Vol 10, No 8 (2016)

This review summarizes the achievements in the field of sterilization in supercritical (SC) media in the last 10 years. In particular, new approaches to carrying out the sterilization process are described and the possible use of a wide range of SC media is shown. In addition, some data on microbial inactivation mechanism are presented and new areas of SC sterilization application are discussed.

The dissolution of acetylsalicylic acid in supercritical carbon dioxide (SC–CO₂) is investigated using Fourier transform IR spectroscopy. Temporal dependences of integrated intensities of the absorption band of acetylsalicylic acid (ASA) near 1199 cm^–1 are measured for various ASA charges placed into a cuvette. Molar fractions of ASA in the saturated solution in SC–CO₂ are determined at a temperature of 40°C and pressures of 10.0 and 15.0 MPa, and the components are 2.10 ± 0.25 × 10^–4 and 11.0 ± 1.5 × 10^–4, respectively.

The process of micronization of levofloxacin (LF, an antibacterial agent of the fluoroquinolone group) by the supercritical antisolvent precipitation technique (SAS) was investigated. It was shown that LF particles of different sizes (from 1 to 10 μm) and of various morphologies (from thin plates to elongated parallelepipeds) can be produced depending on the type of solvent used for conducting micronization. Investigation of the micronized LF preparations using the methods of IR-Fourier spectroscopy, Raman scattering, and circular dichroism showed that the LF micronization caused neither changes in its chemical structure nor racemization. Micronization of LF significantly affects the rate of its dissolution in model systems exhibiting effects dependent on the type of the solvent used for micronization. For example, the highest rate of dissolution at pH 4 was observed for LF preparations micronized with the help of chlorohydrocarbons. It was shown that the rate of dissolution of all micronized LF preparations was higher by 15–30% in comparison with the initial LF, which likely was related to the changes in the degree of crystallinity/amorphousness, as well as of morphologies of microparticles formed in the SAS process.

The solubility of two synthetic asymmetric allyl disulfides, allyl 8-quinolyl disulfide and allyl benzothiazol-2-yl disulfide, in subcritical Freon R22 and Freon R410a, as well as supercritical carbon dioxide, at a temperature of 40°C and a pressure of 200 bar is determined by a dynamic flow method. It is shown that Freon R22 is a promising solvent for practical applications of allyl disulfides.

The spin probe method is used to study the impregnation of polycarbonate (PC) based on bisphenol A, polyethylene oxide (PEO), and crosslinked acrylamide–acrylic acid copolymer (PAA) with organic molecules in sub- and supercritical CO₂ media. Electron spin resonance (EPR) data show that, at 196 bar and 307 K, 2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl (TEMPONE) paramagnetic spin probe molecules penetrate into the PC and PEO matrices, which are, respectively, in the glassy and elastic states under normal conditions. The degree of impregnation of PAA under these conditions is negligibly small. Estimates of the local concentration of probe molecules show that, in the PEO matrix, TEMPONE is distributed much more uniformly than in the PC matrix. Analysis of the effect of temperature on the shape of the EPR spectra of the radical in the polymer matrix shows that, under the same conditions, the mobility of TEMPONE molecules in the PEO matrix is much higher than in the PC matrix. The results suggest that the spin probe method is promising for studying the characteristics of macro- and micro-processes in polymer–supercritical fluid solvent–organic molecule ternary systems.

The supercritical fluid extraction of chlorophylls and carotenoids from marine algae Fucus vesiculosus and Laminaria digitata with carbon dioxide was studied. Complete extraction of carotenoids with carbon dioxide was achieved at a pressure of 250 atm and a temperature of 80°C after 20 min in the absence of a cosolvent. Extraction of chlorophylls in high contents required the use of a polar co-solvent (ethanol, 10–15 vol %) or extraction time increased to 50–90 min (at a co-solvent consumption of <5 vol %). The proposed method is more express compared with extraction in a Soxhlet apparatus.

The oxidation of brown coal continuously fed as part of a coal–water slurry into a counterflowing stream of a supercritical water–oxygen fluid at a temperature difference along the reactor axis of 673–873 K and a pressure of 30 MPa has been studied. It has been found that, in the case of a partial combustion of coal (under conditions of O₂ deficiency), the yield of hydrogen-enriched products increases owing to heat evolution. Under conditions of excess O₂, coal undergoes complete oxidation. In this case, the heat evolved per unit volume of furnace space is about 1.0 MW/m³. It has been shown that the heat consumed for the implementation of the process using external sources can be partially or completely compensated for by heat evolution during homogeneous and heterogeneous combustion coupled with coal thermolysis.