Vol 10, No 7 (2016)

The luminescence features have been studied in ultradisperse CdSe quantum dot-based composites embedded in low-density polyethylene (LDPE) and polytetrafluoroethylene (PTFE) powders using supercritical carbon dioxide. The thermal treatment of the LDPE sample in air is shown to cause the CdSe photoluminescence band shift toward the shorter wavelengths, whereas for PTFE it shifts to the longer wavelengths with a decrease in its integrated intensity. In both cases, the photoluminescence band behavior under near-UV radiation is determined by the photooxidation processes on the CdSe nuclei.

Publications dedicated to the application of supercritical fluids in the advanced processing of plant raw materials for the production of a wide range of products such as ethanol, liquid and gaseous fuels, sugars, organic acids, and oils are reviewed and analyzed. The specific features of the supercritical fluid technologies and their advantages over the existing traditional methods for the processing of plant raw materials are demonstrated.

Derivatives of benzoic and cinnamic acids are secondary metabolites of plants, widely distributed in nature and possessing biological activity. Supercritical fluid chromatography (SFC) is shown to provide a high performance separation of nine most important representatives of this class of compounds with selectivity, dramatically different from the reversed-phase high-performance liquid chromatography. The retention and chromatographic separation parameters of analytes for four stationary phases of different nature are compared and the effect of supercritical fluid parameters and the composition of the mobile phase onto the chromatographic separation are analyzed. The optimal separation is found to be achieved when using a silica-based sorbent with 2-ethylpyridinium linked groups. The proposed approach for determination of the above-mentioned compounds is based on the combination of chromatographic separation with multiwavelength spectrophotometric detection and provides the detection limits in the range of 13.0–51.3 μg/L and the analysis duration about 2.5 min. The approach is successfully tested on real objects, such as different kinds of wine.

Supercritical fluid extraction (SCFE) using carbon dioxide containing tributyl phosphate (TBP), di-(2-ethylhexyl)phosphoric acid (D2EHPA) and their adducts with HNO₃ is applied for extraction of rare earth elements (REE), thorium (Th) and uranium (U) from monazite concentrate (MC) and phosphogypsum (PG). REE extraction from MC and their separation from Th and U are carried out from the product of MC–Na₂CO₃ baking (MCS), which is obtained under microwave irradiation, after which the phosphates of REE, Th and U present in the MC are converted into their oxides. Up to 50% of REE can be recovered as the adducts of TBP and D2EHPA with HNO₃ from the resulting powdered MCS under SCFE conditions, whereas Th and U remain in the solid phase. After a complete dissolution of the MCS residue in the mixture of 4 M HCl and 0.05 M HF, Th and U are quantitatively extracted using supercritical carbon dioxide (SC CO₂) containing D2EHPA and thus separated from the REE that remain in an acidic solution. The conditions of quantitative isolation of REE, Th and U from PG are determined. The schemes for complex processing of MC and PG aimed at REE recovery and their separation from Th and U are suggested.

The solubility of ammonium palmitate in supercritical carbon dioxide is studied in the dynamic regime at 308.15–333.15 K and 10.0–35.0 MPa. Experimental data are described in the framework of the Peng–Robinson equation of state.

A self-consistent approach to the measurement of the solubility of biologically active compounds in supercritical (SC) media based on infrared spectroscopy is exemplified by paracetamol in supercritical CO₂. According to this approach, the integral extinction coefficient for a chosen analytical spectral band is determined from additional spectroscopic measurement of the compound under study dissolved in an inert solvent at known concentrations in the considered range of the parameters of state (pressure and temperature, p and T). The values of solubility of the compound under study in the SC medium are calculated based on the above-obtained values of the extinction coefficient and the analytical values of the integral intensity of the chosen spectral band for saturated solutions in the p,T-range of interest. The method allows one to determine the solubility with a high accuracy even if the measured values are as low as 10^–4–10^–1 mol %.

The process of supercritical fluid encapsulation of pharmaceutical grade risperidone into bioresorbable D,L-polylactide microparticles via the PGSS (Particles from Gas-Saturated Solutions) method was examined. Micronization and changes in the morphology of risperidone crystals during its encapsulation into a polymer plasticized with supercritical carbon dioxide were experimentally observed. This result made it possible to prepare the polymer structures of various dispersities (from 10 to 100 μm) and morphologies containing up to 40 wt % of risperidone without the use of organic solvents. The kinetics of release of risperidone from polymer microparticles in saline solution was studied by UV spectrophotometry. It was shown that the use of D,L-polylactides of various molecular weights makes it possible to achieve a controlled increase in the time of release of risperidone from bioresorbable polymer particles prepared via the PGSS technique up to ten days.

The regeneration of an average temperature isomerization catalyst with supercritical carbon dioxide together with a cosolvent was studied. An effect of regeneration conditions on catalytic activity of the studied catalyst has been defined.

A study of conformational equilibria of ibuprofen in vacuum and supercritical carbon dioxide (SC CO₂) was carried out using methods of quantum chemistry and molecular dynamics simulation. It was found that ibuprofen forms 12 different conformations in SC CO₂, two of them correspond to the structure of the molecule in the crystal lattice of the first and second polymorph. It was shown that the GAFF force field allows one to obtain the characteristics of conformers, almost coinciding with those obtained by quantum chemical calculations at the B3LYP/cc-pVTZ level of theory.

Analysis of the experimental data on the solubility of methylxanthines (theophylline and theobromine) in supercritical (SC) solvent CO₂–methanol at various concentrations of methanol within the framework of the ASL model (Associated Solution + Lattice) based on molecular association theory and the simple lattice model is presented. Hetero-association of methylxanthines with the molecules of methanol is studied by means of ¹H NMR spectroscopy at 313 K. The contribution of molecular association to the solubility of methylxanthines in the mixed SC solvent CO₂–methanol is analyzed. It is shown that the presence of NH group in the molecule of methylxanthine, when passing from caffeine to theophylline or theobromine, leads to an increase of the contribution of the component related to molecular association to solubility.