Search Results (61)

A nonlinear dynamical model for anaerobic digestion (AD) of the soluble organic fraction of municipal solid waste (OFMSW) is analyzed under inhibition conditions. The model incorporates both the acidogenic and methanogenic stages, accounting for substrate and product inhibition in both microbial consortia. Based on the dynamical properties, three stable equilibria are identified and linked to realistic operating scenarios: washout, acidification, and normal (stable) operation. Control schemes are proposed to regulate the organic substrate concentration and achieve the desired operating conditions. These strategies enhance operational stability under both normal and acidification conditions. A linear proportional-integral (PI) control scheme is designed, along with two tuning approaches to ensure closed-loop stability in the presence of external load disturbances, measurement noise, and substrate and product inhibition. The control schemes achieve, under normal operating conditions, 96% removal of soluble organic substrate and 60% removal of volatile fatty acids; under acidification conditions, 72% substrate removal is attained, with a 152% increase in volatile fatty acids relative to the inlet levels. Accordingly, the proposed model and control scheme offer a promising contribution toward improving the operational stability of AD processes treating OFMSW. Full article

Biodiesel is a biofuel commonly produced through transesterification, also known as alcoholysis. In this process, triglycerides react with short-chain alcohols (alkyl–OH), producing a mixture of fatty acid esters and glycerol. These esters and glycerol are only partially miscible, leading to the formation of two liquid phases during product separation. Therefore, it is important to experimentally determine liquid–liquid (LLE) and/or vapor–liquid–liquid equilibrium (VLLE) data to better understand the transesterification process and to support improvements in reaction rate, selectivity, reactor and mixture simulation, optimization, and separation processes. This work aimed to experimentally measure and thermodynamically model the LLE and VLLE of alkyl palmitate + alkyl–OH + glycerol systems at 101.3 kPa. For the LLE at 318.15 K, the binodal curve was determined, and tie-line compositions were measured in a jacketed equilibrium cell. These data were subjected to quality tests and used to calculate separation factors. For the VLLE, calibration curves were constructed, and experimental data were obtained in a modified Othmer ebulliometer and subsequently tested for consistency. Thermodynamic modeling was performed using γ–γ (LLE) and γ–γ–φ (VLLE) approaches with the Non-Random Two-Liquid (NRTL) activity coefficient model. The experimental and modeling results were analyzed using phase diagrams (triangular and 3D prism representations) and showed that it is possible to clearly separate the palmitate-rich and glycerol-rich liquid phases. In the VLLE, it was observed that the alkyl–OH is essentially pure in the vapor phase. For both types of equilibria, deviations in liquid-phase compositions (LLE), bubble temperatures, and vapor-phase compositions were below 2.0%, indicating that the NRTL model is capable of accurately describing the phase behavior of these systems. The phase equilibria of the methyl/ethyl palmitate–methanol/ethanol–glycerol system were studied using molecular dynamics (MD). The analyses based on the radial distribution function (RDF), spatial distribution function (SDF) and interaction energies showed that methanol and ethanol interact more strongly with glycerol than with palmitates. As a result, the glycerol-rich phase contains more methanol or ethanol, which can significantly reduce costs in the biodiesel purification step. Full article

For those who did not follow the invention and development of enantioselective catalysis, this review introduces pertinent historical aspects of the field and presents the scientific concepts of asymmetric bio- and organocatalysis. They are powerful technologies applied in organic laboratories and industry. They realize chiral amplification by converting inexpensive achiral substrates and reagents into enantiomerically enriched products using readily recoverable solvents, if any are used. Racemic substrates can also be deracemized catalytically. More sustainable fabrications are now available that require neither toxic metallic species nor costly reaction conditions in terms of energy, atmosphere control, product purification, and safety. Nature has been the source of the first asymmetric catalysts (microorganisms, enzymes, alkaloids, amino acids, peptides, terpenoids, sugars, and their derivatives). They act as temporary chiral auxiliaries and lower the activation free energy of the reaction by altering the reaction mechanism. Reductions, oxidations, carbon-carbon and carbon-heteroatom bond-forming reactions are part of the process panoply. Asymmetric catalyzed multicomponent and domino reactions are becoming common. Typical modes of activation are proton transfers, hydrogen bonded complex formation, charged or uncharged acid/base pairing (e.g., σ-hole catalysts), formation of equilibria between achiral aldehydes and ketones with their chiral iminium salt or/and enamine intermediates, umpolung of aldehydes and ketones by reaction with N-heterocyclic carbenes (NHCs), phase transfer catalysis (PTC), etc. Often, the best enantioselectivities are observed with polyfunctional catalysts derived from natural compounds, but not always. They may combine to form chiral structures containing nitrogen, phosphorus, sulfur, selenium, and iodine functional moieties. Today, man-made enantiomerically enriched catalysts, if not enantiomerically pure, are available in both enantiomeric forms. Being robust, they are recovered and reused readily. Full article

This study examines the impact of a Science–Technology–Society–Environment (STSE) educational intervention on the teaching of acid–base reactions to 11th-grade students (n = 17). The didactic sequence combined laboratory experiments, real-data analysis, and an interdisciplinary role-play debate, designed to connect chemical concepts with pressing socio-environmental challenges such as ocean acidification, acid rain, and acid mine drainage. Data collection included a pre- and post-test on environmental awareness and semi-structured interviews, enabling the assessment of both conceptual learning and attitudinal change. Significant conceptual gains were observed, with five of eleven test items reaching a normalized Hake gain ≥ 0.70, alongside increased environmental awareness. Qualitative findings further revealed that students valued the real-world context and interdisciplinary integration, reporting enhanced motivation, civic responsibility, and a more meaningful engagement with science. Overall, the results suggest that STSE-based chemistry instruction not only strengthens students’ understanding of acid–base equilibria but also fosters sustainability competencies essential for responsible and informed citizenship in the 21st century. Full article

Soil acid–base status fundamentally regulates biogeochemical cycling and agroecosystem resilience by controlling nutrient solubility, cation exchange, and redox equilibria. However, the long-term evolution of soil pH and its spatial divergence under intensive agricultural expansion remain poorly quantified. Herein, we integrate three nationwide soil surveys (1980, 2012, 2018) encompassing over 190,000 cropland observations into a harmonized 1 km dataset to reconstruct four decades of soil pH change across China. National mean soil pH declined from 7.1 in 1980 to 6.7 in 2012 and 6.6 in 2018, revealing a sustained acidification trend. Nearly one quarter of neutral soils (pH 6.5–7.5) have shifted into acidic classes (<6.5) since 1980, reflecting widespread depletion of soil buffering capacity under intensive fertilization, high rainfall, and carbonate exhaustion. By integrating current pH conditions with standardized pH change rate, we delineate nine bidirectional soil pH risk zones that capture contrasting acidification and alkalization processes along climatic and edaphic gradients. Acidification-prone zones dominate humid southern croplands, whereas alkalization risk prevails in arid northern regions. Our results provide nationally consistent, grid-level evidence of soil acid–base evolution across nearly four decades, offering a quantitative foundation for region-specific soil management to sustain productivity and mitigate environmental risks. Full article

Multilayer and free-standing films self-assembled from water-soluble anionic azo dye acid yellow 9 (AY9) and both poly(allylamine hydrochloride) (PAH) and chitosan (CS) cationic polyelectrolytes were fabricated from water solution using a layer-by-layer (LbL) technique and characterized by UV–Vis and Raman spectroscopy. Observations were made of a strong, unexpected, and highly unusual colour change from deep red to a distinct dark blue upon exposure of the multilayer films to an acidic environment. The colour change was attributed to the multilayer films only and was not observed either for the polymer or the dye alone, or their mixture in water solution, nor when cast as free-standing films. The significant shift to blue colour of the absorption peaks was quantified with UV–Vis spectroscopy, and a proposed explanation is presented based on density functional theory (DFT) calculations exploring possible and most likely acid-base equilibria configurations of the azo dye that result from being self-assembled. Full article

The kynurenine pathway is a significant metabolic route involved in the catabolism of tryptophan, producing various bioactive metabolites with crucial roles as antioxidants in immune regulation and neurobiology. This study investigates the acid-base properties of picolinic acid, kynurenic acid, kynurenine, and 3-hydroxykynurenine, utilizing computational simulations and experimental techniques, including potentiometric and nuclear magnetic resonance titrations. The results reveal distinct pK_a values, with kynurenic acid exhibiting a single dissociation step around 2.4, while kynurenine displays three dissociation steps governed by interactions between its functional groups. Additionally, 3-hydroxykynurenine shows overlapping dissociations in two separate pH regions, suggesting nuanced behavior influenced by its molecular structure. The analysis of intramolecular hydrogen bonding in protonation microspecies across varying pH highlights the relevance of the charge state and hydrogen transfer potential of these metabolites in the context of their radical scavenging ability. At physiological pH, most kynurenine and 3-hydroxykynurenine entities exist in zwitterionic form, with hydrogen bonding stabilizing the aromatic amino group, which may significantly influence their interactions with proteins and reactive oxygen species. This study provides critical insights into the acid-base equilibria of kynurenine pathway metabolites. Full article

In the quest of the pivotal origin of the very strong gas-phase proton basicity for some iminopyrrole derivatives, proposed in the literature on the basis of quantum chemical calculations, the full tautomeric and acid/base equilibria were investigated in vacuo for 2-aminopyrrole exhibiting enamino–imino tautomerism. Thermochemistry of these processes investigated at the Density Functional Theory (DFT) level indicates a lower stability for the imino than for the enamino tautomers. However, the imino N atom in the imino forms displays an exceptionally high basicity, particularly in the minor and rare tautomers containing at least one tautomeric proton at the pyrrole C atom. This explains why derivatives of CH tautomers (being free of prototropy) display exceptionally high gas-phase proton basicity. As predicted by the Maksić group using quantum chemical methods, these derivatives can be considered as good organic imino N-superbase candidates. Unfortunately, some other structures of iminopyrrole derivatives (proposed by the same group) possess labile protons, and, thus, exhibit prototropy, resulting in the transformation into the more stable but less basic aminopyrrole derivatives under synthesis conditions or acid/base equilibria measurements. Full article

Melt crystallization is a promising separation technique that produces ultra-high-purity products while consuming less energy and generating lower CO₂ emissions than conventional methods. However, accurately modeling melt crystallization is challenging due to significant non-idealities and complex phase equilibria in multicomponent systems. This study develops and evaluates two neural network-based surrogate models for acrylic acid melt crystallization: a stand-alone (black-box) model and a thermodynamically guided (hybrid) model. The hybrid model incorporates UNIQUAC-based solid–liquid equilibrium constraints into the learning process. This framework combines first-principles thermodynamic knowledge—particularly activity coefficient calculations and mass balance equations—with multi-output regression to predict key process variables. Both models are rigorously tested for interpolation and extrapolation, with the hybrid approach demonstrating superior accuracy even under operating conditions significantly outside the training domain. Further analysis reveals the critical importance of accurate solid–liquid equilibrium (SLE) data for thermodynamic parameterization. A final case study illustrates how the hybrid approach can quickly explore feasible operating regions while adhering to strict product purity targets. These findings confirm that integrating mechanistic constraints into neural networks significantly enhances predictive accuracy, especially when processes deviate from nominal conditions, providing a practical framework for designing and optimizing industrial-scale melt crystallization processes. Full article

Prototropic conversion (prototropy) for heterocyclic nucleobases was already signaled by Watson and Crick about seventy years ago as one of the reasons for nucleic acids mutations. This isomeric phenomenon has been investigated for neutral derivatives by means of both experimental and theoretical procedures, and their favored tautomers discussed in numerous articles published in the last fifty years. Protonation/deprotonation reactions in the gas phase have also been studied using both quantum-chemical calculations and experimental techniques. Some thermochemical parameters of these processes have been documented. However, prototropy has not always been taken into account in protonation/deprotonation reactions. Most frequently, tautomeric heterocycles have been treated as simple polyfunctional compounds without possible intramolecular protontransfers. Taking into account the lack of data for the complete tautomeric mixtures, quantum-chemical investigations have been undertaken by us about twenty-five years ago for prototropic heterosystems. In this work, the pyrimidine base uracil (U) was chosen. It possesses two identical exo groups (=O/OH) at the 2- and 4-positions, two labile (tautomeric) protons, and five conjugated sites (N1, N3, C5, O7, and O8). Different types of isomerism, prototropy and OH-rotation, were considered for the neutral, protonated, and deprotonated forms. Using quantum-chemical methods, thermochemical stabilities of all possible tautomers-rotamers were examined in vacuo and the potential isomers selected. The selected isomeric mixtures for the neutral and ionic forms were applied for the determination of the thermochemical parameters in the four-step acid/base equilibria: B²⁻ BH⁻ BH₂ BH₃⁺ BH₄²⁺, where BH₂ indicates U. For each step, the microscopic (kinetic) and macroscopic (thermodynamic) acid/base parameters were estimated, and sites of the proton gain and proton loss examined. The similarities and differences between the acid/base equilibria for uracil and other pyrimidine nucleobases were discussed. Full article

Push-pull imines with strong electron donor group(s) display exceptional basicity in the gas phase. Most of them do not exhibit prototropic tautomerism, and gas-phase acid-base equilibria have been already well described and reviewed. Some questions remain for tautomeric systems, particularly for their uncommon forms. As shown by quantum-chemical calculations, some often-neglected tautomers display higher basicity than the thermodynamically favored forms. However, their participation in tautomeric mixtures being in equilibrium is negligible, and their basicity can be impossible to measure in the gas phase by the equilibrium method. During this work, we examined the gas-phase proton basicity for some acyclic and cyclic push-pull organic bases containing the tautomeric amidine or guanidine group. By quantum-chemical calculations, we confirmed the existence of very low amounts of rare tautomeric forms, in particular, those bearing a methylidene (=CH₂) group. We also demonstrated that the alkyl derivatives of rare tautomers, being free of prototropy, can be good candidates as very strong push-pull C bases, i.e., bases protonated on the =CH₂ group. Full article

The acid–base equilibria of cetirizine were investigated with and without the presence of differently charged micelle-forming surfactants (anionic, cationic, nonionic). The pK_a values were potentiometrically determined at 25 °C and at a constant ionic strength (0.1 M NaCl). Experimental data were analyzed by applying the computer program Hyperquad 5.2.15. Based on a shift in the ionization constants (∆pK_a) in micellar solutions against the pK_a values determined in “pure” water under the same conditions, the effects of micelles on the protolytic equilibria of cetirizine were estimated. Applied micelles caused a shift in the protolytic equilibria of all cetirizine ionizable centers, with the piperazine function connected to aliphatic side moiety (∆pK_a1 from −0.47 to +1.42), carboxyl group (∆pK_a2 from −0.92 to +2.02), and piperazine nitrogen connected to phenyl rings (∆pK_a3 from −2.01 to +2.19). Anionic SDS and nonionic Brij 35 micelles caused an increase in the pK_a values of the ionizable centers of cetirizine, while a decrease in the pK_a values was detected under the influence of cationic CTAB and nonionic TX-100 micelles. The change in the ionization pattern by micelles at pH values with biopharmaceutical significance provides indications of possible interactions of cetirizine with biomolecules of different charge and polarity under physiological conditions. Full article

The pH dependence of the free energy level of the flash-induced primary charge pair P⁺I_A⁻ was determined by a combination of the results from the indirect charge recombination of P⁺Q_A⁻ and from the delayed fluorescence of the excited dimer (P*) in the reaction center of the photosynthetic bacterium Rhodobacter sphaeroides, where the native ubiquinone at the primary quinone binding site Q_A was replaced by low-potential anthraquinone (AQ) derivatives. The following observations were made: (1) The free energy state of P⁺I_A⁻ was pH independent below pH 10 (–370 ± 10 meV relative to that of the excited dimer P*) and showed a remarkable decrease (about 20 meV/pH unit) above pH 10. A part of the dielectric relaxation of the P⁺I_A⁻ charge pair that is not insignificant (about 120 meV) should come from protonation-related changes. (2) The single exponential decay character of the kinetics proves that the protonated/unprotonated P⁺I_A⁻ and P⁺Q_A⁻ states are in equilibria and the rate constants of protonation k_on^H +k_off^H are much larger than those of the charge back reaction k_back ~10³ s⁻¹. (3) Highly similar pH profiles were measured to determine the free energy states of P⁺Q_A⁻ and P⁺I_A⁻, indicating that the same acidic cluster at around Q_B should respond to both anionic species. This was supported by model calculations based on anticooperative proton distribution in the cluster with key residues of GluL212, AspL213, AspM17, and GluH173, and the effect of the polarization of the aqueous phase on electrostatic interactions. The larger distance of I_A⁻ from the cluster (25.2 Å) compared to that of Q_A⁻ (14.5 Å) is compensated by a smaller effective dielectric constant (6.5 ± 0.5 and 10.0 ± 0.5, respectively). (4) The P* → P⁺Q_A⁻ and I_A⁻Q_A → I_AQ_A⁻ electron transfers are enthalpy-driven reactions with the exemption of very large (>60%) or negligible entropic contributions in cases of substitution by 2,3-dimethyl-AQ or 1-chloro-AQ, respectively. The possible structural consequences are discussed. Full article

Linoleic acid (LA), the primary ω-6 polyunsaturated fatty acid (PUFA) found in the epidermis, plays a crucial role in preserving the integrity of the skin’s water permeability barrier. Additionally, vegetable oils rich in LA have been shown to notably mitigate ultraviolet (UV) radiation-induced effects, including the production of reactive oxygen species (ROS), cellular damage, and skin photoaging. These beneficial effects are primarily ascribed to the LA in these oils. Nonetheless, the precise mechanisms through which LA confers protection against damage induced by exposure to UVB radiation remain unclear. This study aimed to examine whether LA can restore redox and metabolic equilibria and to assess its influence on the inflammatory response triggered by UVB radiation in keratinocytes. Flow cytometry analysis unveiled the capacity of LA to diminish UVB-induced ROS levels in HaCaT cells. GC/MS-based metabolomics highlighted significant metabolic changes, especially in carbohydrate, amino acid, and glutathione (GSH) metabolism, with LA restoring depleted GSH levels post-UVB exposure. LA also upregulated PI3K/Akt-dependent GCLC and GSS expression while downregulating COX-2 expression. These results suggest that LA induces metabolic reprogramming, protecting against UVB-induced oxidative damage by enhancing GSH biosynthesis via PI3K/Akt signaling. Moreover, it suppresses UVB-induced COX-2 expression in HaCaT cells, making LA treatment a promising strategy against UVB-induced oxidative and inflammatory damage. Full article

Formic acid is a promising candidate fuel that can be produced by reacting renewable hydrogen with carbon dioxide. However, the burning safety characteristics of formic acid–air mixtures have not been fully studied. This paper presents an extensive theoretical study of the adiabatic explosion pressure of formic acid–air premixed laminar flames at various initial conditions (composition of formic acid: 17–38% volume; initial pressure: 0.1–1.5 bar; initial temperature: 333–500 K), using the GASEQ software package. GASEQ software calculates chemical equilibria based on ideal gas behavior and is based on the hypothesis of adiabatic expansion inside a closed containment that allows ideal expansion. The influence of the initial conditions (pressure, temperature, and concentration) of formic acid–air mixtures on the adiabatic explosion pressures, maximum flame temperature, and peak concentrations of the main reaction intermediates is investigated and discussed. It is found that the adiabatic peak explosion pressure (calculated equilibrium pressure) of the studied concentrations decreases with increasing initial temperature and increases linearly with increasing initial pressure. Full article