Estimation of the solubility in supercritical CO2 of α- and δ-tocopherol using Chrastil’ model

Abstract

This work proposes a correlation methodology for solubilities (wi) of α-tocopherol and δ-tocopherol in supercritical CO2, and reports new experimental data at (40, 50, 60) ºC and (9.8–33.6) MPa. The methodology is based on the Gibbs-Duhem test to assess thermodynamic consistency of isothermal solubilities, represented using the Peng-Robinson EoS + Wong-Sandler mixing rule. Chrastil’s model was best-fitted after discarding low-solubility outlier (non-represented by the EoS), high-solubility (>100 g/m3 solute/CO2, CO2 density modified by the solute), and thermodynamically inconsistent data. Additionally, the mean error of log(wi) (with thresholds for data reported of ±0.21 for α-tocopherol, and ±0.27 for δ-tocopherol) and the average bias were computed to discard questionable data sources. α-Tocopherol solubility was 3-to-4 times lower than δ-tocopherol’s at equivalent system conditions. Their solubilities exhibited less dependency on system temperature and CO2 density than reported in literature, possibly due to experimental difficulties in handling comparably high-solubility solutes (>0.5 × 10−3 mol/mol).