Acid-base Equilibrium Constants Research Articles

Water-Polymer Slide Electrification in Polyethylene Films Coated with Amphiphilic Compounds and Its Connection to Surface Properties Dependent on Water-Polymer Interactions.

Slide electrification experiments were performed on low-density polyethylene films (PE) and PE sprayed with five amphiphilic compounds, including antistatic and slip additives. Drops of aqueous solutions were delivered on the films and after sliding spontaneously acquired a net electrical charge (Qdrop), which is dependent on the pH and ionic strength. The slide electrification was detected in pristine PE films and those with five additives. An acid-base equilibrium model, based on the adsorption of hydroxides and protons on surface sites, accounted for the dependence of Qdrop on pH, allowing recovery of the acid-base equilibrium constants and the density of adsorption sites. The model was modified to account for ionic strength effects through activity factors. The surface conductivity, wettability, and friction coefficients were strongly modified by the additives. However, the observed trends are different from those observed in slide electrification, which better correlated with zeta potential determinations. This suggests that the interaction mechanisms among surface water, the considered additives, and the polymer, which are involved in slide electrification and the generation of zeta potential, are different from those associated with other surface processes involving surface water. Although additives are required for changing surface resistivity, friction coefficients, and wettability, the generation of sliding electrical charges in polyethylene is a spontaneous and highly effective process. For one specific additive, a simultaneous decrease in friction coefficients, zeta potential, and Qdrop was observed, assigned to the blockade of hydroxide adsorption sites and water repulsion by the compound.

Analyzing Misconceptions of Acid-Base Topic among Chemistry Education Students in Online Learning Settings: A Case Study

This study aims to identify the misconceptions of 11 third semester Chemistry Education students at Cenderawasih University in relation to online learning during the Covid-19 pandemic. Online learning poses various challenges for both students and lecturers, resulting in incomplete explanations and hindering students' understanding of the material, leading to possible misconceptions. A descriptive qualitative method was employed, utilizing diagnostic tests with a Certainty or Response Index (CRI) and interviews. The findings revealed that 41% of the students had a clear understanding of the acid-base concepts, 30% lacked understanding, and 29% had misconceptions. The students' misconceptions related to six subconcepts in acid-base materials, including acid-base theory, acid-base indicators, acid strength (pH), weak acid-base equilibrium constant (Ka/Kb), calculation of solution pH, and the concept of pH in the environment, with a range of misconceptions from 9-45%. This study highlights the need for addressing misconceptions in online learning, specifically in the acid-base topic, to ensure effective understanding and learning outcomes for students.

PH- and Facet-Dependent Surface Chemistry of TiO2 in Aqueous Environment from First Principles.

TiO2 is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO2 particles at the nanoscale is often complex, spanning from faceted to spherical. In this work, we study the pH- and facet-dependent surface chemistry of TiO2/water interfaces by performing ab initio molecular dynamics simulations with the grand canonical formulation of species in solution. We first determined the acid-base equilibrium constants at the interface, which allows us to estimate the pH at the point of zero charge, an important experimental observable. Then, based on simulated equilibrium constants, we predict the amount of H+, OH-, and adsorbed H2O species present on the surfaces as a function of the pH, a relevant aspect for water splitting semi-reactions. We approximated the complex morphology of TiO2 particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

Surface Hydroxyl Chemistry of Titania- and Alumina-Based Supports: Quantitative Titration and Temperature Dependence of Surface Brønsted Acid-Base Parameters.

Surface hydroxyl groups on metal oxides play significant roles in catalyst synthesis and catalytic reactions. Despite the importance of surface hydroxyls in broader material applications, quantitative measurements of surface acid-base properties are not regularly reported. Here, we describe direct methods to quantify fundamental properties of surface hydroxyls on several titania- and alumina-based supports. Comparing commercially available anatase, rutile, P25, and P90 titania, thermogravimetric analysis (TGA) indicated that the total surface hydroxyl density varied by a factor of 2, and each surface hydroxyl is associated with approximately one weakly adsorbed water molecule. Proton-exchange site densities, determined at 25 °C with slurry acid-base titrations, led to several conclusions: (i) the intrinsic acidity/basicity of surface hydroxyls were similar regardless of the titania source; (ii) differences in the surface isoelectric point (IEP) were primarily attributable to differences in the surface concentration of acid and base sites; (iii) rutile has a higher surface concentration of basic hydroxyls, leading to a higher IEP; and (iv) P25 and P90 titania have slightly higher surface concentrationsof acidic hydroxyls relative to anatase or rutile. Temperature effects on surface acid-base properties are rarely reported yet are significant: from 5 to 65 °C, IEP values change by roughly one pH unit. The IEP changes were associated with large changes to the intrinsic acid-base equilibrium constants over this temperature range, rather than changes in the composition or concentration of the surface sites.

Spectral-Luminescent Study of the Acid–Base Equilibrium of 5-Aminouracil and 6-Aminouracil in Aqueous Solutions

The spectral fluorescence characteristics of 5-aminouracil (5AU) and 6-aminouracil (6AU) in neutral and alkaline aqueous solutions have been studied. With the use of the density functional theory, it has been shown that uracils preferentially dissociate at the N1–H bond. The acid–base equilibrium constants pKa1(5AU) = 9.4 and pKa1(6AU) = 8.95 were determined by a fluorescence method. It was concluded that the ultrashort lifetime of the excited singlets of 5AU and 6AU prevents the measurement of the acid–base equilibrium constant of uracils in an electronically excited state.

Effect of acid-base properties of zirconium dioxide on dissolution kinetics

The processes of metal extraction from ores, in which metals can often be found as part of oxides and sulphides, involve stages of chemical or electrochemical dissolution. Few publications can be found in the literature that describe detailed research studies looking into how the surface phenomena of d-metal oxides are linked to the acid-base properties and the dissolution mechanics. Consequently, it would be of relevance to establish kinetic relationships of the processes related to the extraction of metals from lean ores and secondary raw materials, leaching and beneficiation, surface etching and purification. This paper describes the results of a study that relied on potentiometric titration in an aqueous solution of potassium chloride to understand the dissolution kinetics of zirconium dioxide suspension. Titration curves were produced for different concentrations of the background electrolyte and pH values and a relationship was established between ZrO2 dissolution kinetics and pH of the medium. Acid-base equilibrium constants were calculated at the metal oxide/electrolyte solution interface. The obtained results were used to analyze the distribution of surface particles that form as a result of hydrogen ions adsorption and desorption. Interpretation of the experimental relationships was based on comparison with theoretical calculations. The authors simulated the dissolution process. The paper demonstrates that metal oxides dissolve in stages, and adsorption complexes form in the process, the composition of which is linked to the surface charge. These findings could be useful when studying the kinetics of d-metal oxides, as well as in practical applications related to the dissolution of transition metal oxides in electrolyte solutions.

PH Dependence of MgO, TiO2, and γ-Al2O3 Surface Chemistry from First Principles

We investigate the pH-dependent surface chemistry of three relevant and widely used oxides, MgO, TiO2, and γ-Al2O3, from ab initio molecular dynamics simulations at the level of density functional theory (DFT). We studied the MgO (001), anatase TiO2 (101), and γ-Al2O3 (001) low-index surfaces and considered solvation effects by explicitly simulating the solid/water interfaces. The analysis of the MgO/H2O, TiO2/H2O, γ-Al2O3/H2O interfaces allowed us to access atomistic details of the structure and the effects induced by water on these surfaces. We also investigated the pH dependence by means of the Grand Canonical formulation of species in solution. This allowed us to evaluate acid–base equilibrium constants and the point-of-zero charge (pHPZC) values. The calculated pHPZC values compare well with available experimental measurements. Based on the above result, we also predicted the fraction of charged species (H+ and OH–) and neutral molecules (H2O) present on the surface as a function of pH, a fundamental aspect if one wants to identify suitable catalysts for water splitting reactions.

Electrosurface properties and acid-base equilibria of Ta2O5 and Ta2O5:Eu nanoparticles in NaCl solutions

The electrosurface properties and aggregative stability of nanoparticles (NPs) are key factors that determine their performance and biological or environmental fate. However, there is still a lack of data on the stability of aqueous dispersions of some important NPs for biomedical applications, such as tantalum oxide. In the present work, the effects of pH, electrolyte concentration, temperature, and Eu-doping on the stability of Ta2O5 NPs aqueous dispersions were carefully studied. Surfactant-free Ta2O5 and Ta2O5:Eu NPs were synthesized via a solvothermal method. Stable hydrosols and powder samples were prepared on their basis for a comparative study. The particle size, morphology, and amorphous structure of samples were investigated by transmission electron microscopy and dynamic light scattering. The laser Doppler electrophoresis was used to investigate the pH and temperature dependence of zeta potential of NPs. Obtained results were also discussed within the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory for the first time for tantalum oxide NPs. A turbidimetry study was performed to investigate the coagulation kinetics of Ta2O5 NPs in NaCl solutions. The critical coagulation concentration and stability factor were determined experimentally and via DLVO calculations; the obtained results showed a good agreement. The acid-base surface properties of samples were studied by potentiometric titration, and constants of surface acid-base equilibria were determined within 2pK-modeling.

Charge Regulation at a Nanoporous Two-Dimensional Interface.

In this work, we have studied the pH-dependent surface charge nature of nanoporous graphene. This has been investigated by membrane potential and by streaming current measurements, both with varying pH. We observed a lowering of the membrane potential with decreasing pH for a fixed concentration gradient of potassium chloride (KCl) in the Donnan dominated regime. Interestingly, the potential reverses its sign close to pH 4. The fitted value of effective fixed ion concentration (C̅R) in the membrane also follows the same trend. The streaming current measurements show a similar trend with sign reversal around pH 4.2. The zeta potential data from the streaming current measurement is further analyzed using a 1-pK model. The model is used to determine a representative pK (acid–base equilibrium constant) of 4.2 for the surface of these perforated graphene membranes. In addition, we have also theoretically investigated the effect of the PET support in our membrane potential measurement using numerical simulations. Our results indicate that the concentration drop inside the PET support can be a major contributor (up to 85%) for a significant deviation of the membrane potential from the ideal Nernst potential.

Determination of surface protonation-deprotonation behavior, surface charge, and total surface site concentration for natural, pillared and porous nano bentonite heterostructure

In this paper, the physicochemical properties of natural, pillared and porous nano bentonite heterostructure in the points of view of surface protonation–deprotonation behaviors were studied. Moreover, the structural heterogeneity in each sample based on three-dimensional field-emission scanning electron microscopy (FESEM) plots were investigated to obtain its effect on the formation of surface active sites. To better understand the surface protonation properties and adsorptive capability of natural, pillared and porous clays, it is essential to investigate the surface charge, surface active sites, pH at point of zero charge (pHzpc), and surface acid-base equilibrium constants. Since the information about amphoteric acid-base groups especially in porous clays is still lacking, our main purpose is to obtain the best estimates to compare the surface properties of natural, pillared and porous clays by means of Gran method and surface complexation model from titration data. The 2 pK-diffusion layer model (DLM) was applied to evaluate the contribution and concentration of the various surface site species (such as SOH, SOH2+, and SO−) in the different pH and ionic strengths. This paper indicates considerable new information about surface acid-base reactions, total surface site concentrations, pHzpc measurement, which provides basic knowledge of the underlying adsorption mechanisms. The maximum value of total surface sites concentration (8.84 mmol/L) in ionic strengths of 0.05 M NaCl belonged to porous nano bentonite heterostructure (PBH) with the highest pHzpc at 9.73.

Simulated calculation of surface acid-base equilibrium constants and surface species distribution of the nanoceria suspension

As artificial nano-enzymes and effective adsorbents, CeO2 nanomaterials have been extensively studied for their potential applications in biomedicine and environmental protection. However, the surface acid-base properties of nanoceria suspensions have not been fully studied. In this work, the ceria nanoparticles were synthesized by hydrothermal reaction and calcination. They have a BET specific surface area of 52.01 m2/g. FTIR and XPS showed that there are two amphoteric sites (≡Ce(IV)-OH and ≡Ce(III)-OH) and a trace acidic site (≡Ce(III)-OCOOH) which can act as a site probe. The pH range of the acid-base reaction of surface hydroxyl groups of the CeO2 nanomaterials was determined by Gran plot method. The acidic ionization constants (log K1, log K2), site concentration (log C) of two amphoteric surface hydroxyl sites and pHpzc in three KNO3 supported electrolytes were simultaneously optimized by ProtoFit GUI v2.1 free software using Donnan Shell Model (DSM) and same initial parameters. Their surface species distribution was calculated by PHREEQCI 3.4.0 free software using Donnan model and database (phreeqc.dat). The results show that the proton transfer reaction occurred between ≡Ce(IV)-OH and ≡Ce(III)-OH sites. Therefore, this study reveals the surface acid-base properties, proton transfer behavior and pHpzc of the CeO2 suspension.

Synthesis, characterization and acid-base properties of kaolinite and metal (Fe, Mn, Co) doped kaolinite

Pure kaolinite and metal ion doped kaolinite were synthesized successfully for the first time with different M/Al (M = Mn, Fe, Co) ratios (0, 0.042, 0.087, 0.19) at 220 °C by the hydrothermal method. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and BET techniques (N2/BET) were used for sample characterization. XRD patterns of the products showed that kaolinite was the only crystalline phase synthesized and the lattice parameters varied systematically with metal content. The surface acid-base properties of the pure kaolinite and doped kaolinite were studied by potentiometric titration. Surface site concentration (Hs) and surface site density (Ds) increased as metal ion content in the metal doped kaolinite increased. The relevant acid-base equilibrium constants of synthesized products were calculated by applying the constant capacitance model using computer software WinSGW. The surface pKa values of synthetic kaolinite were calculated as: logKa1 = 1.48; logKa2 = −7.03. The results showed that a small number of metal ions significantly affect the surface acid-base properties of kaolinite, which may explain the origin of variability of the reported surface pKa values of kaolinite.

Adipic Acid Production via Metal-Free Selective Hydrogenolysis of Biomass-Derived Tetrahydrofuran-2,5-Dicarboxylic Acid

Biomass-derived furans offer sustainable routes to adipic acid (AA), a key chemical in Nylon-6,6 synthesis. In this work, we show that tetrahydrofuran-2,5-dicarboxylic acid (THFDCA) is a viable precursor for AA production, achieving up to 89% yield in a metal-free system containing HI and molecular H2 in a propionic acid solvent at 160 °C. Reactivity studies demonstrate that the interplay between HI, H2, and the solvent is essential for effective THFDCA ring opening. By measuring the reaction orders of HI and molecular H2 and calculating an acid–base equilibrium constant in a nonaqueous solvent, we show that HI plays a multifaceted role in the reaction by acting both as a proton source and an iodide source to selectively cleave C–O bonds without overhydrogenation of carboxylic acid groups. Using reactivity studies, kinetic measurements, and first-principles computational insights, we demonstrate that metal-free activation of molecular H2 plays a key role in the reaction, following HI-mediated cleavage of ...

Site- and species-specific hydrolysis rates of cocaine

The hydroxide-catalyzed non-enzymatic hydrolysis of cocaine is quantified in terms of ten site- and species-specific rate constants in connection with also ten site- and species-specific acid-base equilibrium constants, comprising all the twelve coexisting species in solution. This characterization involves the major and minor decomposition pathways via benzoylecgonine and ecgonine methyl ester, respectively, leading to ecgonine, the final product. Hydrolysis has been found to be 10–330 times faster at site 2 than at site 3, depending on the ionization status of the amino moiety and the rest of the molecule. Nitrogen protonation accelerates the hydrolyses approximately ten times both at site 2 and site 3.

Sorption of aromatic acids from aqueous solutions by polymer based on N-vinylpyrrolidone

Kinetic and equilibrium characteristics of the sorption of benzoic, salicylic and ortho-chlorobenzoic acids by a new cross-linked sorbent based on N-vinylpyrrolidone were studied at various pH values of the medium. It was found that the molecular sorption of aromatic carboxylic acids from aqueous solutions occurs in the mixed-diffusion mode and the sorption capacity of the polymer grows with increasing acid-base equilibrium constants of the acids. The advisability of introducing a salting-out agent into sorption systems was confirmed. Effective sorption systems and a method for concentration of aromatic carboxylic acids from aqueous solutions by the new cross-linked sorbent based on N-vinylpyrrolidone were suggested.

Site- and species-specific hydrolysis rates of heroin

The hydroxide-catalyzed non-enzymatic, simultaneous and consecutive hydrolyses of diacetylmorphine (DAM, heroin) are quantified in terms of 10 site- and species-specific rate constants in connection with also 10 site- and species-specific acid-base equilibrium constants, comprising all the 12 coexisting species in solution. This characterization involves the major and minor decomposition pathways via 6-acetylmorphine and 3-acetylmorphine, respectively, and morphine, the final product. Hydrolysis has been found to be 18–120 times faster at site 3 than at site 6, depending on the status of the amino group and the rest of the molecule. Nitrogen protonation accelerates the hydrolysis 5–6 times at site 3 and slightly less at site 6.Hydrolysis rate constants are interpreted in terms of intramolecular inductive effects and the concomitant local electron densities. Hydrolysis fraction, a new physico-chemical parameter is introduced and determined to quantify the contribution of the individual microspecies to the overall hydrolysis. Hydrolysis fractions are depicted as a function of pH.

Constants of acid‒base equilibria in an aqueous amikacin aminoglycoside solution at 298 K

The acid dissociation constants of form pK1 = 7.34 ± 0.01, pK2 = 7.84 ± 0.01, pK3 = 8.77 ± 0.01, pK4 = 9.49 ± 0.01, and pK5 = 10.70 ± 0.02 of cationic amikacin are determined by pH-metric titration at 25°C against the background of 0.1 mol/L KNO3. K1, K2, K3, and K4 correspond to the dissociation of protons coordinated to amino groups, while K5 characterizes the dissociation of the hydroxyl hydrogen atom, testifying to the amphoteric character of amikacin molecules. Applying density functional theory (DFT) with the B3LYP hybrid functional and the 6-311G**++ basis set, the partial charges on the atoms of an amikacin molecule are calculated. It is concluded that the dissociation of H(55)hydrogen atom occurs with a greatest partial charge of +0.53631.

Estimating pH at the Air/Water Interface with a Confocal Fluorescence Microscope.

One way to determine the pH at the air/water interface with a confocal fluorescence microscope has been proposed. The relation between the pH at the air/water interface and that in a bulk solution has been formulated in connection with the adsorption equilibrium and the dissociation equilibrium of the dye adsorbed. Rhodamine B (RhB) is used as a surface-active fluorescent pH probe. The corrected fluorescence spectrum of RhB molecules at the air/water interface with the surface density of 1.0 nmol m(-2) level shows pH-dependent shifts representing an acid-base equilibrium. Two ways to determine the unknown acid-base equilibrium constant of RhB molecules at the air/water interface have been discussed. With surface-tension measurements, the adsorption properties, maximum surface density, and adsorption equilibrium constants were estimated for both cationic and zwitterionic forms of RhB molecules at the air/water interface.

Influence of Solvent Polarity and DNA-Binding on Spectral Properties of Quaternary Benzo[c]phenanthridine Alkaloids.

Quaternary benzo[c]phenanthridine alkaloids are secondary metabolites of the plant families Papaveraceae, Rutaceae, and Ranunculaceae with anti-inflammatory, antifungal, antimicrobial and anticancer activities. Their spectral changes induced by the environment could be used to understand their interaction with biomolecules as well as for analytical purposes. Spectral shifts, quantum yield and changes in lifetime are presented for the free form of alkaloids in solvents of different polarity and for alkaloids bound to DNA. Quantum yields range from 0.098 to 0.345 for the alkanolamine form and are below 0.033 for the iminium form. Rise of fluorescence lifetimes (from 2–5 ns to 3–10 ns) and fluorescence intensity are observed after binding of the iminium form to the DNA for most studied alkaloids. The alkanolamine form does not bind to DNA. Acid-base equilibrium constant of macarpine is determined to be 8.2–8.3. Macarpine is found to have the highest increase of fluorescence upon DNA binding, even under unfavourable pH conditions. This is probably a result of its unique methoxy substitution at C12 a characteristic not shared with other studied alkaloids. Association constant for macarpine-DNA interaction is 700000 M-1.

Modeling pH-zone refining countercurrent chromatography: A dynamic approach

A model based on mass transfer resistances and acid–base equilibriums at the liquid–liquid interface was developed for the pH-zone refining mode when it is used in countercurrent chromatography (CCC). The binary separation of catharanthine and vindoline, two alkaloids used as starting material for the semi-synthesis of chemotherapy drugs, was chosen for the model validation. Toluene/CH3CN/water (4/1/5, v/v/v) was selected as biphasic solvent system. First, hydrodynamics and mass transfer were studied by using chemical tracers. Trypan blue only present in the aqueous phase allowed the determination of the parameters τextra and Pe for hydrodynamic characterization whereas acetone, which partitioned between the two phases, allowed the determination of the transfer parameter k0a. It was shown that mass transfer was improved by increasing both flow rate and rotational speed, which is consistent with the observed mobile phase dispersion. Then, the different transfer parameters of the model (i.e. the local transfer coefficient for the different species involved in the process) were determined by fitting experimental concentration profiles. The model accurately predicted both equilibrium and dynamics factors (i.e. local mass transfer coefficients and acid–base equilibrium constant) variation with the CCC operating conditions (cell number, flow rate, rotational speed and thus stationary phase retention). The initial hypotheses (the acid–base reactions occurs instantaneously at the interface and the process is mainly governed by mass transfer) are thus validated. Finally, the model was used as a tool for catharanthine and vindoline separation prediction in the whole experimental domain that corresponded to a flow rate between 20 and 60mL/min and rotational speeds from 900 and 2100 rotation per minutes.