Carbon Dioxide Expanded Ethanol Extraction: Solubility and Extraction Kinetics of α-Pinene and cis-Verbenol

Abstract

In general, diffusion rates in extractions are enhanced by increasing the temperature. In this study, we instead add compressed liquid carbon dioxide to the extraction phase to accomplish faster mass transfer. The feasibility of using carbon dioxide expanded ethanol (CXE) as the extraction phase was explored, targeting two medium-polar analytes, α-pinene and cis-verbenol in Boswellia sacra tree resin. Hansen solubility parameters (HSP) were first calculated for the analytes and the extraction phases investigated, ethanol, CXE, and supercritical carbon dioxide (scCO2) containing ethanol as a cosolvent. Second, an extraction method with CXE as the extraction phase was optimized using a Box Behnken design, giving optimal conditions of 40 °C, 9.3 MPa, and 0.31 molar fraction of CO2 in ethanol. Third, the developed method was compared with a supercritical fluid extraction (SFE) method and a conventional solid liquid extraction (SLE) method, showing that CXE enables faster and more efficient extraction than both SFE and SLE. In fact, calculations based on Peleg's equation showed that the initial extraction rate of the new method is up to 10 times faster than SFE when using the highest flow rate tested, 3 mL/min. It was also discovered that it is crucial to cool the makeup solvent in the collection system for efficient analyte collection, at least in modern SFE equipment where pressure is regulated by a backpressure regulator. The use of CXE and pertinently also other CO2-expanded liquids in sample preparation shows a great potential in terms of increasing the extraction rate without elevating the temperature.