Dissociation Constants of Protonated Cysteine Species in NaCl Media

The sulfur-containing biomolecule, cysteine has a role in physiological and natural environment because of its strong interactions with metals. To understand these interactions of metals with cysteine, one needs reliable dissociation constants for the protonated cysteine species [ CH(CH₂SH)COOH; H₃B⁺]. The values of dissociated constants, p , for protonated cysteine species (H₃B⁺ H⁺ + H₂B, K ₁; H₂B H⁺ + HB^–,K ₂; HB^– H⁺ + B^2–,K ₃) were determined from potentiometric measurements in NaCl solutions as a function of ionic strength, 0.5–6.0 mol-(kgH₂O)^–1 and between 5, and 45°C. The equations

Volumetric Properties of Binary Mixtures of Acetonitrile and Chloroalkanes at 25°C and Atmospheric Pressure

Excess molar volumes for binary mixtures of acetonitrile + dichloromethane, acetonitrile + trichloromethane, and acetonitrile + tetracloromethane at 25°C have been used to calculate partial molar volumes , excess partial molar volumes , and apparent molar volumes of each component as a function of composition. The V _m ^Evalues are negative over the entire composition range for the systems studied. The applicability of the Prigogine–Flory–Patterson theory was explored. The agreement between theoretical and experimental results is satisfactory for the systems with dichloromethane and tetrachloromethane. For the unsymmetrical behavior of the system with trichloromethane, however, the agreement is poor.     

Thermodynamic Properties of Aqueous Dimethylamine and Dimethylammonium Chloride at Temperatures from 283 K to 523 K: Apparent Molar Volumes, Heat Capacities, and Temperature Dependence of Ionization

Data for the apparent molar volumes of aqueous dimethylamine and dimethylammonium chloride have been determined with platinum vibrating tube densimeters at temperatures 283.15 K T 523.15 K and at different pressures. Apparent molar heat capacities were measured with a Picker flow microcalorimeter over the temperature range 283.15 K T 343.15 K at 1 bar. At high temperatures and steam saturation pressures, the standard partial molar volumes of dimethylamine and dimethylammonium chloride deviate towards positive and negative discontinuities at the critical temperature and pressure, as is typical for many neutral and ionic species. The revised Helgeson-Kirkham-Flowers (HKF) model and fitting equations based on the appropriate derivatives of solvent density have been used to represent the temperature and pressure dependence of the standard partial molar properties. The standard partial molar heat capacities of dimethylamine ionization , calculated from both models, are consistent with literature data obtained by calorimetric measurements at T 398 K to within experimental error. At temperatures below 523 K, the standard partial molar volumes of dimethylamine ionization agree with those of morpholine to within 12 cm³-mol^-1, suggesting that the ionization of secondary amine groups in each molecule is very similar. The extrapolated value for of dimethylamine above 523 K is very different from the values measured for morpholine at higher temperature. The difference is undoubtedly due to the lower critical temperature and pressure of (CH₃)₂NH(aq).     

Effect of solvent on the partial molal volumes and heat capacities of nonelectrolytes

Group contributions to in seven solvents and to in three solvents have been tabulated. The variation of group parameters is discussed in terms of the solvent compressibility coefficient, _T. The scaled particle theory (SPT) is used to calculate cavity contributions to and C _p2 ^o . Interaction contributions are obtained from the cavity terms and and values estimated through the additivity schemes. values are more sensitive to solute-solvent interactions than in water and less sensitive in methanol. The SPT results for heat capacities support the concept of structural promotion by hydrophobic solutes in water.     

Modeling of Thermodynamic Properties of Amino Acids and Peptides Using Additivity and HKF Theory

Relative densities, , and heat capacity ratios, of aqueous L-histidine, L-phenylalanine, L-tyrosine, L-tryptophan, and L-2,3-dihydroxyphenylalanine (L-dopa) have been measured at 15, 25, 40, and 55°C and 0.1 MPa. Apparent molar volumes, V _2,, apparent molar heat capacities, C_P2,, partial molar volumes at infinite dilution, , and partial molar heat capacities at infinite dilution, , have been calculated from these measurements and compared to available literature values. The partial molar properties at infinite dilution for these systems have been added to those previously obtained for amino acids and peptides in water and the combined set used as input to a novel additivity analysis. The model we develop is based upon the equations of state of Helgeson, Kirkham, and Flowers (HKF) and has been constructed with additive parameters. The model may be used to predict thermodynamic properties of many aqueous biochemicals over an extended temperature range. Group contributions to the parameters in our model and effective Born coefficients are reported for 24 aqueous amino acid and peptide systems. Our results are compared to data previously published in the literature.     

Volumetric, Electric, and Magnetic Properties of Thioxanthen-9-one in Aprotic Solvents as Revealed by High-Precision Densitometry, High-Accuracy Refractometry and Magnetic Susceptibility Measurements and by DFT Calculations

High-precision densitometry measurements of solutions of thioxanten-9-one (TX) in 1,4-dioxane, DMSO, toluene, and benzene have been obtained at 293.15, 303.15, 313.15, 323.15, 333.15, and 343.15K. The partial molar volumes of TX ( ) and the corresponding values at infinite dilution ( ) were determined. The partial molar expansibility ( ) of TX at infinite dilution in each solvent is temperature independent. Dynamic electronic polarizabilities of TX in each aprotic solvent were determined by the Singer–Garito approach. These values are in excellent concordance with the theoretical value for TX of 2.611×10⁻²³cm³ estimated here using DFT/B3LYP/6-311++G(d,p). The partial molar volumes of TX at infinite dilution were calculated and interpreted in terms of the Scale Particle Theory (SPT). The solvent influence on the partial molar volume of TX was found to be due mainly to cavity formation and intermolecular dispersion forces.     

Excess partial molar volumes and thermal expansivities in the water-rich region of aqueous 2-butanone

Excess partial molar volumes of 2-butanone V _m ^E (B) and thermal expansivities _p were measured in the water-rich region of aqueous 2-butanone. The composition derivatives of both quantities showed anomalies at about X _B =0.033 (x _B is the mole fraction of B). showed a step anomaly, while exhibited a peak anomaly. The compositions at which these anomalies occurred match those of the step anomalies observed earlier in and in aqueous 2-butanone. These results are discussed in comparison with those obtained previously for aqueous 2-butoxyethanol.Presented at the Symposium, 76^th CSC Congress, Sherbrooke, Quebec, May 30–June 3, 1993, honoring Professor Donald Patterson on the occasion of his 65^th birthday.     

Modeling Heat Capacities of High Valence-Type Electrolyte Solutions with Pitzer's Equations

Apparent molar heat capacities C _p, for 71 rare earth chlorides, nitrates, and perchlorates, alkaline earth and transition metal chlorides, nitrates, and perchlorates, and alkali metal carbonates and sulfates have been fitted to the Pitzer equation for heat capacities. The apparent molar heat capacities at infinite dilution (equal to the standard partial molar heat capacity, ) were used to evaluate a set of best ionic heat capacities, from which improved values of for the electrolytes were calculated. These were then used in the Pitzer equation to reevaluate the higher Pitzer coefficients. The Pitzer coefficients so evaluated can express, in most cases, the behavior of C _p, within experimental error from infinite dilution to the upper limit of the data. Ionic heat capacities have been correlated with the absolute entropies of the ions by statistically assigning the ionic heat capacities to obtain the best linear fit.     

Comprehensive Investigation of Yttrium and Rare Earth Element Complexation by Carbonate Ions Using ICP–Mass Spectrometry

Carbonate stability constants for yttrium and all rare earth elements have been determined at 25°C and 0.70 molal ionic strength by solvent exchange and inductively coupled plasma–mass spectrometry (ICP–MS). Measured stability constants for the formation of and from M³⁺ are in good agreement with previous direct measurements, which involved the use of radio-chemical techniques and trivalent ions of Y, Ce, Eu, Gd, Tb, and Yb. Direct ICP–MS measurements of and formation constants are also in general agreement with modeled stability constants for the metals La, Pr, Nd, Sm, Dy, Ho, Er, Tm, and Lu, based on linear-free energy relationship (LFER). The experimental procedures developed in this work can be used for assessing the complexation behavior of other geochemically important ligands such as phosphate, sulfate, and fluoride.     

Solubility of Oxygen in Some 1-1, 2-1, 1-2, and 2-2 Electrolytes as a Function of Concentration at 25°C

The solubility of oxygen has been measured in a number of electrolytes [(LiCl, KCl, RbCl, CsCl, NaF, NaBr, NaI, NaNO₃, KBr, KI, KNO₃, CaCl₂, SrCl₂, BaCl₂, Li₂SO₄, K₂SO₄, Mn(NO₃)₃)] as a function of concentration at 25°C. The solubilities, mol (kg-H₂O)^–1, have been fitted to a function of the molality m (standard deviation < 3mol-kg^–1)

Densities and Volumetric Properties of Binary Mixtures of Formamide with 1-Butanol, 2-Butanol, 1,3-Butanediol and 1,4-Butanediol at Temperatures between 293.15 and 318.15 K

The densities of binary mixtures of formamide (FA) with 1-butanol, 2-butanol, 1,3-butanediol, and 1,4-butanediol, including those of the pure liquids, over the entire composition range were measured at temperatures (293.15, 298.15, 303.15, 308.15, 313.15 and 318.15) K and atmospheric pressure. From the experimental data, the excess molar volume, V _m ^E, partial molar volumes, and , at infinite dilution, and excess partial molar volumes, and , at infinite dilution were calculated. The variation of these parameters with composition and temperature of the mixtures are discussed in terms of molecular interactions in these mixtures. The partial molar expansivities, and , at infinite dilution and excess partial molar expansivities, and , at infinite dilution were also calculated. The V _m ^E values were found to be positive for all the mixtures at each temperature studied, except for FA + 1-butanol which exhibits a sigmoid trend wherein V _m ^E values change sign from positive to negative as the concentration of FA in the mixture is increased. The V _m ^E values for these mixtures follow the order: 1-butanol < 2-butanol < 1,3-butanediol < 1,4-butanediol. It is observed that the V _m ^E values depend upon the number and position of hydroxyl groups in these alkanol molecules.     

Effect of charge on the standard partial molar volumes and heat capacities of organic electrolytes in methanol and water

Previously developed additivity schemes for nonelectrolytes have been used to estimate and for tetraalkyl and tetraphenyl methanes in methanol and water. Corrections have been applied to the thermodynamic values of these model compounds to account for a variation in size of the central atom, and these were used to ascertain the effect of charge on and of alkyl and phenyl quaternary ions having N, P and B as central atoms. Investigations of R₄NBr, (R=methyl to heptyl) salts show that the charge effect on and of R₄N⁺ ions is large and relatively independent of ion size suggesting that the solvent molecules penetrate the ions. The ability to estimate and of the quaternary ions in the bromide salt solutions has made it possible to make ionic assignments with some confidence; (Br^–) has been evaluated as 19.7±2 and 30.2±7 cm³-mol^–1 and (Br^–) as –83±7 and –68±30 J-K^–1-mol^–1 in methanol and water, respectively. The use of organic ions for making ionic assignments of and is critically examined and comparisons with other assignments are made. The scaled particle theory is employed to divide the heat capacities of electrolytes into cavity and interaction contributions.     

Aqueous Geochemistry of Rare Earth Elements and Yttrium. XII: Potentiometric Stability Constant Determination of Bis-Tris Complexes with La, Nd, Eu, Gd, Yb, Dy, Er, Lu, and Y

Conditional stability constants of 2-[bis(2-hydroxyethyl)amino]-2(hydroxymethyl)-1,3-propanediol (BT) complexes of trivalent rare earth element (Ln) ions (La, Nd, Eu, Gd, Yb, Dy, Er, Lu) and Y were determined potentiometrically in aqueous NaCl solutions at 30°C and 0.1 M ionic strength. Least-squares fitting shows that, at <0.04 molal BT, the complex LnBT³⁺ is dominant, with LnBT₂ ³⁺ forming a secondary complex, where:

First and second thermodynamic mixing functions of ethylbenzene+n-nonane, +n-decane,and+n-dodecane at 25 and 45°C

Isothermal compressibilities _T and isobaric thermal expansion coefficients _p have been determined for mixtures of ethylbenzene+n-nonane, +n-decane, and +n-dodecane at 25 and 45°C in the whole range of composition. The excess functions and have been obtained at each measured mole fraction. The first one is zero for ethylbenzene +n-nonane, positive for ethylbenzene +n-decane, and +n-dodecane and increases with chain length n of the n-alkane. The function is positive for the three studied systems and nearly constant with n. Both mixing functions increase slightly with temperature. From this measurement and supplementary literature data of molar heat capacities at constant pressure C _P , the isentropic compressibilities _S, the molar heat capacities at constant volume C _V and the corresponding mixing functions have been calculated at 25°C. Furthermore, the pressure dependence of excess enthalpy H ^B , at zero pressure and at 25°C has been obtained from our experimental results of and experimental literature values for excess volume V ^E .     

Partial molar heat capacities and volumes of transfer of nucleic acid bases,nucleosides and nucleotides from water to aqueous solutions of sodium and calcium chloride at 25°C

Partial molar heat capacities and volumes of some nucleic acid bases, nucleosides and nucleotides have been measured in 1m aqueous NaCl and CaCl₂ solutions using Picker flow microcalorimeter and a vibrating tube digital densimeter. The partial molar heat capacities of transfer and volumes of transfer from water to the electrolyte solutions were calculated using earlier data for these compounds in water. The values of these transfer parameters are positive. The higher values for transfer to aqueous CaCl₂ solutions reflect the stronger interactions of the constituents of the nucleic acids with Ca⁺² ions than with the Na⁺ ions.     

Chromatographic study of the thermodynamics of solutions of hydrocarbons in liquid crystal solvents. Evidence for order disturbance by the solutes

Gas-chromatographic experiments were carried out in various phases of the solvents 4-acetoxy-N-[4-methoxy-benzylidene]-aniline, dibutoxyazoxybenzene, lithium stearate, dihexoxyazoxybenzene, and diheptoxyazoxybenzene. The solutes were linear, branched and cyclic alkanes, and substituted benzenes. Excess enthalpies, entropies, and free entropies were calculated from net retention volumes. In the nematic liquid crystalline phases the effect of order disturbance was significant in and but it was, by enthalpy-entropy compensation, not demonstrable in . Differences in flexibility and degree of expansion of the solutes did not result in significantly different values of the excess quantities.     

Thermodynamic data for nonmesomorphic solutes at infinite dilution in the nematic and isotropic phases of hexylcyanobiphenyl

Infinite dilution solute activity coefficients _o ² , partial molar excess enthalpies and entropies , and partial molar enthalpies ( ) and entropies ( ) of solution, obtained using gas-liquid chromatography (GLC), are reported for thirty nonmesomorphic solutes in the nematic and isotropic phases of p-n-hexyl-p-cyanobiphenyl (6CB). The solutes studied include normal and branched alkanes, alkenes and hexadienes (with some cis and trans isomers), and benzene. The results corroborate earlier studies on other members of the p-n-alkyl-p-cyanobiphenyl homologous series of liquid crystals. The results demonstrate the effect that solute structure (size, shape, flexibility, polarizability and polarity) has on the solution process. Thermodynamic data for the cis and trans isomers of 2-pentene and 2-hexene are examined. A method for the simultaneous examination of the effects of both solute and solvent structures on the solution process is suggested.     

Group additivity for the standard partial molal heat capacities and volumes of polar compounds in methanol at 25°C

A flow heat capcity calorimeter and a flow vibrating tube densimeter have been used to measure the apparent molal heat capacities and volumes of benzene and 25 polar compounds in methanol at 25°C. These quantities have been extrapolated to infinite dilution to obtain the standard partial molal heat capacities and volumes. The and data have been used in conjunction with an additivity scheme previously determined for alkanes. Group contributions were evaluatd for –OH, –NH₂, –COOH, –C₆H₅, C=O, –COO–, –CONH–, –O–, –S–, and –S₂–. The concentration dependences of _cp and _v of nonelectrolytes in methanol are qualitatively similar but much smaller than in water.