibited the greatest melting point depression (33.7 °C), at approximately 15 MPa. In comparison, melting point depressions (MPD) of the bromide and chloride salts were lower under the same conditions.