Peter R. Tremaine

Deuterium isotope effects on the ionization constant of acetic acid in H2O and D2O by AC conductance from 368 to 548 K at 20 MPa.

Values of the ionization constant of acetic acid in H(2)O and D(2)O (K(HAc) and K(DAc)) and the deuterium isotope effect, ΔpK = pK(DAc) - pK(HAc), have been determined from T = 368 K to T = 548 K at p = 20 MPa, using a flow-through ac conductance cell built at the University of Delaware. Measurements were made on dilute (ionic strength ∼ 10(-4) mol·kg(-1)) solutions of acetic acid, sodium acetate, hydrochloric acid, and sodium chloride in H(2)O and D(2)O, injected in sequence at each temperature and pressure, so that systematic errors in the measured conductance of each solution would cancel. Experimental values for the molar conductivity, Λ, of the strong electrolytes were used to calculate the molar conductivity at infinite dilution, Λ°, using the Fuoss-Hsia-Fernández-Prini (FHFP) equation. These were used to calculate the molar conductivity at infinite dilution for acetic acid which was in turn used to calculate the degree of dissociation and finally the ionization constants of acetic acid. This same procedure was done for the pertinent deuterated solutes in D(2)O. Measured values of log K(HAc), log K(DAc), and ΔpK were obtained to a precision of ±0.008. The present results are in agreement with the only other accurate study at high temperatures and pressures (Mesmer, R. E.; Herting, D. L. J. Solution Chem.1978, 7, 901-913). The deuterium isotope effects, ΔpK, become independent of temperature above ∼420 K, at a value approximately 0.1 unit lower than that at 298 K. These values are ΔpK = 0.43 ± 0.01 and ΔpK = 0.51 ± 0.01, respectively. The temperature dependence of the Walden product ratio, (λ°η)(D(2)O)/(λ°η)(H(2)O), indicates a change in the relative hydration behavior of ions, whereby the effective Stokes radii of the sodium, chloride, and acetate ions in D(2)O relative to H(2)O reverse above ∼423 K. The results also suggest that the greater efficiency of the well-established proton-hopping transport mechanisms for OH(-) and H(3)O(+) at 298 K, relative to OD(-) and D(3)O(+), is significantly reduced as the temperature increases toward 548 K.

Ionization equilibria of acids and bases under hydrothermal conditions

Publisher Summary The properties of acids and bases control much of the aqueous chemistry of geochemical, industrial, and biological systems. The behavior of acids and bases, including the ionization of water itself, under extreme temperature and pressure is less widely known. This chapter aims to present a practical discussion and a compilation of the effects of temperature, pressure, and in some cases, ionic strength on the ionization constants of simple acids and bases, from room temperature to hydrothermal conditions. Raising the temperature and pressure causes the equilibrium of ionization reactions to shift in the direction that favors smaller volumes and greater entropies. Increasing the temperature above about 250oC along the steam saturation pressure curve toward the critical point causes the ionization constants of neutral acids and bases to decrease; increasing the pressure at temperatures above about 250oC causes the ionization constants to increase. The chapter describes the behavior of several classes of inorganic and organic acids and bases.

Raman and ab Initio Investigation of Aqueous Cu(I) Chloride Complexes from 25 to 80 °C

Temperature-dependent Raman studies of aqueous copper(I) chloride complexes have been carried out up to 80 °C, along with supporting ab initio calculations for the species [CuCl(n)(H2O)m](1-n), n = 0-4 and hydration numbers m = 0-6. Normalized reduced isotropic Raman spectra were obtained from perpendicular and parallel polarization measurements, with perchlorate anion, ClO4(-), as an internal standard. Although the Raman spectra were not intense, spectra could be corrected by solvent baseline subtraction, to yield quantitative reduced molar scattering coefficients for the symmetric vibrational bands at 297 ± 3 and 247 ± 3 cm(-1). The intensity variations of these bands with concentration and temperature provided strong evidence that these arise from the species [CuCl2](-) and [CuCl3](2-), respectively. The results from ab initio calculations using density functional theory predict similar relative peak positions and intensities for the totally symmetric Cu-Cl stretching bands of the species [CuCl2(H2O)6](-) and [CuCl3(H2O)6](2-), in which the water is coordinated to the chloride ions. A less intense Raman band at 350 ± 10 cm(-1) is attributed to the symmetric Cu-Cl stretching mode of hydrated species [CuCl(H2O)](0) with six waters of hydration. Temperature- and concentration-independent quantitative Raman molar scattering coefficients (S) are reported for the [CuCl2](-) and [CuCl3](2-)species.

Standard Partial Molar Volumes of Some Aqueous Alkanolamines and Alkoxyamines at Temperatures up to 325 °C : Functional Group Additivity in Polar Organic Solutes under Hydrothermal Conditions

Apparent molar volumes of dilute aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N,N-dimethylethanolamine (DMEA), ethylethanolamine (EAE), 2-diethylethanolamine (2-DEEA), and 3-methoxypropylamine (3-MPA) and their salts were measured at temperatures from 150 to 325 degrees C and pressures as high as 15 MPa. The results were corrected for the ionization and used to obtain the standard partial molar volumes, Vo2. A three-parameter equation of state was used to describe the temperature and pressure dependence of the standard partial molar volumes. The fitting parameters were successfully divided into functional group contributions at all temperatures to obtain the standard partial molar volume contributions. Including literature results for alcohols, carboxylic acids, and hydroxycarboxylic acids yielded the standard partial molar volume contributions of the functional groups >CH-, >CH2, -CH3, -OH, -COOH, -O-, -->N, >NH, -NH2, -COO-Na+, -NH3+Cl-, >NH2+Cl-, and -->NH+Cl- over the range (150 degrees C <or= t <or= 325 degrees C). These allow predictions of the standard partial molar volume of aqueous organic solutes composed of these groups at temperatures up to approximately 310 degrees C and pressures of 10-20 MPa to within a precision of +/-5 cm3 x mol(-1). The model could not be extended to higher temperatures because of uncertainties caused by thermal decomposition. At temperatures above approximately 250 degrees C, the order of the group contributions to Vo2 changes from that observed at 25 degrees C, to become increasingly consistent with the polarity of each functional group. The effect of the dipole moment of each molecule on the contribution to Vo2 from long-range solvent polarization was calculated from the multipole expansion of the Born equation using dipole moments estimated from restricted Hartree-Fock calculations with Gaussian 03 (Gaussian, Inc., Wallingford, CT) and the Onsager reaction-field approximation for solvent effects. Below 325 degrees C, the dipole contribution was found to be less than 2 cm3 x mol(-1) for all the solute molecules studied. At higher temperatures and pressures near steam saturation, the effect is much larger and may explain anomalies in functional group additivity observed in small, very polar solutes.

Non-Complexing Anions for Quantitative Speciation Studies Using Raman Spectroscopy in Fused Silica High-Pressure Optical Cells Under Hydrothermal Conditions.

This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO4−), perchlorate (CIO4−), perrhenate (ReO4−), and trifluoromethanesulfonate, or “triflate” (CF3SO3−) in acidic and neutral solutions at temperatures up to 400 °C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO4− (stable) > ReO4− > CIO4− > CF3SO3− with half-lives (t1/2) > 7 h at 300 °C. In neutral solutions, the order was HSO4− (stable) > CF3SO3− > ReO4− > CIO4−, with t1/2 > 8 h at 350 °C. CF3SO3− was extremely stable in neutral solutions, with t1/2 > 11 h at 400 °C.

Theoretical study of deuterium isotope effects on acid–base equilibria under ambient and hydrothermal conditions

Quantum electronic structure methods are applied for the first time to the study of deuterium isotope effects (DIE) on pKa values under ambient (25 °C, 101.3 kPa) and hydrothermal (250 °C, 20.0 MPa) conditions. This work focuses on sixteen organic acids and explores several methodologies for calculating pKa values and various pKa differences in H2O and D2O under two sets of conditions. Two functionals are considered (B3LYP and BLYP) and solvent effects are accounted for by means of continuum solvation methods (PCM, CPCM, Onsager and SMD). Excellent agreement with experiment is obtained for the calculated DIE (ΔpKa = pKa(D2O) − pKa(H2O)) at the B3LYP-PCM/6-311++G(d,p) level of theory for the two sets of conditions. These values, which are almost constant for a given set of temperature and pressure conditions, are determined by the difference between the Gibbs free energies of formation of the acid and its deuterated form in each solvent. However, accurate predictions under ambient conditions can also be made from zero-point energy differences. The average calculated ΔpKa values under ambient (experimental average: 0.53) and hydrothermal conditions were 0.65 and 0.37, respectively. The mean absolute error between calculated and experimental ΔpKa values under ambient conditions was 0.11. The methodology applied is a very important tool for accurately predicting DIE on pKa values under both ambient and hydrothermal conditions, which can be used to make accurate pKa predictions in D2O.

Formation Constants and Conformational Analysis of Carbamates in Aqueous Solutions of 2-Methylpiperidine and CO2 from 283 to 313 K by NMR Spectroscopy

Quantitative 13C nuclear magnetic resonance (NMR) spectroscopy was used to investigate the speciation in (2-methylpiperidine + H2O + CO2) systems at 283.2-313.2 K. The carbamate of 2-methylpiperidine(2-methylpiperidine- N-carboxylate) was shown for the first time to be a stable species in aqueous solutions. The spectroscopic results were used to obtain temperature-dependant formation constants for the carbamate using a simplified model for the activity coefficients from which the standard molar enthalpy of reaction was estimated. The results were incorporated into a self-consistent chemical equilibrium model, which includes vapor-liquid equilibria and all aqueous species, including the formation of carbamate. The predominant conformation of the sterically hindered carbamate, which was determined using two-dimensional exchange spectroscopy NMR, has the methyl group in the axial orientation and is in agreement with the density functional theory quantum chemical calculations.

Carbamate Formation in the System (2-Methylpiperidine + Carbon Dioxide) by Raman Spectroscopy and X-ray Diffraction

In aqueous solution, 2-methylpiperine (2-MP) has been proposed as a phase-separating amine for carbon-capture applications, whose carbamate is considered to be unstable due to steric hindrance. This paper demonstrates, for the first time, that the carbamate can be synthesized as the salt of the 2-methylpiperidinium cation (2-MPH+) and the 2-methylpiperidine- N-carboxylate anion (2-MPCOO-) by adding carbon dioxide gas to anhydrous liquid 2-MP at CO2/amine ratios α > 0.32 to yield well-defined prismatic crystals. Raman spectra have been measured for anhydrous liquid 2-MP and aqueous solutions of 2-MP and 2-MPH+Cl- at 298 K. The spectra of anhydrous liquid 2-MP, containing dissolved CO2 at lower CO2/amine ratios,. clearly showed the presence of 2-MPH+ and several unidentified bands that were attributed to the carbamate, 2-MPCOO-. Quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory yielded simulated Raman spectra consistent with the experimental spectra. The spectra show the methyl group of both liquid and aqueous 2-MP and that of aqueous 2-MPH+Cl- to be in the equatorial position. The crystal structure shows the same conformations in the solid state and confirms that Raman spectroscopy can be used to determine the conformation of 2-MP species in the liquid state and aqueous solutions.

Apparent Molar Volumes and Standard Partial Molar Volumes of Aqueous Sodium Phosphate Salts at Elevated Temperatures

Densities and apparent molar volumes of aqueous NaH2PO4 (0.1 to 2.1) mol·kg−1, Na2HPO4 (0.1 to 0.5) mol·kg−1 and Na3PO4 (0.4 to 0.6) mol·kg−1 have been determined using a platinum vibrating tube densimeter at temperatures from 473 K to 598 K, 473 K to 570 K, and 373 K to 497 K, respectively, and a pressure of 15 MPa. For the monosodium and disodium phosphate salts, the Pitzer ion-interaction model was used to extrapolate the apparent molar volumes to infinite dilution and to obtain standard partial molar volumes, V°, of NaH2PO4(aq) and Na2HPO4(aq). These new results show that predicted values from the Helgeson−Kirkham−Flowers model overestimate V° for these two species. For the monosodium salt, the difference ranged from 5 cm3·mol−1 at 473 K to 24 cm3·mol−1 at 573 K, and for the disodium salt, the difference was 6 cm3·mol−1 at 473 K and 16 cm3·mol−1 at 570 K. The values for disodium phosphate show evidence of significant contributions from an ion pair, probably NaHPO4−(aq). The apparent molar volumes of N...