Dissolution kinetic study of limonene in supercritical carbon dioxide by in-situ Raman spectroscopy

Abstract: Understanding the dissolution kinetics is important in the experimental determination of solubility, the efficiency of extractions, the rate of reactions and in general any heterogeneous process. Despite the extensive use of supercritical fluids, studies regarding the dissolution kinetics of compounds in supercritical carbon dioxide (scCO2) using analytical methods are limited in the literature. So far, only one work can be found where acetaminophen was used as model compound and in-situ Infrared spectroscopy was used to investigate the dissolution kinetics for two different temperatures at the same pressure. A more common approach involving scCO2 as a solvent is the study of the kinetic of reactions, such as catalyzed organic reactions and polymerizations by the common in-situ spectroscopy techniques (IR, UV-Vis and Raman). In this work, the dissolution kinetic behaviour of limonene in scCO2 was studied at different combinations of pressure and temperature by in-situ Raman spectroscopy. The experiments were carried out inside a high-pressure constant-volume view cell. The instrumental setup consisted of a pressurized system and a linear Raman optical system, where a continuous-wave laser at 3.5 W and a wavelength of 532 nm were used. A peak correspondent to a C=C stretching mode from limonene (583-584 nm) was monitored with time, where the height is directly proportional to the amount solubilized in carbon dioxide (CO2). The experiments were carried out at temperature of 45 ºC and 55 ºC and pressure ranging from 84 to 163 bar. The time of the experiments ranged between 100 and 420 minutes. The time to reach complete dissolution varies significantly with pressure and temperature (from 30 min up to 4 hours). Curve fitting of the processed data showed that the solubility kinetics can be described by a first order exponential model. Dissolution rate values of the fitted curves showed that the rate of dissolution becomes slower with increase of pressure and temperature, in contradiction to conventional dissolution theory for liquid solvents. Experiments to evaluate the stirrer effect on the dissolution showed that for the lowest pressures tested at 45 °C and 55 °C (respectively 84 and 95 bar) stirring accelerates the process significantly, while for higher pressures stirring does not make a difference. These results are an indication that different mechanisms may govern dissolution kinetics, depending on the region of pressure and temperature studied. Thus, the results constitute the first step towards a theoretical understanding of the mechanisms of dissolution kinetics of solutes in supercritical fluids.