Supercritical Water as a Solvent

Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy

Supercritical water provides distinctly different solvation properties compared to what is known from liquid water. Despite its prevalence deep in the Earth's crust and its role in chemosynthetic ecosystems in the vicinity of hydrothermal vents, molecular insights into its solvation mechanisms are still very scarce compared to what is known for liquid water. Recently, neutral metal particles have been detected in hydrothermal fluids and proposed to explain the transport of gold species to ore deposits on Earth. Using ab initio molecular dynamics, we elucidate the solvation properties of small gold species at supercritical conditions. The neutral metal clusters themselves contribute enormous THz intensity not because of their intramolecular vibrations, but due to their pronounced electronic polarization coupling to the dynamical supercritical solvent, leading to a continuum absorption up to about 1000 cm-1. On top, long-lived interactions between the gold clusters and solvation water leads at these supercritical conditions to a sharp THz resonance that happens to be close to the one due to H-bonding in liquid water at ambient conditions. The resulting distinct resonances can be used to analyse the solvation properties of neutral metal particles in supercritical aqueous solutions.

Advanced oxidation processes for water and wastewater treatment - Guidance for systematic future research

Advanced oxidation processes (AOPs) are a growing research field with a large variety of different process variants and materials being tested at laboratory scale. However, despite extensive research in recent years and decades, many variants have not been transitioned to pilot- and full-scale operation. One major concern are the inconsistent experimental approaches applied across different studies that impede identification, comparison, and upscaling of the most promising AOPs. The aim of this tutorial review is to streamline future studies on the development of new solutions and materials for advanced oxidation by providing guidance for comparable and scalable oxidation experiments. We discuss recent developments in catalytic, ozone-based, radiation-driven, and other AOPs, and outline future perspectives and research needs. Since standardized experimental procedures are not available for most AOPs, we propose basic rules and key parameters for lab-scale evaluation of new AOPs including selection of suitable probe compounds and scavengers for the measurement of (major) reactive species. A two-phase approach to assess new AOP concepts is proposed, consisting of (i) basic research and proof-of-concept (technology readiness levels (TRL) 1-3), followed by (ii) process development in the intended water matrix including a cost comparison with an established process, applying comparable and scalable parameters such as UV fluence or ozone consumption (TRL 3-5). Subsequent demonstration of the new process (TRL 6-7) is briefly discussed, too. Finally, we highlight important research tools for a thorough mechanistic process evaluation and risk assessment including screening for transformation products that should be based on chemical logic and combined with complementary tools (mass balance, chemical calculations).

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

The contemporary production of carbon materials heavily relies on fossil fuels, contributing significantly to the greenhouse effect. Biomass is a carbon-neutral resource whose organic carbon is formed from atmospheric CO2. Employing biomass as a precursor for synthetic carbon materials can fix atmospheric CO2 into solid materials, achieving negative carbon emissions. Hydrothermal carbonization (HTC) presents an attractive method for converting biomass into carbon materials, by which biomass can be transformed into materials with favorable properties in a distinct hydrothermal environment, and these carbon materials have made extensive progress in many fields. However, the HTC of biomass is a complex and interdisciplinary problem, involving simultaneously the physical properties of the underlying biomass and sub/supercritical water, the chemical mechanisms of hydrothermal synthesis, diverse applications of resulting carbon materials, and the sustainability of the entire technological routes. This review starts with the analysis of biomass composition and distinctive characteristics of the hydrothermal environment. Then, the factors influencing the HTC of biomass, the reaction mechanism, and the properties of resulting carbon materials are discussed in depth, especially the different formation mechanisms of primary and secondary hydrochars. Furthermore, the application and sustainability of biomass-derived carbon materials are summarized, and some insights into future directions are provided.

Complex phase diagram and supercritical matter

The supercritical region is often described as uniform with no definite transitions. The distinct behaviors of the matter therein, e.g., as liquidlike and gaslike, however, suggest "supercritical boundaries." Here we provide a mathematical description of these phenomena by revisiting the Yang-Lee theory and introducing a complex phase diagram, specifically a four-dimensional (4D) one with complex T and p. While the traditional 2D phase diagram with real temperature T and pressure p values (the physical plane) lacks Lee-Yang (LY) zeros beyond the critical point, preventing the occurrence of criticality, the off-plane zeros in this 4D scenario still induce critical anomalies in various physical properties. This relationship is evidenced by the correlation between the Widom line and LY edges in van der Waals, 2D Ising model, and water. The diverged supercritical boundaries manifest the high-dimensional feature of the phase diagram: e.g., when LY zeros of complex T or p are projected onto the physical plane, boundaries defined by isobaric heat capacity C_{p} or isothermal compression coefficient K_{T} emanates. These results demonstrate the incipient phase transition nature of the supercritical matter.

Intriguing and Facile Preparation Approach of CdO Nanorod-Based Abundant Chitosan for Symmetric Supercapacitors

Abundant chitosan was rationally used for the green fabrication of cadmium oxide nanorods (CdO nanorods) owing to its environmentally benign characteristics, bioavailability, low cost, etc. However, the primary unsubstituted amino group of chitosan interacts with the surface of Cd salt at higher temperatures, resulting in CdO nanorod formation. A one-step hydrothermal technique was adopted in the presence of chitosan. Optical, structural, and morphology techniques characterized CdO nanorods. According to X-ray diffraction crystallography, CdO is well crystallized in the face-centered cubic lattice with an Fm-3m (225) space group. The AC@CdO nanoelectrode demonstrated an outstanding gravimetric capacitance of 320 F g-1 at a current density of 0.5 A g-1, nearly three-fold that of ordinary AC electrodes. The AC electrode and the AC@CdO nanoelectrode retain 90 and 93% of their initial specific capacitance after 10,000 galvanostatic charge discharge cycles. The AC@CdO nanoelectrode has a lower equivalent series resistance value than the AC electrode. Moreover, AC@CdO symmetric supercapacitor devices achieve excellent results in terms of specific energy, specific power, and capacitance retention.

The key role of pretreatment for the one-step and multi-step conversions of European lignocellulosic materials into furan compounds

Nowadays, an increased interest from the chemical industry towards the furanic compounds production, renewable molecules alternatives to fossil molecules, which can be transformed into a wide range of chemicals and biopolymers. These molecules are produced following hexose and pentose dehydration. In this context, lignocellulosic biomass, owing to its richness in carbohydrates, notably cellulose and hemicellulose, can be the starting material for monosaccharide supply to be converted into bio-based products. Nevertheless, processing biomass is essential to overcome the recalcitrance of biomass, cellulose crystallinity, and lignin crosslinked structure. The previous reports describe only the furanic compound production from monosaccharides, without considering the starting raw material from which they would be extracted, and without paying attention to raw material pretreatment for the furan production pathway, nor the mass balance of the whole process. Taking account of these shortcomings, this review focuses, firstly, on the conversion potential of different European abundant lignocellulosic matrices into 5-hydroxymethyl furfural and 2-furfural based on their chemical composition. The second line of discussion is focused on the many technological approaches reported so far for the conversion of feedstocks into furan intermediates for polymer technology but highlighting those adopting the minimum possible steps and with the lowest possible environmental impact. The focus of this review is to providing an updated discussion of the important issues relevant to bringing chemically furan derivatives into a market context within a green European context.

Assessing the Impact and Suitability of Dense Carbon Dioxide as a Green Solvent for the Treatment of PMMA of Historical Value

Surface cleaning of plastic materials of historical value can be challenging due to the high risk of inducing detrimental effects and visual alterations. As a result, recent studies have focused on researching new approaches that might reduce the associated hazards and, at the same time, minimize the environmental impact by employing biodegradable and green materials. In this context, the present work investigates the effects and potential suitability of dense carbon dioxide (CO₂) as an alternative and green solvent for cleaning plastic materials of historical value. The results of extensive trials with CO₂ in different phases (supercritical, liquid, and vapor) and under various conditions (pressure, temperature, exposure, and depressurization time) are reported for new, transparent, thick poly(methyl methacrylate) (PMMA) samples. The impact of CO₂ on the weight, the appearance of the samples (dimensions, color, gloss, and surface texture), and modifications to their physicochemical and mechanical properties were monitored via a multi-analytical approach that included optical microscopy, Raman and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopies, and micro-indentation (Vickers hardness). Results showed that CO₂ induced undesirable and irreversible changes in PMMA samples (i.e., formation of fractures and stress-induced cracking, drastic decrease in the surface hardness of the samples), independent of the conditions used (i.e., temperature, pressure, CO₂ phase, and exposure time).

Cryo-Induced Cellulose-Based Nanogel from Elaeis guineensis for Antibiotic Delivery Platform

Cryo-induced hydrogel from cellulose is a new class of biomaterials for drug delivery, cell delivery, bone and skin tissue engineering for cell proliferation and regeneration applications. This research aimed to synthesize cryo-induced hydrogel from cellulose and carboxymethyl cellulose (CMC) produced from empty bunch's cell wall of Elaeis guineensis. First, the experiment was to produce cellulose-rich material using hot-compressed water extraction followed by alkaline delignification and bleaching with H₂O₂. The obtained bleached EFB cellulose was used as the substrate for CMC, and the optimal condition with the highest degree of carboxyl substitution (DS) of 0.75 was achieved when varying NaOH and monochloroacetic acid concentration as well as etherification temperature using fractional factorial design. For cryogelation study, hydrogels were synthesized from cellulose, CMC and beta-cyclodextrin (β-CD) by dissolving cellulose-based matrix in a NaOH/urea system, and the cellulose (CEL) solution was frozen spontaneously at -40 °C followed by high speed mixing to loosen cellulose fibrils. Epichlorohydrin (ECH) and Polyethylene glycol diglycidyl ether (PEGDE) were used as a cross-linker. First, the ratio of cellulose and CMC with different amounts of ECH was investigated, and subsequently the proper ratio was further studied by adding different crosslinkers and matrices, i.e., CMC and β-CD. From the result, the ECH crosslinked CMC-CEL (E-CMC-CEL) gel had the highest swelling properties of 5105% with the average pore size of lyophilized hydrogel of 300 µm. In addition, E-CMC-CEL gel had the highest loading and release capability of tetracycline in buffer solution at pH 7.4 and 3.2. At pH 7.4, tetracycline loading and release properties of E-CMC-CEL gel were 65.85 mg g^-1 dry hydrogel and 46.48 mg g^-1 dry hydrogel (70.6% cumulative release), respectively. However, at pH 3.2, the loading and release capabilities of Tetracycline were moderately lower at 16.25 mg g^-1 dry hydrogel and 5.06 mg g^-1 dry hydrogel, respectively. The findings presented that E-CMC-CEL hydrogel was a suitable material for antibiotic tetracycline drug carrying platform providing successful inhibitory effect on Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa, respectively.

Limiting Conductivities of Strong Acids and Bases in D2O and H2O: Deuterium Isotope Effects on Proton Hopping over a Wide Temperature Range

The molar conductivity (Λ°) of hydrochloric acid, potassium hydroxide, and sodium hydroxide has been measured in both light and heavy waters from 298 to 598 K at p = 20 MPa using a high-precision flow-through alternating current (AC) conductance instrument. The results were used to explore the deuterium isotope effect on ionic transport by proton hopping mechanisms under hydrothermal conditions. Extrapolations of published transport number data to elevated temperature were used to calculate the individual ionic contributions (λ°) for H3O+, D3O+, OH-, and OD-, from which the excess molar conductivities due to proton hopping were calculated. These are the first reported values for the excess conductivities for D3O+ and OD- at temperatures above 318 K. The excess conductivities indicate a strong deuterium isotope effect whereby the transport of D3O+ by proton hopping is reduced by ∼33% relative to H3O+, and OD- is reduced by over 60% relative to OH-, over the entire temperature range. A well-defined maximum in the excess conductivities of D3O+ and H3O+ at ∼420 K suggests that the Eigen cation (H2O)4H+ and the Zundel transition-state cation (H2O)2H+ are destabilized at elevated temperatures as the three-dimensional, tetrahedrally hydrogen-bonded networks in water break down. The less pronounced maximum for OD- and OH- suggested that their Eigen and Zundel anions, (H2O)3OH- and (H2O)OH-, are less destabilized in the two-dimensional networks and chains that dominate the "structure" of liquid water under these conditions.

Extraction of Phytochemicals from Maypole Apple by Subcritical Water

The Maypole apple is a new, promising species of small apples with a prominent flavor and deep red flesh and peel. This study divided Maypole apples into outer flesh, inner flesh, and peel, and used subcritical water at 100–175 °C for 10–30 min to extract various phytochemicals (procyanidin B2 (PB2), 5-caffeoylquinic acid (5CQA), and epicatechin). The obtained Maypole apple extracts and extraction residues in this work were analyzed using a SEM, HPLC, FT-IR, and UV-Vis spectrophotometer. Under different subcritical water extraction conditions, this work found the highest extraction rate: to be PB2 from the peel (4.167 mg/mL), 5CQA (2.296 mg/mL) and epicatechin (1.044 mg/mL) from the inner flesh. In addition, this work regressed the quadratic equations of the specific yield through ANOVA and found that temperature is a more significant affecting factor than extraction time. This aspect of the study suggests that phytochemicals could be obtained from the Maypole apple using the new extraction method of subcritical water.

Main Strategies for the Synthesis of meso-Arylporphyrins

meso-Arylporphyrins as most accessible tetrapyrrole macroheterocycles have always been the focus of attention from researchers concerned with practically useful properties of these compounds. The first syn­theses of meso-arylporphyrins date back to about 90 years ago. Up to now, the yields of these compounds have been improved from 5 to 80%. The present review analyzes different ways and strategies for the synthesis of meso-aryl-substituted porphyrins. The most efficient methods that can be scaled up to an industrial level have been identified.

Review on metal dissolution characteristics and harmful metals recovery from electronic wastes by supercritical water

Supercritical water (SCW) technology can be applied as an efficient and environment-friendly method to recover toxic or complex chemical wastes. Separation and chemical reactions under supercritical conditions may be realized by changing the temperature, pressure, and other operating parameters to adjust the physical and chemical properties of water. However, salt deposition and corrosion are often encountered during the treatment of inorganic substances, which will hinder the commercial applications of SCW technology. The solubility of salt in high pressure/temperature water forms the theoretical basis for studying the recovery of metal salts in supercritical water and understanding salt deposition. Therefore, this work systematically and objectively reviews different research methods used to analyze salt solubility in high pressure/temperature water, including the experimental method, prediction theoretical modeling, and computer simulation method; the research status and existing data of this parameter are also analyzed. The purpose of this review is to provide ideas and references for follow-up research by providing a comprehensive overview of salt solubility research methods and the current situation. Suggestions for more efficient metal recovery through technology integration are also provided.

Baseline Data of Low-Density Polyethylene Continuous Pyrolysis for Liquid Fuel Manufacture

The recycling of end-of-life plastics is a problem, since small parts can be returned into circulation. The rest is burned, landfilled or recycled into low-quality heating oil by pyrolysis methods. The disadvantages of this method are the need to dispose the formed by-product, pyrolytic carbon, the poor quality of produced liquid fuel and the low productivity of the method associated with the periodicity of the process. In this work, methods of thermogravimetry and chromatography–mass spectrometry (GC-MS) have been used to study the co-pyrolysis products of low-density polyethylene (LDPE) and oxygen-containing substances at the pressures of 4–8 MPa and temperatures of 520–620 °C. Experiments have highlighted the conditions needed for producing of high-quality liquid fuel. Initial data have been prepared for the design of a continuous pyrolysis reactor to dispose polymer waste for the production of bio-oil which would be available to enter the petrochemical products market.

Dissolution of cellulose into supercritical water and its dissolving state followed by structure formation from the solution system

Cellulose was treated with supercritical water at 668 K and 25 MPa for 0.04 s in this study. The cellulose/water system was transparent at room temperature for a while after supercritical water treatment before a precipitate gradually appeared over several hours. The precipitation process was monitored by synchrotron X-ray scattering. The scattering functions of fractal systems and flat-like structures were utilized to explain the experimentally observed small-angle scattering profiles. Immediately after supercritical water treatment, the cellulose appeared to dissolve with a fractal dimension D of approximately 1, indicating that the cellulose molecules were rigid, followed by aggregation into a 5-nm-thick flat-like structure. The flat-like structure was determined to be similar to the molecular sheets observed during the early stages of precipitation in the cellulose/aqueous sodium hydroxide and cellulose/aqueous lithium hydroxide/urea systems. Resultant regenerated cellulose had high crystallinity, large crystal size, and a low degree of polymerization.

Hydrogen production rates of aluminum reacting with varying densities of supercritical water

Aluminum particles, spanning in size from 10 μm to 3 mm, were reacted with varying densities of water at 655 K. The density of the water is varied from 50 g L⁻¹ to 450 g L⁻¹ in order to understand the effect of density on both reaction rates and yields. Low-density supercritical water is associated with properties that make it an efficient oxidizer: low viscosity, high diffusion, and low relative permittivity. Despite this, it was found that the high-density (450 g L⁻¹) supercritical water was the most efficient oxidizer both in terms of reaction rate and hydrogen yield. The 10 μm powder had a peak reaction rate of approximately 675 cm_H2³ min⁻¹ g_Al⁻¹ in the high-density water, and a peak reaction rate below 250 cm_H2³ min⁻¹ g_Al⁻¹ in the low- and vapour-density water. A decline in peak reaction rate with decreasing water density was also observed for the 120 μm powder and the 3 mm slugs. These findings imply that the increased collision frequency, a property of the high-density water, outpaces reduction in the reaction enhancing properties associated with low-density supercritical water. Hydrogen yield was minimally affected by decreasing the oxidizer density from 450 g L⁻¹ to 200 g L⁻¹, but did drop off significantly in the vapour-density (50 g L⁻¹) water.

Hydrogen production rates of aluminum reacting with various densities of supercritical water are determined using a novel method. High-density supercritical water is found to be the most efficient oxidizer, in terms of both reaction rates and yields.

A review of green solvent extraction techniques and their use in antibiotic residue analysis

Antibiotic residues are being continuously recognized in the aquatic environment and in food. Though the concentration of antibiotic residues is typically low, adverse effects on the environment and human health have been observed. Hence, an efficient method to determine numerous antibiotic residues should be simple, inexpensive, selective, with high throughput and with low detection limits. Liquid-based extractions have been exceedingly used for clean-up and preconcentration of antibiotics prior to chromatographic analysis. In order to make methods more green and environmentally sustainable, conventional hazardous organic solvents can be replaced with green solvents. This review presents sampling strategies as well as comprehensive and up-to-date methods for chemical analysis of antibiotic residues in different sample matrices. Particularly, solvent-based sample preparation techniques using green solvents are discussed along with applications in antibiotic residue analysis.

On the Different Faces of the Supercritical Phase of Water at a Near-Critical Temperature: Pressure-Induced Structural Transitions Ranging from a Gaslike Fluid to a Plastic Crystal Polymorph

The present study reports a systematic analysis of a wide variety of structural, thermodynamic, and dynamic properties of supercritical water along the near-critical isotherm of T = 1.03T_c and up to extreme pressures, using molecular dynamics and Monte Carlo simulations. The methodology employed provides solid evidence about the existence of a structural transition from a liquidlike fluid to a compressed, tightly packed liquid, in the density and pressure region around 3.4ρ_c and 1.17 GPa, introducing an alternative approach to locate the crossing of the Frenkel line. Around 8.5 GPa another transition to a face-centered-cubic plastic crystal polymorph with density 5.178ρ_c is also observed, further confirmed by Gibbs free energy calculations using the two-phase thermodynamic model. The isobaric heat capacity maximum, closely related to the crossing of the Widom line, has also been observed around 0.8ρ_c, where the local density augmentation is also maximized. Another structural transition has been observed at 0.2ρ_c, related to the transformation of the fluid to a dilute gas at lower densities. These findings indicate that a near-critical isotherm can be divided into different domains where supercritical water exhibits distinct behavior, ranging from a gaslike one to a plastic crystal one.

Development of alginate@tin oxide-cobalt oxide nanocomposite based catalyst for the treatment of wastewater

In this study, tin oxide‑cobalt oxide nanocatalyst was prepared by a simple method, which grew in spherical particles with an average diameter of 30 nm. Tin oxide-cobalt oxide was further wrapped in alginate polymer hydrogel (Alg@tin oxide-cobalt oxide), and both materials were utilized as nanocatalysts for the catalytic transformation of different pollutants. Tin oxide-cobalt oxide and Alg@tin oxide-cobalt oxide nanocatalysts were tested for the catalytic reduction of 4-nitrophenol, congo red, methyl orange, methylene blue (MB) and potassium ferricyanide in which sodium borohydride was used as a reducing agent. Tin oxide-cobalt oxide and Alg@tin oxide-cobalt oxide nanocatalysts synergistically reduced MB in shorter time (2.0 and 4.0 min) compared to other dyes. The reduction conditions were optimized by changing different parameters. The rate constants for MB reduction were calculated and found to be 1.5714 min^-1 and 0.6033 min^-1 using tin oxide-cobalt oxide and Alg@tin oxide-cobalt oxide nanocatalysts, respectively. Implementing Alg@tin oxide-cobalt oxide nanocatalyst toward MB reduction in real samples proved its efficacy in sea and well water samples. The catalyst could be easily recovered, recycled and revealed a minimal loss of nanoparticles, which offering a competition and replacement with reputable commercial catalysts.

Deep carbon cycle constrained by carbonate solubility

Earth's deep carbon cycle affects atmospheric CO₂, climate, and habitability. Owing to the extreme solubility of CaCO₃, aqueous fluids released from the subducting slab could extract all carbon from the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca²⁺ in carbonates is replaced by Mg²⁺ and other cations during subduction. Here we determined the solubility of dolomite [CaMg(CO₃)₂] and rhodochrosite (MnCO₃), and put an upper limit on that of magnesite (MgCO₃) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilise [Formula: see text]% ([Formula: see text] Mt/yr) of subducted carbon from subducting slabs.

Two-step droplet formation in monodisperse nanodroplet generation in quenched hydrothermal solution as revealed by spontaneous transformation of nanodroplets to swollen micelles in octane‑in‑water nanoemulsions

HYPOTHESIS

Monodisperse nanodroplet generation in quenched hydrothermal solution (MAGIQ) is a newly developed bottom-up process for preparing nanoemulsions. In this process, homogeneous solutions of oil in supercritical water are quenched by adding cold water containing a surfactant to induce rapid phase-separation, during which oil molecules self-assemble to form nano-sized oil droplets. The droplet size in MAGIQ is known to be influenced by the interplay of the phase-separation dynamics, coalescence kinetics of the droplets, and adsorption kinetics of the surfactant on the droplet surface; however, the primary stages of the droplet formation are still elusive.

EXPERIMENTS

Octane‑in‑water nanoemulsions containing 0.5, 1, and 3 vol% octane were prepared by the MAGIQ method. Their ripening was studied by dynamic light scattering, and the phase diagram was established.

FINDINGS

The nanoemulsions containing 0.5 and 1 vol% octane transformed to thermodynamically stable microemulsions containing swollen micelles, whereas the nanoemulsion containing 3 vol% octane underwent Ostwald ripening. The initial formation of the nano-sized droplets in the former was ascribed to a unique mechanism of droplet formation in MAGIQ-the droplets are first formed by the phase separation of homogeneous binary solutions of oil in supercritical water and then stabilized upon surfactant adsorption.

Confinement-Controlled Aqueous Chemistry within Nanometric Slit Pores

In this Focus Review, we put the spotlight on very recent insights into the fascinating world of wet chemistry in the realm offered by nanoconfinement of water in mechanically rather rigid and chemically inert planar slit pores wherein only monolayer and bilayer water lamellae can be hosted. We review the effect of confinement on different aspects such as hydrogen bonding, ion diffusion, and charge defect migration of H⁺(aq) and OH^-(aq) in nanoconfined water depending on slit pore width. A particular focus is put on the strongly modulated local dielectric properties as quantified in terms of anisotropic polarization fluctuations across such extremely confined water films and their putative effects on chemical reactions therein. The stunning findings disclosed only recently extend wet chemistry in particular and solvation science in general toward extreme molecular confinement conditions.

Aerogels in drug delivery: From design to application

Aerogels are the lightest processed solid materials on Earth and with the largest empty volume fraction in their structure. Composition versatility, modularity, and feasibility of industrial scale manufacturing are behind the fast emergence of aerogels in the drug delivery field. Compared to other 3D materials, the high porosity (interconnected mesopores) and high specific surface area of aerogels may allow faster loading of small-molecule drugs, less constrained access to inner regions of the matrix, and more efficient interactions of the biological milieu with the polymer matrix. Processing in supercritical CO₂ medium for both aerogel production (drying) and drug loading (impregnation) has remarkable advantages such as absence of an oxidizing environment, clean manufacture, and easiness for the scale-up under good manufacturing practices. The aerogel solid skeleton dictates the chemical affinity to the different drugs, which in turn determines the loading efficiency and the release pattern. Aerogels can be used to increase the solubility of BCS Class II and IV drugs because the drug can be deposited in amorphous state onto the large surface area of the skeleton, which facilitates a rapid contact with the body fluids, dissolution, and release. Conversely, tuning the aerogel structure by functionalization with drug-binding moieties or stimuli-responsive components, application of coatings and incorporation of drug-loaded aerogels into other matrices may enable site-specific, stimuli-responsive, or prolonged drug release. The present review deals with last decade advances in aerogels for drug delivery. An special focus is paid first on the loading efficiency of active ingredients and release kinetics under biorelevant conditions. Subsequent sections deal with aerogels intended to address specific therapeutic demands. In addition to oral delivery, the physical properties of the aerogels appear to be very advantageous for mucosal administration routes, such as pulmonary, nasal, or transdermal. A specific section devoted to recent achievements in gene therapy and theranostics is also included. In the last section, scale up strategies and life cycle assessment are comprehensively addressed.

Two-Component Dynamics and the Liquidlike to Gaslike Crossover in Supercritical Water

Molecular-scale dynamics in sub- to supercritical water is studied with inelastic x-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: a low-frequency (LF), gaslike component and a high-frequency (HF) component arising from the O-O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid- to gaslike dynamics with rapid changes around the Widom line.

Supercritical Water is not Hydrogen Bonded

Thinking about water is inextricably linked to hydrogen bonds, which are highly directional in character and determine the unique structure of water, in particular its tetrahedral H-bond network. Here, we assess if this common connotation also holds for supercritical water. We employ extensive ab initio molecular dynamics simulations to systematically monitor the evolution of the H-bond network mode of water from room temperature, where it is the hallmark of its fluctuating three-dimensional network structure, to supercritical conditions. Our simulations reveal that the oscillation period required for H-bond vibrations to occur exceeds the lifetime of H-bonds in supercritical water by far. Instead, the corresponding low-frequency intermolecular vibrations of water pairs as seen in supercritical water are found to be well represented by isotropic van-der-Waals interactions only. Based on these findings, we conclude that water in its supercritical phase is not a H-bonded fluid.

Synthesis of Micro- and Nanoparticles in Sub- and Supercritical Water: From the Laboratory to Larger Scales

The use of micro- and nanoparticles is gaining more and more importance because of their wide range of uses and benefits based on their unique mechanical, physical, electrical, optical, electronic, and magnetic properties. In recent decades, supercritical fluid technologies have strongly emerged as an effective alternative to other numerous particle generation processes, mainly thanks to the peculiar properties exhibited by supercritical fluids. Carbon dioxide and water have so far been two of the most commonly used fluids for particle generation, the former being the fluid par excellence in this field, mainly, because it offers the possibility of precipitating thermolabile particles. Nevertheless, the use of high-pressure and -temperature water opens an innovative and very interesting field of study, especially with regards to the precipitation of particles that could hardly be precipitated when CO2 is used, such as metal particles with a considerable value in the market. This review describes an innovative method to obtain micro- and nanoparticles: hydrothermal synthesis by means of near and supercritical water. It also describes the differences between this method and other conventional procedures, the most currently active research centers, the types of particles synthesized, the techniques to evaluate the products obtained, the main operating parameters, the types of reactors, and amongst them, the most significant and the most frequently used, the scaling-up studies under progress, and the milestones to be reached in the coming years.

ReaxFF molecular dynamics simulations of electrolyte-water systems at supercritical temperature

We have performed ReaxFF molecular dynamics simulations of alkali metal-chlorine pairs in different water densities at supercritical temperature (700 K) to elucidate the structural and dynamical properties of the system. The radial distribution function and the angular distribution function explain the inter-ionic structural and orientational arrangements of atoms during the simulation. The coordination number of water molecules in the solvation shell of ions increases with an increase in the radius of ions. We find that the self-diffusion coefficient of metal ions increases with a decrease in density under supercritical conditions due to the formation of voids within the system. The hydrogen bond dynamics has been interpreted by the residence time distribution of various ions, which shows Li⁺ having the highest water retaining capability. The void distribution within the system has been analyzed by using the Voronoi polyhedra algorithm providing an estimation of void formation within the system at high temperatures. We observe the formation of salt clusters of Na⁺ and K⁺ at low densities due to the loss of dielectric constants of ions. The diffusion of ions gets altered dramatically due to the formation of voids and nucleation of ions in the system.

Thermodynamics of mixing methanol with supercritical CO2 as seen from computer simulations and thermodynamic integration

The changes in extensive thermodynamic quantities, such as volume, energy, Helmholtz free energy and entropy, occurring upon mixing liquid methanol with supercritical CO₂, are calculated using Monte Carlo simulations and thermodynamic integration for all eight combinations of four methanol and two CO₂ potential models in the entire composition range at 313 K. The obtained results are also compared with experimental data whenever possible. The transition of the system from liquid to a supercritical state is found to occur at this temperature around a CO₂ mole fraction value of 0.95 with all model combinations considered. This liquid to supercritical transition is always accompanied by positive Helmholtz free energy of mixing values and, consequently, by the non-miscibility of the two components. Furthermore, both this non-miscibility around the liquid to supercritical transition and also the miscibility of the two components below this transition, in the liquid regime, are found to be primarily of the energetic rather than entropic origin; the entropy of mixing turns out to be very close to zero, and around the liquid to supercritical transition even its qualitative behaviour is strongly model dependent. Finally, it is found that the methanol expansion coefficient is not sensitive to the details of the potential models, and it is always in excellent agreement with the experimental data. On the other hand, both the volume and the energy of mixing depend strongly on the molar volume of neat CO₂ in the model being used, and in this respect the TraPPE model of CO₂ [J. J. Potoff and J. I. Siepmann, AIChE J., 2001, 47, 1676] performs considerably better than that of Zhang and Duan [Z. Zhang and Z. Duan, J. Chem. Phys., 2005, 122, 214507].

Carbon dioxide, bicarbonate and carbonate ions in aqueous solutions under deep Earth conditions

We investigate the effect of pressure, temperature and acidity on the composition of water-rich carbon-bearing fluids under thermodynamic conditions that correspond to the Earth's deep crust and upper mantle. Our first-principles molecular dynamics simulations provide mechanistic insight into the hydration shell of carbon dioxide, bicarbonate and carbonate ions, and into the pathways of the acid/base reactions that convert these carbon species into one another in aqueous solutions. At temperatures of 1000 K and higher, our simulations can sample the chemical equilibrium of these acid/base reactions, thus allowing us to estimate the chemical composition of diluted carbon dioxide and (bi)carbonate ions as a function of acidity and thermodynamic conditions. We find that, especially at the highest temperature, the acidity of the solution is essential to determine the stability domain of CO₂vs. HCO₃^-vs. CO₃^2-.

Facile sub-/supercritical water synthesis of nanoflake MoVTeNbOx-mixed metal oxides without post-heat treatment and their catalytic performance

A fast and simple sub-/supercritical water synthesis method is presented in this work in which MoVTeNbO_x-mixed metal oxides with various phase compositions and morphologies could be synthesized without post-heat treatment. It was demonstrated that the system temperature for synthesis had a significant influence on the physico-chemical properties of MoVTeNbO_x. Higher temperatures were beneficial for the formation of a mixed crystalline phase containing TeVO₄, Te₃Mo₂V₂O₁₇, Mo₄O₁₁ and TeO₂, which are very different from the crystalline phases of conventional Mo–V–Te–Nb-mixed metal oxides. While at lower temperatures, Mo₄O₁₁ was replaced by Te. At high temperature, the as-prepared samples presented distinct nanoflake morphologies with an average size of 10–60 nm in width and exhibited excellent catalytic performances in the selective oxidation of propylene to acrylic acid. It is illustrated that the large specific surface area, presence of Mo₄O₁₁ and superficial Mo⁶⁺ and Te⁴⁺ ions are responsible for the high propylene conversion, while suitable acidic sites and superficial Nb⁵⁺ ions improved the selectivity to acrylic acid.

Nanoflake MoVTeNbO_x prepared by sub-/supercritical water exhibit excellent catalytic performance in the selective oxidation of propylene to acrylic acid.

Assessing the properties of supercritical water in terms of structural dynamics and electronic polarization effects

Evolution of water's structural dynamics from ambient liquid to supercritical dense liquid-like and dilute gas-like conditions.

Water as a tuneable solvent: a perspective

Water is the most sustainable solvent, but its polarity limits the solubility of non-polar solutes. Confining water in hydrophobic nanopores could be a way to modulate water solvent properties and enable using water as tuneable solvent (WaTuSo).