Search Results (204)

The development of sustainable catalysts from renewable resources is a key challenge for reducing the cost of industrial catalytic processes and waste valorization. In this work, low-cost heterogeneous active catalysts were prepared based on pyrolyzed forest residues, forming valuable porous support materials (Biochar) able to efficiently accommodate the highly active heteropolyacid HPW₁₂. Further, magnetic functionality was incorporated in the novel catalytic materials by the impregnation of NiFe₂O₄. The resulting magnetic composites were characterized by FTIR-ATR, SEM-EDS, ICP-OES, BET, XRD, potentiometric titration and magnetometry. The novel HPW₁₂@NiFe₂O₄@Biochar composites were able to valorize the glycerol to produce the fuel additive solketal with high conversion and high selectivity after only 3 h of reaction via acetalization reaction with acetone. The biochar catalytic composite prepared from cork presented higher pore size than the same prepared from forest biomass. This property was crucial to achieve the best conversion (89%) and the highest solketal selectivity (96%). Additionally, reusability capacity was verified, supporting the potential of the cork-pyrolyzed-based composites as potential low-cost catalytic material to produce fuel additives, such as solketal, under sustainable conditions. This may contribute one step further toward a future with greener energy, increasing the viability of biodiesel industry waste. Full article

Profiling the blood–brain barrier (BBB) permeability of bioactive molecules during early drug development is critical for optimizing their pharmacokinetic profile. The in vivo ability of a compound to cross the BBB is measured by the log BB parameter; however, its determination requires costly and time-consuming animal experiments. This study aimed to develop a novel in vitro method for high-throughput prediction of log BB values. The approach combines experimental data from open-tubular capillary electrochromatography (CEC) and automated potentiometric titrations, including the CEC retention factor (k′), electropherograms, and physicochemical parameters pK_a and log D_7.4. The k′ parameter reflects BBB permeability using a capillary internally coated with liposomes that mimic a biological membrane. Preliminary CEC analyses were conducted for 25 neutral drugs at pH 7.4, revealing a promising correlation between the permeability parameters log k and log BB. The validation was extended to 57 ionized drugs, with additional determination of pK_a and log D_7.4. A regression model was developed: log BB = −2.45 + 0.1k′ + 0.3logD_7.4 + 0.27pK_a (R² = 0.64). Furthermore, the analysis of CEC electropherograms enabled the machine learning-based rapid classification of compounds using Dynamic Time Warping, k-Nearest Neighbors, and the Bag-of-SFA-Symbols in Vector Space model, yielding an accuracy of 0.81 and an F1_weighted score of 0.8. Full article

Proton binding (i.e., charging) isotherms of halloysite nanotubes (HNT) were determined from cycled acid-base potentiometric titrations in KCl solution at constant ionic strengths (0.01, 0.10, 1.00 mol dm⁻³). The isotherms measured in the pH cycle from 3 to 11 and back exhibit a pronounced hysteresis with respect to the direction of pH change, which is accurately reproducible when the cycle is repeated. The hysteresis is absent if the cycled titration is performed within a narrow pH range between 5 and 9. These results align with the dissolution rates of alumina and silica, which form the two surfaces of the rolled kaolinite sheet in HNT, and clearly point to reversible partial dissolution-deposition processes in the HNT interior during a titration cycle, outside the above pH range (alumina dissolution below pH ≈ 5 and silica dissolution above pH ≈ 8.5). In the studied titration experiments, these processes produce partially dissolved surface-bound, rather than completely dissolved species (reversible surface etching). Under the applied conditions, reversible surface etching is less pronounced in the acidic part of the titration cycle. Charging isotherms recorded in the decreasing pH titrations at varied ionic strength exhibit a common intersection point very close to zero charge (point of zero charge) around pH ≈ 8.1, characteristic for an amphoteric solid surface. These isotherms were reasonably well fitted by applying the surface protonation model in the HNT interior, which invokes the Stern model of the electric double layer (EDL), by summing the surface charges calculated for alumina and silica as separate components (surfaces). The model surface charge isotherms for alumina surface in the HNT interior exhibit a point of zero charge at pH = 9.0, while the silica surface has a negative charge above pH > 8.5, which is in very good agreement with the values reported in the literature: as for these two surfaces, thus for kaolinite nanoparticles. The best-fit protonation site density for both surfaces is equal to 8.0 nm⁻², while the best-fit intrinsic pK_a for alumina and silica surfaces of HNT are equal to 9.0 and 8.5, respectively. The pH-dependence of electrophoretic mobility, measured by means of electrophoretic light scattering, reveals a more acidic behavior of the outermost silica surface than within the inner HNT phase, which is consistent with the literature result reported for kaolinite. The results reported herein confirm that the inner and outer surfaces of the HNT are oppositely charged below pH < 8.0 and negatively charged above that value, and importantly, they reveal new details about the protonation affinities and EDL parameters at active surfaces of HNT, important for the colloidal stability of HNT suspensions and the functionalization of HNT through the electrostatic binding of active molecules. Full article

The synthesis, characterization, and equilibrium studies of complexes of selected lanthanide ions Eu(III), Gd(III), and Tb(III) with the ligand 1,3-bis(3-bromo-5-chlorosalicylideneamino)-2-propanol (H₃L) are reported. It was found that in the solid state, the complexes with the formulas [Eu(H₃L)₂(NO₃)₃], [Gd(H₃L)₂(NO₃)₃], and [Tb(H₃L)₂(NO₃)₃] are formed. In solution, complexes with stoichiometries of Ln(III):H₃L 1:1 and 1:2 were obtained. The ligand H₃L was isolated in crystalline form, and its molecular structure and conformation were determined by single-crystal X-ray diffraction analysis. The compounds were further characterized by elemental analysis, infrared spectroscopy, ¹H NMR, ¹³C NMR techniques, and mass spectrometry (ESI), confirming the formation of the Schiff base group. Stability constants of the complexes in solution were determined using potentiometric titration, providing insights into the metal-ligand binding equilibria. In addition, the spectroscopic properties of the ligand and its lanthanide(III) ion complexes were investigated by UV-Vis spectroscopy, which confirmed ligand-to-metal charge transfer interactions, as well as by luminescence measurements. The luminescence studies revealed inefficient energy transfer in [Eu(H₃L)₂(NO₃)₃] complexes, while no transfer was observed in [Tb(H₃L)₂(NO₃)₃] systems at any pH value. This behavior is attributed to the large energy gap between the ligand triplet state and the lowest resonant levels of the studied lanthanide ions. Full article

An extensive study on the interactions between O-phosphorylethanolamine (PEA) and O-phosphorylcholine (PPC), Cu²⁺ and Zn²⁺, is thoroughly described. The formation constants were determined at different temperatures (15 ≤ t/°C ≤ 37) and ionic strengths (0.15 ≤ I/mol L⁻¹ ≤ 0.97) by potentiometric titrations. For the Zn²⁺-PEA/-PPC systems, speciation models were also confirmed by ¹H NMR titrations at t = 25 °C and I = 0.15 mol L⁻¹ in NaCl. Sequestering abilities were calculated under different temperatures and physiological conditions. Density Functional Theory (DFT) calculations along with enhanced sampling of the conformational space were performed aimed to better elucidate the Cu²⁺-, Zn²⁺- PEA/PPC molecular interactions and their relative stabilities. Overall, both experiments and computer simulations showed that the complex species involved in the Cu²⁺–PEA system exhibited a significant and selective stability, particularly in conditions simulating cerebrospinal fluid. While the binding molecular mechanisms were elucidated via DFT supplemented by automized conformational search, the computational binding energies trend qualitatively follows the experimental logK behavior across the Cu²⁺-, Zn²⁺- PEA/PPC complexes. These results highlight the potential physiological role of PEA in modulating free copper levels and regulating its redox activity in pathological conditions, such as Wilson’s Disease (WD). Full article

Surfactants are used in various washing applications with potential negative environmental and health impacts. The ion-pair 1,3-dioctadecyl-1H-1,2,3-triazol-3-ium-tetraphenylborate (DODTA–TPB) was used to fabricate the potentiometric sensor for the quantification of anionic surfactants. The computational analysis of the DODTA⁺–TPB⁻ adduct reveals a dynamic, thermodynamically favorable interaction driven primarily by hydrophobic C–H∙∙∙π contacts and the flexibility of the C-18 chains, rather than electrostatic or π–π stacking forces. These findings, supported by the MM-PBSA, RDF, and structural analyses, align with broader trends in molecular recognition and provide a foundation for designing advanced ion-pair-based sensors. The sensor showed advanced analytical properties to anionic surfactants with low interfering effects of selected anions. The response of the SDS was investigated in the range from 8.1 × 10⁻⁸ M to 1.0 × 10⁻² M, with a slope of −59.2 mV and a limit of detection (LOD) of 3.1 × 10⁻⁷ M; and DBS was in the range of 8.1 × 10⁻⁸ M to 2.5 × 10⁻³ M with a slope of −57.5 mV and an LOD of 5.9 × 10⁻⁷ M. The sensor was tested on potential interfering ions. Potentiometric titrations of technical-grade anionic surfactants had high recovery rates from 100.2 to 100.4%. The recovery test for spiked samples of surface waters was from 94.2 to 96.5%. The sensor was tested on commercial samples containing anionic surfactants, and the results were compared and showed a good agreement with the two-phase titration method. Full article

Mining activities have long-term impacts on the environment even after the active stage. Historical mines developed in the 19th and 20th centuries for tin, copper, and mainly iron ore are located in the Pitkäranta area (Karelia, Russia). These objects are considered in our research as natural–anthropogenic sites of long-term water–rock interaction. Waters from flooded mines are the subject of this research. Redox conditions, pH, dissolved oxygen content, conductivity, and water temperature were determined during field work. The chemical composition of natural waters was determined by ICP-MS, ICP-AES, ion chromatography, potentiometric titration, and spectrophotometry. Our investigation showed that the mine waters are fresh and predominantly calcium–magnesium hydrocarbonate; most samples showed elevated sulfate ion contents. Circumneutral pH values and the absence of extremely high concentrations of heavy metals indicate neutral mine drainage. However the calculation of the accumulation coefficient showed the highest levels for siderophile elements relative to the corresponding data of the geochemical regional background. Moreover, zinc has the highest content in the series of heavy metal(loid)s considered. The maximum concentration of zinc was determined in the water of one of the shafts of the Lupikko mine, i.e., 5205 µg/L. The accumulation of heavy metals occurs in the process of long-term interaction of water–rock–organic matter under conductive redox conditions. Overall, the research highlighted the relevance of investigating the geochemistry of historical mines in the Pitkäranta area both from the perspective of environmental safety and the preservation of mining sites for scientific and educational purposes. Full article

The acid number evaluates the degree of deterioration of lubricating oil. Existing methods for evaluating the performance degradation of lubricating oils are mostly based on the detection of traditional physical and chemical indicators, which often only reflect a single dimension of the degradation process, thus affecting the accuracy and repeatability of the results. Integrating multi-dimensional information can more comprehensively reflect the essence of degradation, which can improve the accuracy and reliability of the evaluation results. Mid-infrared spectroscopy is an effective means of monitoring the acid number. In this study, a combination of infrared spectroscopy quantitative analysis and chemometrics was used. The oil sample data was divided into training set and validation set by the Kennard–Stone method. In the experiment, a Fourier transform infrared spectrometer equipped with an attenuated total reflection accessory (ATR-FTIR) was used to collect spectral data of the samples in the wavenumber range of 1750–1700 cm⁻¹ (this range corresponds to the characteristic absorption of carboxyl groups and is directly related to the acid number). Meanwhile, a G20S automatic potentiometric titrator was used to determine the acid number as a reference value in accordance with GB/T 7304. The study compared various preprocessing methods. A regression prediction model between the spectra and acid number was established using partial least squares regression (PLSR) within the selected wavenumber range, with the root mean square error of cross-validation (RMSECV), root mean square error of prediction (RMSEP), and coefficient of determination (R) as evaluation indicators. The experimental results showed that the PLSR model established after preprocessing with second derivative combined with seven-point smoothing exhibited the optimal performance, with an RMSECV of 0.00505, an RMSEP of 0.14%, and an R of 0.9820. Compared with the traditional titration method, this prediction method is more suitable for real-time monitoring of production lines or rapid on-site screening of equipment. It can in a timely manner warn of the deterioration trend of lubricating oil, reduce the risk of equipment wear caused by oil failure, and provide efficient technical support for lubricating oil life management. Full article

Renewable fuels produced from animal- and plant-based edible oils have emerged as an alternative to oil and natural gas. Burgeoning interest in renewables can be attributed to the rapid depletion of fossil fuels caused by the global energy demand and the environmental advantages of renewables, specifically reduced emissions of greenhouse gases. An important property of the feedstock that is crucial for the conversion of edible oils to renewable fuels is the total acid number (TAN), as even a small increase in TAN for the feedstock can lead to corrosion of the catalyst in the refining process. Currently, the TAN is determined by potentiometric titration, which is time-consuming, expensive, and requires the preparation of reagents. As part of an effort to promote the use of renewable fuels, a partial least squares regression method with orthogonal signal correction to remove spectral information related to the sample background was developed to determine the TAN from the mid-infrared (IR) spectra of the feedstock. Digitally blended mid-IR spectral data were generated to fill in regions of the PLS calibration where there were very few samples. By combining experimental and digitally blended mid-IR spectral data to ensure adequate sample representation in all regions of the spectra–property calibration and better understand the spectra–property relationship through the identification of sample outliers in the original data that can be difficult to detect because of swamping, a PLS regression model for TAN (R² = 0.992, cross-validated root mean square error = 0.468, and bias = 0.0036) has been developed from 118 experimental and digitally blended mid-IR spectra of commercial feedstock. Thus, feedstock whose TAN value is too high for refining can be flagged using the proposed mid-IR method, which is faster and easier to use than the current titrimetric method. Full article

The interactions of Fe³⁺ with some ligands (Tranexamic (TXA⁻), Indole-3-acetic (IAA⁻), and Aminomethylphosphonic (AMPA²⁻) acids) of biological and environmental interest were studied. The speciation studies were performed in NaNO_3(aq) and NaCl_(aq) using potentiometric and, only for IAA⁻, spectrophotometric titrations at T = 298.15 K and 0.01 ≤ I/mol dm⁻³ ≤ 1.0. The proposed speciation models are as follows: Fe(TXA)H³⁺, Fe(TXA)₂⁺, Fe(TXA)(OH)⁺, and Fe(TXA)(OH)_2(aq) for TXA⁻; Fe(IAA)²⁺ for IAA⁻; and Fe(AMPA)H₂³⁺, Fe(AMPA)H²⁺, and Fe(AMPA)⁺ for AMPA²⁻. A comparison of logβ for the common FeL species gives logβ_FeIAA = 6.56 and logβ_FeAMPA = 14.84 (at I = 1.00 mol dm⁻³ and T = 298.15 K), suggesting that AMPA²⁻ has a higher complexing ability towards Fe³⁺ than IAA⁻. The dependence on the ionic strength of the formation constants was modeled by means of a Debye–Hückel type equation and the SIT model, whilst the sequestering ability of the investigated ligands towards Fe³⁺ was quantified at various pHs, ionic strengths, and in the different supporting electrolytes by means of an empirical pL_0.5 parameter. To complete this study of the behavior of the different Fe³⁺/ligand systems, various simulations in biological fluids and natural waters were conducted. Full article

The SAXS pattern and pore volume of SBA-15 are not affected by aging SBA-15 with water, dilute acetic acid, and ammonia up for to 1 month, while the specific surface area is substantially depressed in interactions with basic solutions. The SEM images indicate pits on the side surfaces of SBA-15 particles in interactions with basic solutions. Aging of SBA-15 in ammonia solutions results in cavities formed by the collapse of walls between neighboring hexagonal channels in the SBA-15. This phenomenon is discussed with a special emphasis on its possible effect on the potentiometric titration curves. Especially, a standard procedure, in which the SSA measured before the titration is used to calculate the σ₀, is compared with a modified procedure, in which the SSAs measured before and after the titration are used to estimate the SSA for each data point separately. Full article

Research on using biochar as an adsorbent of contaminants in aqueous matrices has gained significant relevance in recent years due to the surface chemistry and porous structure of biochar, which facilitate the retention of a wide range of pollutants. This study explores the adsorption performance of a novel biochar produced from agave bagasse—a readily available agro-industrial waste in Mexico—through low-temperature pyrolysis. The biochar was evaluated for its capacity to remove conventional water quality parameters (chemical oxygen demand (COD), nitrates (NO₃⁻), total nitrogen (TN), total phosphorus (TP), ammonium (NH₄⁺), turbidity, apparent color, and true color) from water samples collected from the polluted Bustillos Lagoon in Chihuahua, Mexico. Additionally, the removal of emerging pharmaceutical contaminants, specifically acetaminophen (Act) and diclofenac (Dfc), was assessed in synthetic aqueous solutions. Potentiometric titration analyses revealed a significant contribution of surface acidity in the adsorption of pharmaceutical derivatives, highlighting the relevance of functional groups retained during low-temperature pyrolysis. The biochar derived from agave bagasse (BBAF1) was tested in a fixed-bed column system and compared with two commercial activated carbons (CACCF2 and CVCF3). The BBAF1 biochar achieved average removal efficiencies ranging from 50% to 90% for all conventional parameters. In contrast, those of ACT and DFC were between 0.43 and 0.67 mg g⁻¹ (59–85%) and 0.34 and 0.62 mg g⁻¹ (37–79%), respectively, demonstrating their potential as an adsorbent material for improving water quality. This work supports the development of circular economic strategies by valorizing agricultural residues while offering an effective solution to environmental pollution challenges. Full article

This work studies the effect of mesostructured silica–zirconia–tungstophosphoric acid (SiO₂-ZrO₂-TPA) composites used as catalysts in the synthesis of nifedipine by the Hantzsch methodology. The selectivity for nifedipine is determined, along with that of secondary products that may form depending on the reaction conditions. The materials were synthesized via the sol–gel method and characterized by N₂ adsorption–desorption isotherms, infrared spectroscopy (FT-IR), ³¹P solid-state nuclear magnetic resonance (NMR-MAS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectra (XPS), and potentiometric titration. The characterization results from the XPS spectra showed that as the Si/Zr ratio drops, the Si-O-Si signal size decreases, while the Zr-O signal size increases. Characterization by titration indicated that an increase in the total acidity of the material, resulting from support modification with tungstophosphoric acid (H₃PW₁₂O₄₀, TPA), enhances the reaction yield. The catalytic activity in the solvent-free Hantzsch reaction was evaluated under thermal heating and microwave irradiation. The experiments conducted at 80 °C achieved a maximum yield of 57% after 4 h of reaction using the Si20Zr80TPA30 catalyst (50 mg), while by microwave heating, the yield significantly improved, reaching 77% in only 1 h of reaction. This catalyst exhibited stability and reusability without significant loss of activity up to the third cycle. Depending on the type of material and the reaction conditions, it is possible to modify the selectivity of the reaction, obtaining a 1,2-dihydropyridine isomeric to nifedipine. Reaction intermediates and other minor secondary products that may be formed in the process were also evaluated. 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

The complexation behavior of lanthanide(III) ions with terephthalic acid (1,4-benzene-dicarboxylic acid) in 0.01 M KNO₃ aqueous solutions was studied across a broad pH range and at two metal-to-ligand ratios using potentiometric titration combined with photoluminescence spectroscopy. Chemometric analysis of titration curves enabled the determination of relative molar fractions, stability constants, and probable stoichiometry of the formed complexes. In solutions with a 1:2 metal-to-ligand ratio, bis-complexes (two terephthalate ligands per lanthanide ion) predominated, while ligand-rich conditions favored the formation of tetra-complexes (four ligands per metal ion). In alkaline media, bis-complexes transform into mixed hydroxy-terephthalate species. Meanwhile, for the tetra-complexes, the addition of NaOH results in the formation of lanthanide ion hydroxo complexes without organic ligands. The structural diversity of these complexes, driven by the terephthalate ligand’s tendency to maximize denticity, suggested dimeric or oligomeric configurations. The stability constants and structural features of complexes in solution were found to align with those of known solid-state lanthanide–terephthalate polymers, highlighting their potential as models for polymeric structures. Full article