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.

Popular Abstract

Carbon dioxide (CO2) is a colourless and odourless gas present in all living organisms and the entire atmosphere on Earth. The molecule is formed by one carbon and two oxygen atoms. It is an essential molecule for plants to make the photosynthesis process, which produces the oxygen we breathe. It is also the main product of plant and animal respiration and burning process such as forest fires and volcanic eruptions.1 Levels of carbon dioxide would be well-regulated in the atmosphere if human interference did not exist. Nowadays, increasing levels of the gas as a result of mankind’s activities are intensifying the greenhouse effect, a natural phenomenon essential for making Earth’s atmosphere warm enough to support life, and therefore making carbon dioxide to be seen as a global issue.
In the chemistry field, on the other hand, carbon dioxide is not seen as a threat but a solution. At a certain pressure and temperature (74 bar and 31ºC), that is above our surrounding conditions but still quite mild for chemical applications, CO2 can reach a state called supercritical and become a fluid. Beyond these values, CO2 present improved properties than as a gas, allowing it to work as a solvent. This means that some substances can be dissolved in it to a certain extent. Compared to toxic organic solvents commonly used in chemistry, such as hexane, CO2 is non-toxic, non-flammable, cheaper, abundant, renewable and therefore a better option.Examples of industrial applications with CO2 as a supercritical fluid involve extraction processes, such as removal of caffeine from coffee (decaffeination), particle size control of active pharmaceuticals ingredients (in order to improve its deliver in the human body), surfaces cleaning and textile dyeing.
In the scientific area, very little is known about how fast substances are completely dissolved in supercritical CO2 (scCO2) with time, which are called kinetic studies. Pressure and temperature changes can also affect the dissolution speed. Understand such a system can help chemical processes by determining the best conditions to save time, control amount of substance used and improve yield, all relevant for an industry, since money is involved.
One such study concerns the use of high pressure and temperature. The mixture of substance+scCO2 is commonly placed inside a sealed vessel to keep the conditions. Now imagine how hard it would be if a small amount of this mixture had to be taken out of the vessel to be analyzed without affecting the pressure and temperature. To overcome this problem, there is technique called in situ, that is analyzing the mixture directly inside the vessel. In the present work, in-situ Raman spectroscopy technique will be used. Spectroscopy is any study that involves the interaction of a matter with light. In the case of Raman, the light source is a laser, like the ones used as pointers, but at a much higher power. Basically, a powerful laser will be pointed to the mixture and through interactions undetectable by the naked eye, the energy of the laser will be split into lower energies, same is to say that the light will be scattered. This scattering is very specific for each substance and it is seen in form of peaks in a spectrum, which can be called the ID or fingerprint of your substance in the Raman.
In this work dissolution kinetics of limonene in scCO2 was studied at different combinations of pressure and temperature inside a vessel by in-situ Raman spectroscopy. The curve fitting of the processed data showed that the solubility kinetics can be described by a first order exponential equation. Dissolution rate values of the fitted curves showed that the kinetic of solubility is faster at lower pressures and temperatures.

• author: Buzolin, Cristiana
• supervisor: Irene Rodriguez Meizoso
• organization: Department of Chemistry
• course: KEMR36 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: limonene, Raman spectroscopy, supercritical carbon dioxide, dissolution kinetics, solubility, analytical chemistry, analytisk kemi
• language: English
• id: 8894677
• date added to LUP: 2016-11-15 13:59:17
• date last changed: 2016-11-15 13:59:17