Supercritical Carbon Dioxide Solutions Research Articles

A new strategy to fabricate polyvinyl alcohol (PVA) foam with low thermal conductivity using PVA-citric acid solution in supercritical carbon dioxide

Polymer foams with low density have a great potential in the field of heat preservation due to good thermal insulation properties. In this work, a new method for the preparation of polyvinyl alcohol (PVA) foam with low density using PVA-citric acid (CA) solution based on supercritical carbon dioxide (scCO2) foaming technology was proposed. This approach introduced abundant water as co-blowing agent and utilized the crosslinking between CA and PVA to regulate the viscosity of solution, thereby the low density PVA foam with stable morphology was prepared. The effects of saturation time, PVA content, CA content, temperature and pressure on PVA foaming behavior were investigated. Meanwhile, the use of water as assistant solvent was explored to increase the co-blowing content and weaken the diffusion of water molecules to CO2 in the PVA-CA solution, which resulted in the successful fabrication of PVA foams with lower density and low thermal conductivity. The results showed that the minimum densities of PVA before and after addition of assistant solvent were 0.070 g/cm3 and 0.047 g/cm3, respectively. Moreover, PVA foam fabricated by the addition of assistant solvent showed the thermal conductivity of 29.68 mW/mK and the compression modulus of 2.21 MPa.

Wear-Resistant Hydrophobic Coatings from Low Molecular Weight Polytetrafluoroethylene Formed on a Polyester Fabric

The paper presents a comparative analysis of the surface structure and morphology of hydrophobic coatings formed on a polyester fabric from various types of low molecular weight tetrafluoroethylenes by a new method. The low molecular weight compounds used include: ultrafine polytetrafluoroethylene of the FORUM® trademark prepared through thermo-gas-dynamic decomposition of industrial polytetrafluoroethylene waste, and tetrafluoroethylene telomers, synthesized by radiation-chemical initiation from fluoromonomers in acetone, butyl chloride and trimethylchlorosilane solutions. The formation of coatings consists in deposition of low molecular weight tetrafluoroethylenes from solutions in supercritical carbon dioxide and organic solvents. The contact angle and water absorption of the polyester fabric with coatings containing the specified water-repellents are determined. The resistance of the achieved effect to various types of wear—washing, dry cleaning and abrasionis evaluated. The hydrophobic properties of the fabric are found to be affected by the coating plasticity characterized by the stiffness coefficient. The importance of this indicator to targeted changes in fabric properties is proved. All the considered types of low molecular weight polytetrafluoroethylene (PTFE) are shown to be effective water-repellents for the polyester fabric.

AI-PCSAFT approach: New high predictive method for estimating PC-SAFT pure component properties and phase equilibria parameters

In this work, a new approach based on the association of Artificial intelligence method (AI) and PC-SAFT equation of state is applied to conceive a model for estimating the solubility of solid drugs in supercritical carbon dioxide. Neuro-equation of state approach (NES) is the new technique that takes benefit from the advantages of both ANN and PC-SAFT equation of state. The new method decomposes into three main stages, first the optimization of direct ANN for predicting solids-scCO2 phase equilibria (where 15 binary systems are used), then the ANN inverse is performed to be an alternative to group contribution methods (GCMs) for estimating the pure components and physical properties (reduce the uncertainty committed in estimating these properties) and enhance the PCSAFT equation of state to estimate phase equilibria parameters and finally, ANN-PCSAFT approach is used to estimate the solubility of 213 solid solutes in supercritical carbon dioxide. The performance strategy has been carried out using a linear regression analysis of the predicted versus experimental outputs, as an indication of the predictive ability of the developed method.The new approach is successfully applied to the phase equilibria modeling for 213 binary systems with high accuracy (the comparison in terms of average absolute relative deviation (AARD %) showed a variation from 2 to 6%) and allowed to enhance the phase equilibria modeling by reducing the number of optimized parameters and surpass the main drawbacks faced in this area mainly the non-availability of physical properties and EOS pure component properties and the limitation of the equation of state.

Supercritical fluid extraction with cosolvent of alkaloids from Lupinus mutabilis Sweet and comparison with conventional method

AbstractThe purpose of this study was to evaluate the use of supercritical fluid extraction with cosolvent (SFE‐co) as an alternative process in the extraction of lupin alkaloids. A sample of crushed Lupinus mutabilis Sweet seeds, with a high content of alkaloids, was treated using aqueous extraction (AQE) and SFE‐co, where the effect of the operating conditions and the solvent in the SFE‐co were evaluated applying a factorial design. Results showed that both methods reduced the content of total alkaloids in the lupin seed, ensuring its safe consumption. SFE‐co got an extraction yield (39.19 ± 0.14 mg/g) comparable with AQE (39.22 ± 0.16 mg/g) at a temperature of 323 K, a pressure of 27 MPa, using as a solvent supercritical carbon dioxide (scCO2) and dilute ethanol solution, but limited to a fraction of the treated sample. SFE‐co, unlike AQE, was able to keep the nutritional value of the lupin seeds, reducing the time process from 235 hr (AQE) to 2.5 hr, assuring the non‐use of an excessive amount of water and hazardous solvents. Moreover, the fractions formed in the extraction vessel by SFE‐co allowed the understanding of the extraction mechanism of different components from a complex matrix related to their chemical polarity.Practical ApplicationsThis study shows that it is possible to remove lupin alkaloids with green technology as supercritical fluid extraction with cosolvents, reaching extraction yields comparable with the conventional method (aqueous extraction), keeping the nutritional value of the sample, saving time and resources. However, working with a larger quantity of material than the usual used in a laboratory showed an inhomogeneous extraction of alkaloids from the sample, limiting the optimal extraction yield to one of the fractions formed. This reveals a future problem in the use of this technology at an industrial level, where the characteristics of the extracts are not only important but also the material treated since, in the lupin industry, the debittered seed and the released alkaloids are useful in the food and pharmaceutical industries, respectively.

Machine learning models for the prediction of diffusivities in supercritical CO2 systems

The molecular diffusion coefficient is fundamental to estimate dispersion coefficients, convective mass transfer coefficients, etc. Since experimental diffusion data is scarce, there is significant demand for accurate models capable of providing reliable diffusion coefficient estimations.In this work we applied machine learning algorithms to develop predictive models to estimate diffusivities of solutes in supercritical carbon dioxide. A database of experimental data containing 13 properties for 174 binary systems totaling 4917 data points was used in the training of the models. Five machine learning algorithms were evaluated and the results were compared with three commonly used classic models.The best results were found using the Gradient Boosted algorithm which showed an average absolute relative deviation (AARD) of 2.58 % (pure prediction). This model has five parameters: temperature, density, solute molar mass, solute critical pressure and solute acentric factor. For the same dataset, the classic Wilke-Chang equation showed AARD of 12.41 %. The developed model is provided as command line program.

Deposition of Ni-Based Thin Films in Supercritical Carbon Dioxide Solution Via Flow-Type Reaction System

Ni-based films have been extensively studied owing to their potential application in several fields such as metal coatings 1, microelectronic devices 2, magnetic devices 3, and catalysis 4. One of the techniques to deposit Ni thin films is supercritical fluid chemical deposition (SFCD) that offers the potential for the capability of high conformal step coverages and high-aspect-ratio features at materials dimensions 5-6. This technique includes the dissolution of precursor in supercritical fluids, adsorption of precursor and reducing agent onto the substrate surface, the reaction between adsorbed precursor and a reducing agent to its metal or metal oxide form on the substrate surface, and desorption of hydrogenated ligands from the substrate into the supercritical fluids phase. These process steps typically proceed under supercritical carbon dioxide (scCO2) solution due to its relatively moderate critical temperature (31.1°C) and pressure (7.38 MPa) 7 that has high molecular density, high diffusivity, low viscosity and zero surface tension 8.There are two basic type reaction systems for the SFCD technique namely batch-type and flow-type systems 9. In a typical batch-type system, a substrate, a fixed amount of precursor, CO2, and H2 gas are together charged in a closed reactor. The temperature and pressure could ramp up/down because of there is no inlet or outlet in this system. Consequently, the unreacted precursor or the reaction by-product accumulates in the reactor which may make the final product in poor film quality. To overcome this problem, we present here the deposition of Ni thin films in scCO2 solution from Ni(hfac)2 ·3H2O precursor through hydrogen reduction with a variety of independent variables via a flow-type reaction system. In this system, a precursor, CO2, and H2 are continuously supplied at a fixed flow rate, concentration, and pressure into a tubular flow reactor. Furthermore, we also employed the deposition of Ni-Sn alloy thin films with various compositions by flow-type SFCD reaction system, and then their activity was evaluated.The raw materials used in this study were Si(100) wafers having a TiN/SiO2 layer on top, Ni(hfac)2.3H2O, and (i-C3H7)4Sn as Ni and Sn precursor sources respectively, liquid CO2, hydrogen gas, and acetone. Liquid CO2 was cooled and pressurized above a CO2 critical pressure. H2 gas with a certain pressure was added to the scCO2 solution using a gas mixing unit. The precursor solution was added to scCO2 using a high-pressure pump. The substrate was placed inside a tubular flow reactor. And the pressure of this system was adjusted by a back-pressure regulator. All the process parameters of Ni thin film deposition are consisting of precursor concentration, hydrogen concentration, and growth temperature.Ni thin films were successfully deposited onto TiN/SiO2 film by the SFCD technique. Fig. 1 shows the XRD patterns of Ni films deposited at various precursor concentrations and temperatures. This shows that the Ni thin films were highly (111) structure-oriented. For the XRD patterns of Ni-Sn alloy thin films, the Ni(111) and Ni(200) peaks after added an Sn ratio was slightly shifted and became low intensity that might indicate the formation of the Ni-Sn alloys in a wide composition range. Based on the precursor concentration dependence, the curve trend lines in this study indicated Langmuir-Hinshelwood-type adsorption that the Ni growth rate found to be a zero-order reaction. At temperature dependence, the increase in H2 concentration resulted in the Ni growth rate increase, indicating the activation energy decreased with increasing H2 concentration.

Silver Composites of Ultradisperse Polytetrafluoroethylene and Its Fractions in Supercritical Carbon Dioxide: Synthesis and Structural Study

Ultradisperse polytetrafluoroethylene is separated into fractions soluble and insoluble in supercritical carbon dioxide. The initial polymer and its soluble and insoluble fractions are impregnated with silver(I) (1,5-cyclooctadiene)-1,1,1,5,5,5-hexafluoroacetylacetonate in supercritical carbon dioxide solution for 6.6 h under thermodynamic conditions at 65°С and 8 MPa followed by reduction of the metal in an atmosphere of hydrogen for 6 h at 65°С and 1.2 MPa. According to X-ray fluorescence analysis, the content of the metal attains 2.4 wt %. Ag-polymer composites are studied by X-ray fluorescence spectroscopy, transmission electron microscopy, small-angle X-ray scattering, and X-ray photoelectron spectroscopy. It is shown that silver in the form of nanoparticles with a size of 1.0–7.5 nm is localized largely in the soluble low molecular weight fraction.

Artificial Neural Network and Support Vector Regression Applied in Quantitative Structure-property Relationship Modelling of Solubility of Solid Solutes in Supercritical CO2

In this study, the solubility of 145 solid solutes in supercritical CO<sub>2</sub> (scCO<sub>2</sub>) was correlated using computational intelligence techniques based on Quantitative Structure-Property Relationship (QSPR) models. A database of 3637 solubility values has been collected from previously published papers. Dragon software was used to calculate molecular descriptors of 145 solid systems. The genetic algorithm (GA) was implemented to optimise the subset of the significantly contributed descriptors. The overall average absolute relative deviation MAARD of about 1.345 % between experimental and calculated values by support vector regress SVR-QSPR model was obtained to predict the solubility of 145 solid solutes in supercritical CO<sub>2</sub>, which is better than that obtained using ANN-QSPR model of 2.772 %. The results show that the developed SVR-QSPR model is more accurate and can be used as an alternative powerful modelling tool for QSAR studies of the solubility of solid solutes in supercritical carbon dioxide (scCO<sub>2</sub>). The accuracy of the proposed model was evaluated using statistical analysis by comparing the results with other models reported in the literature.

Formation of Dispersed Particles of Tungsten Oxide and Deposition of Platinum Nanoparticles on Them Using Organometallic Precursors from Solutions in Supercritical Carbon Dioxide

Abstract—A one-pot synthesis of dispersed platinum-tungsten composites has been developed. Initially, using tungsten hexacarbonyl as a precursor in solution in supercritical CO2 in the presence of oxygen as an oxidizer promoting thermal decomposition of the precursor, dispersed particles of tungsten oxide with a grain size of about 100 nm and an aggregate size of 200–500 nm have been synthesized. Then, these dispersed particles have been used as a substrate for the subsequent deposition of platinum nanoparticles, which includes two stages: (1) forming a film of an organometallic precursor by deposition from a solution in supercritical CO2 and (2) thermal decomposition of the precursor. Two types of platinum precursors, the dimethyl(1,5-cyclooctadiene)platinum and platinum hexafluoroacetylacetonate, which are both soluble in supercritical CO2, are typically used in the practice of similar studies. The size of the platinum nanoparticles formed using the former and latter precursors are 2.3 ± 0.7 nm and 3.5 ± 0.8 nm, respectively. The composites obtained are characterized by a relatively narrow size distribution of the noble metal nanoparticles and their uniform distribution over the surface of the particles of the dispersed carrier. Such materials may be of interest for (electro)catalysis tasks.

Conformational transitions of poly(ethylene oxide) in supersonic jets of supercritical CO2

The supersonic gas jets are proposed as a tool for researching conformational transitions of polymer chain molecules in untraditional (supercritical) fluids. The working fluid of the jet and the solvent for polymers in this case are supercritical fluids. This method is used for the microencapsulation of TiO2 nanoparticles in poly(ethylene oxide) (Mv = 8,000,000). The suspension of TiO2 nanoparticles in the polymer solution in supercritical carbon dioxide is expanded into vacuum or background gas. The substrate of the jet resulting from rapid expansion was investigated by SEM. Along the axis of the jet, zones of coil-globule-coil transitions were found. The position and extent of these zones can be controlled by varying the conditions in the reactor (pressure, temperature, co-solvent concentration) and background gas pressure in the nozzle chamber.

Polymorphism and conformations of mefenamic acid in supercritical carbon dioxide

In this paper we have studied the conformations of mefenamic acid molecules in its saturated solution in supercritical carbon dioxide being in contact with bottom phase containing different polymorphs of mefenamic acid under isochoric heating conditions in the temperature range of 80–220 °C. For this purpose, we used in situ IR and Raman spectroscopy as well as micro IR and micro Raman spectroscopy and quantum chemical calculations. We have found that there is a correlation between the polymorphic modifications of the crystalline form of mefenamic acid and its conformers in CO2 fluid phase. The results of this work have shown that it is possible to monitor the polymorphic transformation of the crystalline phase of mefenamic acid by screening its molecular conformations in a fluid solution. This finding opens up the possibilities to manage the obtaining of certain polymorph of mefenamic acid with high purity degree from its fluid solution by means of variation of thermodynamic parameters of this solution.

Synthesis and hydrophobic properties of poly[(perfluoro-tret-hexyl)bytylmethacrylate

A new fluorinated polymethacrylate with perfluoro-tret-hexyl groups in the side chain has been prepared by radical polymerization. Polymer films applied from solutions in supercritical carbon dioxide enables the preparation of superhydrophobic coatings.

Hydrogen-Bonded Complexes of p-Hydrobenzoic Acid and Its Derivatives with a Polar Cosolvent in Supercritical Carbon Dioxide

The results of numerical modeling of p-hydroxybenzoic acid, methylparaben, and propylparaben solutions in methanol-modified supercritical carbon dioxide were presented. The composition, structure, and formation energy of the solute–cosolvent hydrogen-bonded complexes were determined. The duration of the existence of hydrogen bonds via individual paraben atoms was evaluated based on the data of Car–Parrinello ab initio molecular dynamics simulation, and the potential of mean force of interaction with methanol was calculated taking into account the medium. It was shown that, despite the presence of several potential centers of specific interaction, p-hydroxybenzoic acid and its derivatives form hydrogen bonds mainly via carboxyl and hydroxyl protons, respectively.

Superhydrophobic coatings on textiles based on novel poly(perfluoro-tert-hexylbutyl methacrylate-co-hydroxyethyl methacrylate) copolymer deposited from solutions in supercritical carbon dioxide

A novel 6,6,7,7,8,8,8-Heptafluoro-5,5-bis(trifluoromethyl)octyl methacrylate monomer is synthesized. Corresponding homopolymers and copolymers with hydroxyethyl methacrylate are obtained by means of radical polymerization in supercritical carbon dioxide using azobisisobutyronitrile as an initiator. The resulting homopolymers are highly soluble in supercritical carbon dioxide. Solubility of copolymers is decreased with increase in hydroxyethyl methacrylate content. Simultaneous deposition of copolymers with diisocyanate crosslinking agents from supercritical carbon dioxide solutions allows one to obtain uniform coating of individual fibers of cotton/polyester fabrics. Such coatings show higher water contact angles and delayed transition from Cassie-Baxter to Wenzel state during droplet evaporation than 6,6,7,7,8,8,8-Heptafluoro-5,5-bis(trifluoromethyl)octyl methacrylate or perfluorohexylethyl methacrylate homopolymer coatings.

The Coil–Globule Transition in Microencapsulation of TiO2 Nanoparticles in a Supercritical CO2 Jet

An experimental investigation was made of microencapsulation of titanium dioxide nanoparticles in PEG 8000 polyethylene glycol by pulsed expansion of a jet of a polymer solution in supercritical carbon dioxide into either a vacuum or a background gas (helium). In both cases, at a given initial temperature in the mixing reactor and a given cosolvent concentration, an electron microscopic study was carried out to determine the distance from the nozzle exit to the point in the jet axis in the vicinity of which the coil–globule transition was detected. The dependence of the position of the transition point on the initial temperature and the cosolvent (ethanol) concentration was characterized.

Hydrophobic Properties of Thin Films of Comb-Shaped Perfluorohexylethyl Methacrylate-Polydimethylsiloxane Copolymers Deposited from Supercritical Carbon Dioxide Solutions

Comb-shaped copolymers of perfluorohexylethyl methacrylate and methacryloxypropyl-terminated polydimethylsiloxane are synthesized by radical polymerization in supercritical carbon dioxide, solubility of the copolymers in supercritical carbon dioxide is studied, and hydrophobic properties of thin films obtained via precipitation of the copolymers from trifluorotrichloroethane and supercritical carbon dioxide solutions on substrates are examined. On the basis of water and dimethyl sulfoxide contact angle measurements, the specific free surface energy of the formed films is calculated. It is shown that the thin films of the copolymers have a lower surface energy and are characterized by a smaller water contact angle hysteresis than the films based on homopolymer poly(perfluorohexylethyl methacrylate). A comparative testing of coatings based on the homopolymer and copolymer deposited from solutions in supercritical carbon dioxide on the surface of nylon fabrics is performed. It is found that copolymer-coated fabrics have on average higher water contact angles.

Adsorption equilibria of paeoniflorin and albiflorin on cyano-silica column from supercritical carbon dioxide/ethanol

Abstract Adsorption equilibria of paeoniflorin and albiflorin on a cyano-silica column (CN column) from the solution of supercritical carbon dioxide (scCO2) modified with ethanol were studied. The adsorption capacity at 308.15 K, 313.15 K, 318.15 K and 323.15 K under pressures corresponding to carbon dioxide/ethanol densities from 0.347 g·cm− 3 to 0.662 g·cm− 3 were determined using the elution by characteristic point method (ECP). The effects of temperature and pressure on the solute loading were investigated. The results showed that the lower the temperature, the higher the adsorption capacity. With the decrease of density of scCO2, the adsorption capacity strengthens. The maximum adsorption capacity of paeoniflorin (albiflorin) on the CN column was 15.24 mg·ml− 1 (31.14 mg·ml− 1) in the range of 0–1.84 mg·ml− 1 (0–1.67 mg·ml− 1) of paeoniflorin (albiflorin) standard solution. The adsorption capacity of albiflorin was twice as much as that of paeoniflorin under the same conditions. Adsorption data of paeoniflorin and albiflorin could be well described by the Langmuir model and Freundlich model. Compared with the two model fitting results, the adsorption of paeoniflorin and albiflorin belonged to the monolayer adsorption under conditions of 308.15–323.15 K and 10–17 MPa.

Solubility of polystyrene with various molecular weights in subcritical 1,1,1,2‐tetrafluoroethane: experiment and modified model

AbstractThe solubility of polystyrene with molecular weight of 100 000 and 260 000 g mol−1 was measured at temperatures from 313 to 333 K and at pressures from 5.0 to 13.0 MPa in subcritical 1,1,1,2‐tetrafluoroethane (R134a). The effects of pressure, temperature and molecular weight on the solubility of polystyrene were investigated. Meanwhile, the solubility of polystyrene was correlated using six density‐based semi‐empirical models (Chrastil, A‐L, K‐J, S‐S, M‐S‐T and Bartle). The M‐S‐T model was used to verify the self‐consistency of the experimental data, and the enthalpy values of polystyrene, including ΔHtotal, ΔHsub and ΔHsol, were estimated through the Chrastil and Bartle models. In addition, a modified M‐S‐T model was proposed through a detailed study of semi‐empirical model formulas and verified by the solubility of polystyrene in subcritical R134a and 38 other solid solutes in supercritical carbon dioxide and subcritical R134a. © 2018 Society of Chemical Industry

Measuring the Solubility of Anthracene and Chrysene in Supercritical Fluid Carbon Dioxide Using Static Solubility Apparatus Directly Interfaced Online to Supercritical Fluid Chromatography

A continuously stirred high pressure solubility vessel has been directly interfaced online to a supercritical fluid chromatography (SFC) system using a single valve recirculation interface. The recirculation pump built into the interface maintains a constant flow of fresh supercritical fluid carbon dioxide solution from the solubility vessel through the interface sample injection valve in a return flow circuit. A series of progressively more concentrated anthracene and chrysene solutions were separately prepared by incrementing the pressure within the solubility vessel in controlled steps. Prior to performing online SFC solubility measurements for each polyaromatic hydrocarbon, in situ checks were made to establish that fully saturated solutions were formed at each different pressure step. The method also involves the utilization of a self-validation experimental procedure to check the accuracy of solubility measurements. For anthracene, solubility data was obtained at 313, 323, 333, and 343 K through the...

Durable crosslinked omniphobic coatings on textiles via supercritical carbon dioxide deposition

A novel method of creation durable water- and oil- repellent coatings on flexible substrates (such as textiles) by simultaneous deposition of perfluoroalkyl methacrylate/hydroxyalkyl methacrylate statistical copolymers with a diisocyanate crosslinking agent from solutions in supercritical carbon dioxide (SC CO2) is proposed. The morphology of fluropolymer films deposited on nylon fibers from 1,1,2- trichlorotrifluoroethane and from SC CO2 with/without hexamethylene diisocyanate as a crosslinking agent is compared using scanning electron microscopy. Repellent properties of the films are analysed by water contact angle measurements and by performing standard water spray test as well as hydrocarbon resistance tests before and after multiple home laundry cycles. Enhanced uniformity of the SC CO2 deposited coatings results in improved durability and liquid repelling properties in comparison to coatings deposited from a liquid solvent.