Journal of Supercritical Fluids最新文献

Flexible silicone rubber (SR) foams with tunable cellular morphologies were fabricated via supercritical CO₂ foaming. Conductive carbon black (CB) modified at a low cost and with high complex viscosity was preferentially selected. The effects of the pore size and void fraction on the electrical conductivity and dielectric permittivity of the SR/CB composite foams were carefully investigated. The pore size of the foams affected the specific shielding effectiveness (SSE) and absorption coefficient (A), whereas the variation in the void fraction did not generate evident changes. In comparison with its solid counterpart, a foam with a pore diameter of 59.9 μm showed a 50 % decrease in density, 31.6 % increase in absorptivity, and 95.7 % increase in SSE, demonstrating a considerable electromagnetic interference shielding effectiveness (EMI SE). Decreases in the pore size and void fraction of the foams improved compression modulus and strength. In addition, sample preparation process was simplified, making industrial production easier.

Rosmarinus officinalis L. is one of the most widely exploited medicinal plants for its bioactive compounds, including rosmarinic acid (RA), carnosic acid (CA) and carnosol (CAR). This study aims to selectively extract these compounds, using supercritical fluids extraction with carbon dioxide and ethanol as co-solvents, based on the Hansen solubility parameters approach. This approach predicted that CA and CAR are highly soluble in the supercritical fluid. However, a weak interaction was predicted between RA and this fluid. The experimental findings indicated the highest amounts of RA and CA were observed at 150 bar and 80°C, resulting in yields of 3.43 mg/g dry weight (DW), and 18.9 mg/g DW, respectively. Favourable extraction conditions for CAR were recorded at 250 bar and 40°C, with a content of 19.1 mg/g DW. Evaluation of antioxidant activity using DPPH tests revealed that extracts obtained at 250 bar at 40 and 80°C rich in CAR have a stronger antioxidant power.

This study investigates the estimation of solute solubility in supercritical carbon dioxide (SC-CO₂) within a pressure and temperature range of 80 bar to 490.29 bar and 308 K to 423 K. We propose a novel empirical model that establishes a correlation between relevant parameters and the targeted solubility. A feature importance algorithm facilitated the development of this empirical model. The model’s accuracy is comprehensively evaluated using 40 published experimental datasets, with an average absolute relative deviation (AARD) of 9.9 %. It demonstrates superior performance compared to 12 previously established models. Furthermore, a fine-tuned artificial neural network (ANN) is developed to harness the unique capabilities of machine learning techniques. The ANN outperforms the proposed model, achieving a significantly lower AARD% of 4.38. This outcome emphasizes the potential of machine learning techniques, particularly ANNs, for achieving superior accuracy.

This study investigates the hydrothermal synthesis of KNbO₃-NaNbO₃ particles, which contains compositions of interest as lead-free piezoelectric materials. Precursor solutions containing Nb₂O₅, NaOH/KOH were hydrothermally reacted at 190 ⁰C for 15 hours using either a conventional reactor or a reactor with stirring. For 50 % KOH or lower, the product was cubic or elongated cuboid/belt-shaped NaNbO₃, whereas solely KOH resulted in hexagonal plate-like KNbO₃. When 75 % KOH was used, the product was a mixture of large K₅Na₃Nb₆O₁₉·9 H₂O hexagonal plates and smaller NaNbO₃ cubes. We hypothesized this phase segregation was induced by highly concentrated microenvironments, and stirring would prevent that. Accordingly, stirring hydrothermal treatment of 75 % KOH solution produced only K₅Na₃Nb₆O₁₉·H₂O hexagonal plates, which could be transformed to K_0.7Na_0.3NbO₃ by calcination. By reducing the hydrothermal treatment time to 1 – 6 hours we isolated reaction intermediates, and based on them propose a kinetics-controlled mechanism for hydrothermal reactions of Nb₂O₅ and KOH/NaOH.

Amphoteric oxides can be challenging to produce in phase pure form due to their sensitivity to the pH of the reaction mixture. The high-pressure, high-temperature reaction conditions of hydrothermal processes make the pH difficult to control. Here we report a simple approach to obtain nanoparticles of amphoteric oxides, where cheap metal nitrate precursors are reacted in alcohol (2-propanol) water mixtures (25 vol%:75 vol%) to maintain a neutral pH under solvothermal conditions. Phase pure SnO₂ nanoparticles were synthesized in a continuous flow solvothermal reactor at 250 °C, 300 °C, 350 °C and 400 °C with sizes ranging from 3 nm to 7 nm. The nanoparticles were characterized using powder X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy, ¹¹⁹Sn MAS NMR spectroscopy, Brunauer-Emmett-Teller nitrogen adsorption, UV–VIS, and as anode material in CR2032 Li-ion battery cells. To demonstrate generality, amphoteric oxides ZnO, Bi₂O₃, Cr₂O₃ and γ-AlOOH were also synthesized.

The electrochemical carbon dioxide reduction reaction (CO2RR) is of increasing importance for the development of a closed carbon cycle. Here, electrolytes are often used to increase the electrical conductivity and thus the current density. In addition, the use of compressed CO₂ and electrolyte-containing organic solvents suppresses hydrogen evolution. However, the electrical conductivity of CO₂-saturated electrolyte solutions is mostly unknown. Therefore, their electrical conductivity was investigated in this work at different electrolyte concentrations, pressures (up to 150 bar) and temperatures (298.15 K and 343.15 K). In addition, the simplified Mean Spherical Approximation (MSA-simple) model, supported by ePC-SAFT for density modeling, was used to successfully model the conductivity. The results show that CO₂-saturated electrolyte solutions generally exhibit lower conductivities than CO₂-free electrolyte solutions. However, at very low electrolyte and CO₂ concentrations, the conductivities were higher in CO₂-saturated electrolyte solutions than in CO₂-free electrolyte solutions, which is a new finding in the literature.

Supercritical carbon dioxide (SCCO₂) fracturing significantly enhances shale gas recovery, which is influenced by particle size. We soaked shale in SCCO₂ and investigated the impact of SCCO₂ on different particle sizes. Large-particle shales showed the largest percentage changes in specific surface area and total pore volume (54.11 %, 87.87 %; 58.59 %, 76.32 %) followed by small-particle size shales. This trend was also observed in other pore structure parameters. The particle-size effect is: Large-particle shale, with abundant microfractures, enhances SCCO₂ flow and pore alteration. Small-particle shale's high specific surface area facilitates SCCO₂ penetration. Medium-particle shale is less affected due to balanced interactions of these factors. Methane is primarily found in large and medium pores and microfractures. Methane adsorption in shale mainly involves multi-layer adsorption. Following SCCO₂ treatment, pore fractures narrowed, increasing the proportion of methane molecules adsorbed as a single-layer. This study is crucial for evaluating the fracturing effects on shale gas wells.

A supercritical CO₂ assisted process was used to perform a comparative study between liposomes and niosomes. Process operating conditions were fixed at 100 bar and 40 °C, and the produced vesicles were characterized in terms of mean diameter, size distribution, ζ-potential, and stability over time. Mean diameters of liposomes and niosomes were similar (130 ± 37 nm for liposomes and 141 ± 36 nm for niosomes) and both systems were stable after 1 month from production. Ascorbic acid (AA) was loaded in both kinds of formulation. AA encapsulation efficiency was equal to 92 % and 99 % for liposomes and niosomes, respectively, and DPPH-activity was larger than 90 % in both vesicular systems. Drug release tests revealed that AA was released in 120 min and 240 min from liposomes and niosomes, respectively, due to a different compactness of the vesicle bilayer.

We here report the first occurrence of a preferential cocrystallization process assisted by compressed carbon dioxide. Our study focuses on the Nefiracetam-Mandelic acid conglomerate forming system. The cocrystal conglomerate was successfully crystallized using compressed CO₂ as antisolvent, and acetone, ethyl acetate, or isobutanol as solvents for starting solutions in the so-called GAS process. We then focus on acetone to successfully perform preferential cocrystallization supported by compressed CO₂. A critical step in the design of the process is the adaptation of the set-up to allow the introduction of enantiopure seeding inside the reactor. Based on the use of seeded rings, we test several seeding configurations to enhance enantioseparation through the simultaneous crystallization of both enantiomers in two different areas of the reactor.

Arthrospira platensis polyphenolic natural extracts have several benefits for human health. Due to their instability and reduced shelf-life, these extracts are usually encapsulated. This work analyzed the supercritical emulsion extraction (SEE) process for the production of polymeric nanoparticles loaded with A. platensis polyphenolic extracts. The objective of this work was to evaluate a powdered encapsulation system to overcome the drawbacks of liquid systems, which are those mainly used for microalgae extract stabilization. An optimization study was performed investigating different process parameters and comparing the obtained particles in terms of particle size distribution, morphology, and encapsulation efficiency. The process permitted the successfull production of loaded polymeric nanoparticles with an encapsulation efficiency up to 74 % in the best operating conditions. The SEE process allowed a better control of morphology and particle size distribution of the nanometric particles operating in a continuous mode.