Thermodynamic Solution Model Research Articles

Kinetics of alkaline hydrolysis of organic esters and amides in neutrally‐buffered solution

AbstractReaction rate constants for the hydrolysis of organic esters and amides were determined at temperatures of 100–240°C in aqueous solutions buffered at pH values between 5.5 and 7.3. Experiments are modeled assuming alkaline hydrolysis with a thermodynamic solution model included to account for the temperature dependence of hydroxide ion concentration. In most cases, the ester hydrolysis second order rate constants agree well with published values from experiments in strongly basic solutions at pH values from 11 to 14 and temperatures from 25–80°C, despite the large extrapolations required to compare the data sets. The amide hydrolysis rate constants are about one order of magnitude higher than the extrapolated results from other investigators, but the reaction rate increased proportionally with hydroxide ion concentration, suggesting that an alkaline hydrolysis mechanism is also appropriate. These data establish the validity of the alkaline hydrolysis mechanism and can be used to predict hydrolysis reaction rates in neutrally‐buffered solutions such as many groundwater and geothermal fluids.

Importance of considerations of mixing properties in establishing an internally consistent thermodynamic database: thermochemistry of minerals in the system Mg2SiO4-Fe2SiO4-SiO2

A thermodynamic solution model is developed for minerals whose compositions lie in the two binary systems Mg2SiO4-Fe2SiO4 and Mg2Si2O6-Fe2Si2O6. The formulation makes explicit provision for nonconvergent ordering of Fe2+ and Mg2+ between M1 and M2 sites in orthopyroxenes and non-zero Gibbs energies of reciprocal ordering reactions in both olivine and orthopyroxene. The calibration is consistent with (1) constraints provided by available experimental and natural data on the Fe-Mg exchange reaction between olivine and orthopyroxene ± quartz, (2) site occupancy data on orthopyroxenes including both crystallographic refinements and Mossbauer spectroscopy, (3) enthalpy of solution data on olivines and orthopyroxenes and enthalpy of disordering data on orthopyroxene, (4) available data on the temperature and ordering dependence of the excess volume of orthopyroxene solid solutions, and (5) direct activity-composition determinations of orthopyroxene and olivine solid solutions at elevated temperatures. Our analysis suggests that the entropies of the exchange [Mg(M2)Fe(M1)⇔Fe(M2)Mg(M1)] and reciprocal ordering reactions [Mg(M2)Mg(M1)+ Fe(M2)Fe(M1)⇔Fe(M2)Mg(M1)+Mg(M2)Fe(M1)] cannot differ significantly (± 1 cal/K) from zero over the temperature range of calibration (400°–1300° C). Consideration of the mixing properties of olivine-orthopyroxene solid solutions places tight constraints on the standard state thermodynamic quantities describing Fe-Mg exchange reactions involving olivine, orthopyroxene, pyralspite garnets, aluminate spinels, ferrite spinels and biotite. These constraints are entirely consistent with the standard state properties for the phasesα-quartz,β-quartz, orthoenstatite, clinoenstatite, protoenstatite, fayalite, ferrosilite and forsterite which were deduced by Berman (1988) from an independent analysis of phase equilibria and calorimetric data. In conjunction with these standard state properties, the solution model presented in this paper provides a means of evaluating an internally consistent set of Gibbs energies of mineral solid solutions in the system Mg2SiO4-Fe2SiO4-SiO2 over the temperature range 0–1300° C and pressure interval 0.001–50 kbars. As a consequence of our analysis, we find that the excess Gibbs energies associated with mixing of Fe and Mg in (Fe, Mg)2SiO4 olivines, (Fe, Mg)3Al2Si3O12 garnets, (Fe, Mg)Al2O4 and (Fe, Mg)Fe2O4 spinels, and K(Mg, Fe)3AlSi3O10(OH)2 biotites may be satisfactory described, on a macroscopic basis, with symmetric regular solution type parameters having values of 4.86±0.12 (olivine), 3.85±0.09 (garnet), 1.96±0.13 (spinel), and 3.21±0.29 kcals/gfw (biotite). Applications of the proposed solution model demonstrate the sensitivity of petrologic modeling to activity-composition relations of olivine-orthopyroxene solutions. We explore the consequences of estimating the activity of silica in melts forming in the mantle and we develop a graphical geothermometer/geobarometer for metamorphic assemblages of olivine+orthopyroxene+quartz. Quantitative evaluation of these results suggests that accurate and realistic estimates of silica activity in melts derived from mantle source regions,P-T paths of metamorphism and other intensive variables of petrologic interest await further refinements involving the addition of “trace” elements (Al3+ and Fe3+) to the thermodynamic formulation for orthopyroxenes.

LiX (X=Br, F) salt doping effect in lithium borophosphate glasses

In an attempt to enhance the conductivity of lithium conducting glasses, the “mixed former effect” and the halide salt doping effect have been combined. Thus, borophosphate glasses Li2O−B2O3−P2O5 doped with LiX (X=Br, F) have been characterized by the measurements of Tg and the electrical conductivity. The variation of the electrical conductivity versus salt doping is interpreted with a regular thermodynamic solution model, in which the base glass Li2O−B2O3−P2O5 is the solvent and the salt LiX the solute.

The Pb-Sn-Te phase diagram and its application to the liquid phase epitaxial growth of Pb 1− xSn xTe

The ternary phase diagram of Pb 1− x Sn x Te has been calculated using a modified simple solution thermodynamic model. Interaction parameters between the binary solids and between elements in the liquid have been determined by fitting a polynomial in temperature to experimental liquidus and solidus data. The resulting calculation is in excellent agreement with extensive measurements of the solidus. The principal features of interest are that for x ≲ 0.18, the Sn composition in the solid is approximately the same as that in the liquid and independent of growth temperature. As the Sn content of the liquid increases to 0.30, the solid composition, x, saturates to a value dependent primarily on the growth temperature with the largest values of x corresponding to the highest growth temperature.

Phase Relations and Liquid Structure in the System As ‐ Sb2Se3 ‐ Se

The phase equilibria for the ternary system were determined by sealed system DTA and quench methods. The and ternary sections are true binary systems exhibiting simple eutectic behavior. The general phase relationships are discussed and the liquidus behavior and liquid structure of the selenium rich part of the ternary are treated in detail. A nonregular thermodynamic solution model is used to describe the solution behavior and calculate the size of structural units present in the ternary liquid. A physical interpretation, in terms of these structural units, is given for the observed incipient liquid immiscibility.