Equations for the Partition of Neutral Molecules, Ions and Ionic Species from Water to Water–Ethanol Mixtures

Equations set up for the transfer of neutral solutes from water to water?Cethanol mixtures can also be used to correlate the transfer of ions and ionic species, as log₁₀ P, where P is the partition coefficient. Only two additional terms are required in the equations, one for anions and one for cations. The extended equations can fit log₁₀ P values for anions and cations with a standard deviation of about 0.2 to 0.3 log units for transfer of 41 anions and cations from water to various water?Cethanol mixtures from 10?% ethanol to 100?% ethanol by volume. The log₁₀ P values for carboxylate anions and protonated amine cations as obtained from the variation of pK _a with solvent are quite compatible with log₁₀ P values for simple anions and cations.     

Influence of inorganic anions on the tautomeric equilibria of N-substituted aminoazobenzenes — The evidence for ionic association

The influence of inorganic anions: perchlorate, chloride, sulphate, and hydrogen sulphate on the acid-base and tautomeric equilibria of 4-aminoazobenzene (AAB) and its N-ethyl (EAAB), N,N-diethyl (DEAAB), and N-phenyl (PhAAB) derivatives was investigated by UV-spectrophotometry in 80%:20% vol. ethanol:water system. The possible equilibrium models were evaluated by means of the nonlinear confluence analysis program STOICHIO in order to explain the observed dependence of the electronic spectra on acid and salt concentration. It was found that all the anions considered form 1:1 associates with both ammonium and azonium tautomers of the protonated aminoazobenzenes. The corresponding association constants were generally about one order of magnitude greater for SO ₄ ^2– than for the univalent ions. The azonium species were found to form more stable associates in all the cases except SO ₄ ^2– , which explains the observed shift of the tautomeric equilibrium towards the azonium form on increasing the mineral acid or salt concentration. Based on the obtained values of the molar absorption coefficients all but the PhAAB associates were found to be of the ionic pair type, with proton transfer taking place only in the last case. This was due to the greater basicity SO ₄ ^2– , compared with PhAAB. The association constant values obtained are discussed in terms of the tendency of the anions studied to associate, as well as in terms of the effect of solvation on the stability of the ionic pairs formed.     

A Branched Pore Model Analysis for the Adsorption of Acid Dyes on Activated Carbon

A new branched-pore adsorption model has been developed using an external mass transfer coefficient, K _f, an effective diffusivity, D _eff, a lumped micropore diffusion rate parameter, K _b, and the fraction of macropores, f, to describe sorption kinetic data from initial adsorbent-adsorbate contact to the long-term adsorption phase. This model has been applied to an environmental pollution problem—the removal of two dyes, Acid Blue 80 (AB80) and Acid Red 114 (AR114), by sorption on activated carbon. A computer program has been used to generate theoretical concentration-time curves and the four mass transfer kinetic parameters adjusted so that the model achieves a close fit to the experimental data. The best fit values of the parameters have been determined for different initial dye concentrations and carbon masses. Since the model is specifically applicable to fixed constant values of these four parameters, a further and key application of this project is to see if single constant values of these parameters can be used to describe all the experimental concentration-time decay curves for one dye-carbon system.The error analysis and best fit approach to modeling the decay curves for both dye systems show that the correlation between experimental and theoretical data is good for the fixed values of the four fitted parameters. A significantly better fit of the model predictions is obtained when K _f, K _b and f are maintained constant but D _eff is varied. This indicates that the surface diffusivity may vary as a function of surface coverage.     

Reevaluation of electronic polarization energies in organic molecular crystals

Effective electronic polarization energies for positive P⁺_eff and negative P⁻_eff charge carriers in polyacene crystals have been reevaluated. By comparing the P⁺_eff and P⁻_eff values obtained from the analyses of reported energy parameters with calculated data, it is shown that the widely accepted assumption that P⁺_eff = P⁻_eff in polyacene crystals is not valid. By applying recently reported data on optical E^opt_G and adiabatic E^ad_G energy gap values, the contribution of molecular (vibronic) polarization in the total effective polarization energies W⁺_eff and W⁻_eff has been estimated, and modified energy diagrams for polyacene crystals have been presented. Further, due to practically constant values of observed and calculated P⁺_eff and P⁻_eff in polyacenes, the gap energies between positive and negative charge carrier conduction levels have been estimated for several related aromatic hydrocarbons.     

Calculations of pKa values of carboxylic acids in aqueous solution using density functional theory

With the specific aim of calculating the acidity equilibrium constant (K_a) of carboxylic acids in aqueous solution we investigated the solute-solvent interactions of these acids and their corresponding anions. The pK_a (−lg K_a) values have been calculated using density functional theory (DFT). The polarized continuum model (PCM) is used to describe the solvent. Using these methods, we successfully predicted the pK_as of 66 carboxylic acids in aqueous with the average error of 0.5 in pK_a units. Two different thermodynamic cycles have been studied. The theoretical values are in better agreement with the experimental results for those acids with moderate strength of acidity with the pK_a value higher than 3.     

Avoided-crossing molecular-beam spectroscopy of CH3SiF3

The avoided-crossing molecular-beam method has been applied to CH₃SiF₃ in the ground torsional state. Stark and hyperfine rotational anticrossings have been studied, along with barrier anticrossings in which the zero-field energy differences depend only on the torsionial splittings. For υ = 0, pure rotational spectra were measured forJ = 13 ← 12 and 14 ← 13 with a mm-wave spectrometer and for J = 1 ← 0 with the molecular-beam spectrometer. Stark and Zeeman studies have been carried out with conventional beam techniques. From a simultaneous analysis of existing microwave data for υ ? 2 and the current measurements, it was found that the moment of inertia of the methyl top I_α = 3.170(2) amu A², the effective rotational constants A^eff = 4059.522(22) Mhz, and the effective height of the barrier V₃^eff = 413.979(14) cm^?1. The true values of A and V₃ have been obtained within certain approximations. The rotational constant B and several distortion constants including D_K were evaluated. In addition to determining the electric dipole moment μ 2.33938(14) D, the data yielded values for the distortion dipole constants μ_D and μ_J, and the molecular g-factors g_| and g_⊥.     

Microcalorimetric study of the binding of methotrexate and its metabolites to thymidylate synthase

Direct microcalorimetric measurements allow determination of both the δH and association constant of biological complexes ifK _a value does not exceed 10⁶ M ^?1. For higherK _a values, δH can obviously be determined; this paper describes an original microcalorimetric method that permits determination of such high association constants. This method is based on the analysis of the competitive effect between two ligands having the same binding site in their receptor. As an example, the affinity constant for thymidylate synthase of a novel antifolate, CB 3717. was found to be 1.4 · 10⁷ M ^?1 using methotrexate polyglutamate MTX-G2 (K _a=2.3·10⁵ M ^?1) as competitor.     

Autoprotolysis constants and standardization of the glass electrode in acetonitrile-water mixtures. Effect of solvent composition

Standard e.m.f.s for the cell GE/HCl/AgCl/Ag/Pt (GE=glass electrode) in acetonitrile-water mixtures containing 0–70% (w/w) of acetonitrile were obtained. Values of the autoprotolysis constant, K_ap, of these mixed solvents were also determined from e.m.f. measurements of the cell GE/KCl + KS/AgCl/Ag/Pt. The influence of variations in the solvent composition on pK_ap values was evaluated. Over the whole of the composition range studied the pK_ap values were linearly correlated with the mole fraction of acetonitrile and with the reciprocal of the relative permittivity of solvent mixtures. Linear relationships were also obtained for pK_ap values vs. the Kamlet-Taft π^★ polarizability/dipolarity parameter in the range 0–50% (w/w).     

An electrochemical investigation of the redox properties of lumiflavin in acidic media

The electrochemistry of lumiflavin in acidic media (1% to 40% sulfuric acid) has been investigated using cyclic voltammetry and bulk electrolysis in conjunction with UV—Vis absorption spectroscopy. The observed stabilization of flavin radicals in strongly acidic media has been explained on the basis of differences in medium effects on fully reduced flavin cations (FH⁺₄) as compared with protonated oxidized (FH⁺₂) and radical (FH^∓+₃) species. The equilibrium constant for the comproportionation reaction between protonated lumiflavin species (FH⁺₂ + FH⁺₄ → 2 FH^∓+₃) has been determined. Acid dissociation constants for FH⁺₂ and FH⁺₄ have also been evaluated. These data are used in conjunction with the pK_a reported for FH^∓+₃ to estimate the value of the comproportionation constant for the neutral lumiflavin species.     

Studies on the effect of ethanol on the solvation behaviour of thiamine hydrochloride in water

The specific conductance of Thiamine hydrochloride in water and water-ethanol mixtures was measured at different temperatures 283, 293, 303 and 313 K. Kraus-Bray and Shedlovsky models of conductivity were used for analyzing conductance data. The limiting molar conductance, association constant K _a, energy of activation of the rate process E _a, and related thermodynamic parameters have been determined. Based on viscosity of the solvent, Walden product and Stokes molecular radius were also been determined. Born model of ion solvation was proved. Standard thermodynamic parameters of association (G _a, H _a and S _a) were calculated and discussed. The results show the decrease in limiting molar conductance and increase in K _a with decrease in relative permittivity. Such computed values have been used to discuss the solvation behaviour of Thiamine hydrochloride in water-ethanol mixtures.     

On the influence of ionic strength on the equilibrium constant of an ion-molecule reaction

Concentration formation constants for the 18-crown-6-sodium ion complex in anhydrous methanol solutions were measured as a function of the ionic strength of the solution. The K_c values remained reasonably constant for I_c ? 0.05 mol dm^?3 At higher ionic strengths the K_c values begin to decrease. The infinite-dilution formation constant was 2.2 × 10⁴.     

Thermodynamic study of ion exchange reaction to predict ionic selectivity using ion exchange resin Indion-850

The thermodynamic study was carried out to predict the selectivity behaviour of ion exchange resin Indion-850 towaed various inorganic anions like chloride, iodide, sulfate, and oxalate. The equilibrium constant K _std values for the ion exchange reactions were calculated at different temperatures from which the enthalpy values where obtained. The K _std values were observed to increase with the rise in temperature, indicating endothermic ion exchange reactions. The thermodynamic data obtained here reveal that iodide ions were more strongly retained on the resin surface compared to chloride ions. Also the low enthalpy and high equilibrium constant values for oxalate ions indicate the greater selectivity of ion exchange resin for oxalate ions as compared to sulfate ions.     

Solvent effect of ionic liquids on the distribution constant of 2-thenoyltrifluoroacetone and its nickel(II) and copper(II) chelates and the evaluation of the solvent properties based on the regular solution theory

Distribution constants of 2-thenoyltrifluoroacetone (Htta) and its Ni(II)and Cu(II) chelates between 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C_nmimTf₂N; n = 4, 6, and 8) as ionic liquid (IL) and an aqueous phase were determined. The enol fraction of Htta in ILs was spectrophotometrically measured to calculate the distribution constant of the enol form (K_D,HE) of Htta. The K_D,HE values in ILs were evaluated by comparing those in various molecular solvents such as alkanes, aromatic hydrocarbons, chlorohydrocarbons, ethers, ketones, and esters previously reported on the basis of the regular solution theory (RST). It was elucidated that the IL solutions of Htta (enol) can be taken as apparently regular solutions as expected in the organic solvents. On the other hand, the effect of ILs on the distribution constant of metal(II) chelates (K_D,M) was fairly complicated. The Cu(tta)₂-IL solutions behaved like the alkane and aromatic hydrocarbon solutions but the Ni(tta)₂-IL (C₄mimTf₂N) like ether and ketone solutions. In the Ni(II) case, some specific interactions between the Ni(II) chelate and IL was suggested. Finally, the solubility parameters of ILs were calculated using K_D,HE by RST and were in good agreement with the literature values obtained by the enthalpy of vaporization.     

Comparison of retention behavior of oligolysine and oligoarginine in ion-pairing chromatography using heptafluorobutyric acid

This paper describes the retention behavior of oligolysine and oligoarginine peptides of different lengths as a function of heptafluorobutyric acid (HFBA) concentration in ion-pairing reversed-phase chromatography in isocratic elution. A mixture of oligolysine and a mixture of oligoarginine with number of amino acid residues (dp) from two to eight were conveniently prepared by one-pot protease-catalyzed synthesis. Analysis of the logarithm of the retention factor k as a function of [HFBA] for each oligopeptide component, using a closed pairing model, provided values for (1) number (n) of paired HFBA anions per peptide molecule, (2) equilibrium constant (K _ip,m) for ion pairing between oligopeptides and HFBA anions, and (3) product of the phase ratio and the distribution constant of the paired oligopeptide between the mobile and stationary phases (βK _d,ip). We found that βK _d,ip of oligoarginine is larger compared with oligolysine having the same dp. A linear relationship was obtained for ln βK _d,ip as a function of n?+?g?·?dp. By optimizing constant g separately for oligolysine and oligoarginine, we determined that g is larger for oligoarginine, in agreement with the higher hydrophobicity of arginine residues. Plotting the fraction of paired oligoarginine and oligolysine as a function of [HFBA] shows that the cooperative effect in forming ion pairs is greater for oligoarginine than oligolysine.

Recognition of amino acids and anions by a Zn(Ⅱ)-methylazacalix[4]pyridine complex

As a powerful macrocyclic host molecule with unique conformation and cavity structure that are fine-tuned by the bridging nitrogen atoms, methylazacalix[4]pyridine (MACP-4) has been shown to selectively recognize Zn2+ and form stable Zn(Ⅱ)-MACP-4 complexes both in solid state and solution with an association constant up to 5.97 (logKs). The molecular recognition of Zn(Ⅱ)-MACP-4 complexes towards various amino acids and anions with different geometry was investigated by using the spectral titration methods a...     

Acid-base chemistry of omeprazole in aqueous solutions

Omeprazole is a potent anti-acid drug. Its absorption and mode of action are closely related to its prototropic behavior. In the present study, omeprazole samples from different sources and in different forms were studied spectrophotometrically to obtain pK_a values. In the neutral to alkaline pH region, two consistent pK_a values of 7.1 and 14.7 were obtained from various samples. The assignment of these pK_a values was realized by comparison with the prototropic properties of N(1)-methylated omeprazole substituted on the nitrogen at the 1-position of the benzimidazole ring, which was found to have a pK_a of 7.5. The omeprazole pK_a of 14.7 is assigned to the dissociation of the hydrogen from the 1-position of the benzimidazole ring and the pK_a of 7.1 is assigned to the dissociation from the protonated pyridine nitrogen of omeprazole. The results presented are at variance with those of earlier work.     

Chemical standards in ion mobility spectrometry

Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37-250 °C with (H₂O)_nH⁺ as the reactant ion. All three compounds yield a protonated molecule but only 2,4-dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K₀) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K₀ for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K₀ values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized.     

Cyclodextrin assisted nanophase determination of alkaloid salts

The poor water solubility of the free base and the high dissociation constant (K_a) hinder mainly the assay of alkaloid salts. We have elaborated an enviroment friendly method that can be carried out in aqueous media. The stability difference of the cyclodextrin (CD) complexes of free and protonated bases were used for this purpose. The base is included into the hydrophobic cavity of the CD (which serves as an apolar solvent phase on molecular level) and its solubility in water is increased. Since the base forms more stable inclusion complex than its protonated species, the pK_a is decreased and the potentiometric titration is promoted by this way, too. Six different hydrohalide alkaloid salts have been investigated and the most appropriate CDs were chosen (depending on the size of the molecules and/or substituents). The results of the assays agree well with those obtained by the direct nonaqueous titrations. The stability constants of the inclusion complexes have been also computed.     

Sample temperatures during outdoor and laboratory weathering exposures

Effective exposure temperatures (T_eff) in Arizona were calculated from hourly or 10-min parsed irradiation data along with ambient, black panel, and sample temperatures. The T_eff represents a constant temperature that creates the same amount of photodegradation as the naturally varying temperature and provides a benchmark temperature for making lifetime predictions from accelerated laboratory exposures. The annual ambient and black panel T_eff at a Wittmann, Arizona site were 30 °C and 42 °C, respectively, assuming that the photodegradation has an activation energy (E_a) of 21 kJ/mol (5 kcal/mol). T_eff was only weakly dependent on E_a over the range of 10-40 kJ/mol (3-10 kcal/mol). Samples exposed as van sunroofs were found to have T_eff that were offset from the black panel temperatures by a constant amount for the entire year. Thus, measurements of sample and black panel need to be made for only a few weeks to determine the offset and give the annual sample T_eff if the annual black panel T_eff is known. Light-colored samples probably are better compared with the ambient temperatures. Sample temperatures in xenon arc exposures usually are higher than the outdoor T_eff, so Arrhenius temperature corrections need to be carried out to relate accelerated to outdoor exposures. Temperatures in xenon arc exposure tests often correspond more closely to maximum temperatures that samples might encounter for only a few hours per year.