Excess Volume Of Mixing Research Articles

A revised model for activity–composition relations in solid and molten FePt alloys and a preliminary model for characterization of oxygen fugacity in high-pressure experiments

Abstract. We present new models for the activity of iron (γFe) in solid face-centered cubic (fcc) and liquid FePt alloy at high temperature and pressure to facilitate their use as sliding buffer redox sensors under extreme conditions. Numerous experimental studies of γFe in FePt alloy at 100 kPa have produced a wide spread of values. By favoring high-temperature studies that are more likely to have produced equilibrium measurement and excluding experiments for compositions and temperatures that probably encountered ordered or unmixed low-temperature phases, we regress an asymmetric Margules activity–composition model with parameters WFePtfcc=-121.5±2.1 kJ mol−1 and WPtFefcc=-93.3±4.3 kJ mol−1. These values are close to the widely used model of Kessel et al. (2001), but for Pt-rich compositions they predict larger Fe activities and correspondingly more reduced oxygen fugacities. Activity–composition relations in liquid FePt are calibrated from direct measurements of activities and, most sensitively, from the trace of the Fe–Pt liquidus. Together, these yield asymmetric Margules parameters of WFePtliq=-124.5 kJ mol−1 and WPtFeliq=-94.0 kJ mol−1. The effects of pressure on both fcc and liquid FePt alloy are considered from excess-volume relations. Both solid and liquid alloy display significant positive excess volumes of mixing. Extraction of the excess volume of mixing for fcc FePt alloy requires filtering data for ordered low-temperature phases and corrections for the effects of magnetostriction on Fe-rich compositions which exhibit “Invar” behavior. Applied at high temperatures and pressures, both solid and liquid FePt alloys have strongly negative deviations from ideality at low pressure, which become closer to ideal at high pressure. These models provide a provisional basis for the calculation of aFe in high-temperature, high-pressure experiments that, when combined with estimates of aFeO, allow characterization of fO2 under conditions relevant to magma oceans, core formation, and differentiation processes in the lower mantle of Earth or on other terrestrial planets. Improvements in these models require new constraints on the equation of state of FePt fcc alloy and documentation of the high-pressure melting relations in the system Fe–Pt.

Metastable Phase Formation in Electrodeposited Co-Rich Co-Cu and Co-Ni Alloys

In a previous work [El-Tahawy et al., J. Magn. Magn. Mater. 560, 169660 (2022)], we reported that from a sulfate type bath, hcp-Co can be electrodeposited at high pH and low current density and investigated the structure and magnetoresistance (MR) characteristics of such hcp-Co electrodeposits. Based on this earlier work, Co-rich Co-Cu and Co-Ni alloy electrodeposits were prepared under the same conditions by adding varying amounts of CuSO4 and NiSO4, respectively, to the CoSO4 bath. According to the results of detailed structural studies by various X-ray diffraction (XRD) geometries, in both the Co-Cu and Co-Ni systems an hcp phase formed exclusively up to about 2 at% of the alloying element. Above this concentration, a significant fcc phase fraction appeared in Co-Cu and a minor fcc fraction in Co-Ni up to about 8 at%. This means that the destabilization effect of Cu on hcp-Co is higher than that of Ni. Although the reduction of the stability of hcp-Co with increasing Cu and Ni content is a well-known phenomenon, a quantitative comparison of this effect in Co-Cu and Co-Ni alloys is missing from the literature. The measured lattice constants are analyzed in comparison with Vegard’s law for the Co-Cu and Co-Ni element pairs deduced from data previously reported for the hcp and fcc phases of all three pure elements. For Co-rich Co-Ni alloys, the concentration dependence of the lattice parameters was found to follow Vegard’s law for both the hcp and fcc phases due to the miscibility of the two components. For the Co-rich Co-Cu alloys, the data indicate a positive deviation from Vegard’s law for both the hcp and fcc phases in agreement with the known similar behavior of fcc Co-Cu alloys for the whole composition range. The positive deviation from Vegard’s law in the Co-Cu system is due to the excess mixing volume required for solid solution alloy formation of these immiscible elements in either phases. The MR data are discussed in the light of the observed phases and of the MR parameters reported in our previous work on the hcp and fcc phases of pure Co.

Apparent molar volume anomaly in water-dimethyl sulfoxide liquid mixtures. Molecular dynamics computer simulations

We have studied the composition dependence of density of liquid water-DMSO mixtures at different temperatures by using the isobaric-isothermal (NPT) molecular dynamics computer simulations. The non-polarizable semi-flexible, P1 and P2 models for the DMSO molecule combined with the TIP4P-2005 water model are considered. The excess mixing volume and the apparent molar volumes of the species are reported. We have established that the P1-TIP4P-2005 model for the mixture provides a very good description of the location of the minimum of apparent molar volume for DMSO species indicating the anomaly. Most important is that the temperature interval where the hydrophobic effect exists, is correctly captured with this modelling, in contrast to the P2-TIP4P-2005 model.

Revisiting the composition dependence of the properties of water-dimethyl sulfoxide liquid mixtures. Molecular dynamics computer simulations

We have revisited the composition dependence of principal properties of liquid water-DMSO mixtures by using the isobaric-isothermal molecular dynamics computer simulations. A set of non-polarizable semi-flexible models for the DMSO molecule combined with the TIP4P-2005 and TIP4P/ε water models is considered. We restrict calculations to atmospheric pressure, 0.1013 MPa, and room temperature, 298.15 K. Composition trends of density, excess mixing volume and excess mixing enthalpy, partial molar volumes and partial molar enthalpies of species, apparent molar volumes are reported. Besides, we explore composition trends of the self-diffusion of species, the static dielectric constant and the surface tension. Evolution of the microscopic structure of the mixture with composition is analyzed in terms of radial distributions functions, coordination numbers and the fractions of hydrogen-bonded molecules. We intend to capture the peculiarities of mixing the species in the mixture upon the DMSO molar fraction and the anomalous behaviors, if manifested in each of the properties under study. The quality of several combinations of the models for species is evaluated in detail to establish the possibility of necessary improvements.

The Soret coefficients of the ternary system water/ethanol/triethylene glycol and its corresponding binary mixtures

Thermodiffusion in ternary mixtures is considered prototypic for the Soret effect of truly multicomponent systems. We discuss ground-based measurements of the Soret coefficient along the binary borders of the Gibbs triangle of the highly polar and hydrogen bonding ternary DCMIX3-system water/ethanol/triethylene glycol. All three Soret coefficients decay with increasing concentration, irrespective of the choice of the independent component, and show a characteristic sign change as a function of temperature and/or composition. With the exception of triethylene glycol/ethanol at high temperatures, the minority component always migrates toward the cold side. All three binaries exhibit temperature-independent fixed points of the Soret coefficient. The decay of the Soret coefficient with concentration can be related to negative excess volumes of mixing. The sign changes of the Soret coefficients of the binaries allow to draw far-reaching conclusions about the signs of the Soret coefficients of the corresponding ternary mixtures. In particular, we show that at least one ternary composition must exist, where all three Soret coefficients vanish simultaneously and no steady-state separation is observable.Graphic abstract

A new activity model for Fe\u2013Mg\u2013Al biotites: I\u2014Derivation and calibration of mixing parameters

A new activity model for Fe–Mg–Al biotites is formulated, which extends that of Mg–Al biotites (Dachs and Benisek, Contrib Mineral Petrol 174:76, 2019) to the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system. It has the two composition variables XMg = Mg/(Mg + Fe2+) and octahedral Al, and Fe–Mg and Mg–Al ordering variables resulting in five linearly independent endmembers: annite (Ann, K[Fe]M1[Fe]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, phlogopite (Phl, K[Mg]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered Fe–Mg biotite (Obi, K[Fe]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered eastonite (Eas, K[Al]M1[Mg]2M2[Al]2T1[Si]2T2O10(OH)2, and disordered eastonite (Easd, K[Al1/3Mg2/3]M1[Al1/3Mg2/3]2M2[Al]2T1[Si]2T2O10(OH)2. The methods applied to parameterize the mixing properties of the model were: calorimetry, analysis of existing phase-equilibrium data, line-broadening in powder absorption infrared (IR) spectra, and density functional theory (DFT) calculations. For the calorimetric study, various biotite compositions along the annite–phlogopite, annite–siderophyllite (Sid, K[Al]M1[Fe]2M2[Al]2T1[Si]2T2O10(OH)2), and annite–eastonite joins were synthesized hydrothermally at 700 °C, 4 kbar and logfO2 of around − 20.2, close to the redox conditions of the wüstite–magnetite oxygen buffer at that P–T conditions. The samples were characterised by X-ray powder diffraction (XRPD), energy-dispersive scanning electron microprobe analysis, powder absorption IR spectroscopy, and optical microscopy. The samples were studied further using relaxation calorimetry to measure their heat capacities (Cp) at temperatures from 2 to 300 K. The measured Cp/T was then integrated to get the calorimetric (vibrational) entropies of the samples at 298.15 K. These show linear behaviour when plotted as a function of composition for all three binaries. Excess entropies of mixing are thus zero for the important biotite joins. Excess volumes of mixing are also zero within error for the three binaries Phl-Ann, Ann-Sid, and Ann-Eas. KFMASH biotite, therefore, has excess enthalpies which are independent of pressure and temperature (WGij = WHij). A least-squares procedure was applied in the thermodynamic analysis of published experimental data on the Fe–Mg exchange between biotite and olivine, combined with phase-equilibrium data for phlogopite + quartz stability and experimental data for the Al-saturation level of biotite in the assemblage biotite–sillimanite–sanidine–quartz–H2O to constrain enthalpic mixing parameters and to derive enthalpy of formation values for biotite endmembers. For Fe–Mg mixing in biotite, the most important binary, this gave best-fit asymmetric Margules enthalpy parameters of WHAnnPhl = 14.3 ± 3.4 kJ/mol and WHPhlAnn = −8.8 ± 8.0 kJ/mol (3-cation basis). The resulting asymmetric molar excess Gibbs free energy (Gex) departs only slightly from ideality and is negative at Fe-rich and positive at Mg-rich compositions. Near-ideal activity–composition relationships are thus indicated for the Ann–Phl binary. The presently used low value of − 2 kJ/mol for the enthalpy change of the reaction 2/3 Phl + 1/3 Ann = Obi is generally confirmed by DFT calculations that gave − 2 ± 3 kJ/mol for this ∆HFe–Mg order, indicating that Fe–Mg ordering in biotite is weak. The large enthalpy change of ∆HMg-Al disorder = 34.5 kJ/mol for the disordering of Mg and Al on the M sites in Eas (Dachs and Benisek 2019) is reconfirmed by additional DFT calculations. In combination with WHPhlEas = 10 kJ/mol, which is the preferred value of this study describing mixing along the Phl–Eas join, Mg–Al disordering over the M sites of biotite is predicted to be only significant at high temperatures > 1000 °C. In contrast, it plays no role in metamorphic P–T settings.

The lattice parameter – composition relationship of the body centered cubic uranium-niobium alloys

In-situ time-of-flight neutron diffraction experiments were performed on uranium-niobium alloy with 6 wt% Nb to study the lattice parameter-composition relationship for the body centered cubic (bcc) alloys. Based on lattice-parameter measurements of Nb-supersaturated γs phase over the range of 450 – 790 °C, an improved Vegard's-type relationship was established for determination of Nb concentrations at elevated temperatures. Neutron diffraction data were also collected as a function of aging time over the 450 - 600 °C range when γs phase decomposed isothermally into the mixture of orthorhombic α-U and Nb-rich bcc phase, as well as at room-temperature on the alloys aged ex-situ at 500 °C up to five years. From these measurements, the average Nb concentrations in the bcc phase were determined based on the Rietveld refinements of weight fraction and mass conservation relations (lever rule). Over the 15at% – 78at% range of Nb concentrations that correspond to different experimental aging times, the lattice parameters at constant temperatures exhibit a nonlinear S-shaped variation with Nb concentration, and the associated excess volumes of mixing can be described by a subregular solution model of the Redlich–Kister type of polynomial. Over the full range of composition the S-shaped deviation from Vegard's law can be modeled using a combination of an elastic continuum model and a perturbation to the radii of the solute atoms in the solvent, suggesting that electronic interactions between solute and solvent atoms could play an important role in the compositional dependence of lattice parameter for the γ-phase U-Nb alloys. While Vegard's law is a straightforward and reasonably good approximation for the bcc solid solutions in the U-Nb system, the Nb concentrations determined from the weight fraction refinements of diffraction data provide internally consistent, mass-conserving estimates of solute redistribution for the monotectoid reaction.

Diffusivity of Gas Into Bitumen: Part I—Analysis of Pressure-Decay Data With Swelling

Summary The oil-production rate of in-situ heavy-oil-recovery processes involving the injection of gaseous hydrocarbons partly depends on the diffusivity of the gas in the bitumen. The gas diffusivities required to model these processes are determined indirectly from models of mass-transfer experiments. However, the data in the literature are scattered partly because different methods and model assumptions are used in each case. In this work, the pressure-decay method is examined with a focus on accounting for swelling and the dependence of the diffusivity on the solvent content. To assess these issues, the diffusion of gaseous propane into bitumen is measured at conditions where significant swelling occurs. A numerical model is developed for the pressure-decay experiment that accounts for swelling (including excess volumes of mixing) and variable diffusivity. For gases, such as propane, with a relatively high solubility in bitumen, the error in the calculated diffusivity reached 25% when swelling was not included in the model. The error in the height of the gas/oil interface reached 15%. Nonideal mixing had no effect on the calculated diffusivity and only a small effect on the height of the interface. It was found that the diffusion data from a single experiment could be modeled equally well with a constant or a solvent-concentration-dependent (or viscosity-dependent) diffusivity. However, the apparent constant diffusivities for each experiment were different, confirming their dependence on the solvent content. The constant diffusivity approximately correlated to the viscosity of the oil. A larger data set is required to fully develop and test a correlation, and this work will be presented in Part II of this study (Richardson et al. 2019).

On the Timescale of Magma Ocean Solidification and Its Chemical Consequences: 1. Thermodynamic Database for Liquid at High Pressures

AbstractWe present a new method to construct an internally consistent thermodynamic model using a compilation of high‐pressure melting experiments. The steepest descent method and Monte Carlo sampling are combined to constrain all model parameters simultaneously instead of determining each parameter sequentially from relevant experiments. Our approach is applied to the published melting experiments on mantle materials to obtain the thermodynamic parameters of the MgO‐FeO‐SiO2 ternary system. Inversion with the subsets of experimental data is conducted as well to investigate the source of discrepancy among existing studies, and the key parameters are found to be the thermal expansivity of SiO2 and the excess volume of mixing between MgO and SiO2. Mixing between FeO and SiO2 is only constrained with large uncertainty, which could also imply that oxides with low concentrations have minimal effects on melting. Constraining the thermodynamics of MgO and SiO2 will be important for a better understanding of mantle melting at high pressures.

Molecular dynamics simulation for the test of calibrated OPLS-AA force field for binary liquid mixture of tri-iso-amyl phosphate and n-dodecane.

Tri-isoamyl phosphate (TiAP) has been proposed to be an alternative for tri-butyl phosphate (TBP) in the Plutonium Uranium Extraction (PUREX) process. Recently, we have successfully calibrated and tested all-atom optimized potentials for liquid simulations using Mulliken partial charges for pure TiAP, TBP, and dodecane by performing molecular dynamics (MD) simulation. It is of immense importance to extend this potential for the various molecular properties of TiAP and TiAP/n-dodecane binary mixtures using MD simulation. Earlier, efforts were devoted to find out a suitable force field which can explain both structural and dynamical properties by empirical parameterization. Therefore, the present MD study reports the structural, dynamical, and thermodynamical properties with different mole fractions of TiAP-dodecane mixtures at the entire range of mole fraction of 0-1 employing our calibrated Mulliken embedded optimized potentials for liquid simulation (OPLS) force field. The calculated electric dipole moment of TiAP was seen to be almost unaffected by the TiAP concentration in the dodecane diluent. The calculated liquid densities of the TiAP-dodecane mixture are in good agreement with the experimental data. The mixture densities at different temperatures are also studied which was found to be reduced with temperature as expected. The plot of diffusivities for TiAP and dodecane against mole fraction in the binary mixture intersects at a composition in the range of 25%-30% of TiAP in dodecane, which is very much closer to the TBP/n-dodecane composition used in the PUREX process. The excess volume of mixing was found to be positive for the entire range of mole fraction and the excess enthalpy of mixing was shown to be endothermic for the TBP/n-dodecane mixture as well as TiAP/n-dodecane mixture as reported experimentally. The spatial pair correlation functions are evaluated between TiAP-TiAP and TiAP-dodecane molecules. Further, shear viscosity has been computed by performing the non-equilibrium molecular dynamics employing the periodic perturbation method. The calculated shear viscosity of the binary mixture is found to be in excellent agreement with the experimental values. The use of the newly calibrated OPLS force field embedding Mulliken charges is shown to be equally reliable in predicting the structural and dynamical properties for the mixture without incorporating any arbitrary scaling in the force field or Lennard-Jones parameters. Further, the present MD simulation results demonstrate that the Stokes-Einstein relation breaks down at the molecular level. The present methodology might be adopted to evaluate the liquid state properties of an aqueous-organic biphasic system, which is of great significance in the interfacial science and technology.

Energetics of a Uranothorite (Th1–xUxSiO4) Solid Solution

High-temperature oxide melt solution calorimetric measurements were completed to determine the enthalpies of formation of the uranothorite, (USiO4)x–(ThSiO4)1–x, solid solution. Phase-pure samples with x values of 0, 0.11, 0.21, 0.35, 0.71, and 0.84 were prepared, purified, and characterized by powder X-ray diffraction, electron probe microanalysis, thermogravimetric analysis and differential scanning calorimetry coupled with in situ mass spectrometry, and high-temperature oxide melt solution calorimetry. This work confirms the energetic metastability of coffinite, USiO4, and U-rich intermediate silicate phases with respect to a mixture of binary oxides. However, variations in unit cell parameters and negative excess volumes of mixing, coupled with strongly exothermic enthalpies of mixing in the solid solution, suggest short-range cation ordering that can stabilize intermediate compositions, especially near x = 0.5.

On the composition dependence of thermodynamic, dynamic and dielectric properties of water-dimethyl sulfoxide model mixtures. NPT molecular dynamics simulation results

We have systematically examined available molecular dynamics (MD) simulation results for thermodynamic, dynamic and dielectric properties of water-dimethyl sulfoxide (DMSO) mixtures with the purpose of analyzing their discrepancies with experimental data. To get a more profound insight into the behavior of these mixtures, we have performed constant pressure-constant temperature simulations in the entire composition range at 1bar and 298.15K and compared the obtained results with experimental data and previous findings. The SPC-E water model was used in combination with the P1, P2 and OPLS united atom models for dimethyl sulfoxide. The dependencies of density, of excess mixing volume and potential energy, of the self-diffusion coefficients of species, and of the dielectric constant on the mixture composition are discussed. The evolution of the hydrogen bonding network was analyzed in terms of the average number of hydrogen bonded molecules and of the distribution of bonding states. In conclusions we discuss the capability of the united atom models for DMSO to adequately reproduce an ample set of properties of these mixtures and outline future work.

Excess mixing volume, microstrain, and stability of pyrope-grossular garnets

Synchrotron X-ray diffraction (XRD) was used to measure the unit-cell parameters of synthetic pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12), and four intermediate garnet solid solutions. Garnets synthesized dry in multi-anvil (MA) apparatus at 6 GPa show irregular asymmetric positive quenchable excess volumes, with binary Margules parameters W V grossular = 2.04 ± 0.14 cm3/mol, and W V pyrope = 4.47 ± 0.15 cm3/mol. The two-parameter Margules equation is only an approximate description of the excess volumes in the pyrope-grossular garnets from this study and from the literature. The values for intermediate garnets in this report are roughly a factor of ~3 larger than previous literature reports of excess volumes of garnets grown at piston-cylinder (PC) pressures (2–4 GPa) with hydrothermal assistance. The discrepancy between our large dry excess volumes and the smaller hydrothermally assisted syntheses previously reported in the literature may be due in part to the hydrothermal assistance. When we do damp syntheses, we too get small excess volume. Although dampness produces smaller excess volumes, the mechanism by which this is achieved remains to be discovered. Does dampness relieve microstrains to relax excess volumes? Analysis of synchrotron X-ray diffraction profiles by using Williamson-Hall plots to test for microstrain shows that peak width does change with dampness and with garnet composition. Microstrain in the garnet structure, rather than grain size variation, is the principal reason for the observed XRD peak broadening. Damp garnets at Py60Gr40 have less microstrain and lower excess volume than dry Py60Gr40, in accord with the working hypothesis that dampness is responsible for diminished excess volume through strain relief. However garnets with compositions Py80Gr20 and Py20Gr80, close to the negligibly strained end-members pyrope (Py100) and grossular (Gr100), have large microstrains but relatively small excess volume. This is in contrast to Py40Gr60, which has the largest excess volume but almost no microstrain. (The end-members have no excess volume, by definition, and little microstrain.) This uncorrelated behavior demonstrates that microstrain in general is not directly related to the excess volumes, whatever else dampness may do to a specific composition, for instance introduce a small amount of clinopyroxene into the mode that partitions Ca preferentially away from garnet. Thus the mechanism responsible for the difference between our large excess volumes and the smaller ones in the literature may not be as complete or as simple as dampness relieves microstrain and relaxes excess volume. Our dry intermediate garnets have already been relieved of their microstrain by some other mechanism than dampness and still have large excess volume. The state of Ca-Mg ordering may also change, and we show that this may have a small effect on the excess volume through experiments using variable synthesis time and temperature. A potential test for whether our large, complex excess volumes, of whatever origin, are real (or not) is whether the rise of the solvus with pressure is better described by our large excess volumes or those smaller ones in the literature. We observe garnet phase exsolution at 8 GPa and show it to be reversible, at a temperature consistent with theoretical calculation using the large mixing volume presented in the current study.

On the composition dependence of thermodynamic, dynamic and dielectric properties of water-methanol model mixtures. Molecular dynamics simulation results

We have investigated thermodynamic and dynamic properties as well as the dielectric constant of water-metha\-nol model mixtures in the entire range of composition by using constant pressure molecular dynamics simulations at ambient conditions. The SPC/E and TIP4P/Ew water models are used in combination with the OPLS united atom modelling for methanol. Changes of the average number of hydrogen bonds between particles of different species and of the fractions of differently bonded molecules are put in correspondence with the behavior of excess mixing volume and enthalpy, of self-diffusion coefficients and rotational relaxation times. From the detailed analyses of the results obtained in this work, we conclude that an improvement of the description of an ample set of properties of water-methanol mixtures can possibly be reached, if a more sophisticated, carefully parameterized, e.g., all atom, model for methanol is used. Moreover, exploration of parametrization of the methanol force field, with simultaneous application of different combination rules for methanol-water cross interactions, is required.

Non-empirical phase equilibria in the Cr–Mo system: A combination of first-principles calculations, cluster expansion and Monte Carlo simulations

In this work, the low temperature part of the Cr–Mo phase diagram is calculated by a combination of first-principles calculations, cluster expansion, and Monte Carlo simulations without the use of empirical parameters. In brief, the miscibility gap in the bcc phase is computed through lattice Monte Carlo simulations with the energetics of the Cr–Mo system represented by cluster expansion Hamiltonians, wherein the Hamiltonians are firstly constructed using first-principles calculations of formation energies for supercells of various bulk compositions. It is demonstrated that the onset (critical) temperature of the miscibility gap is preliminarily computed to be Tc = 1720 K, and is significantly decreased to 1150 K once excess vibrational contribution to the free energy is considered. The resulting solid-state miscibility gap of the Cr–Mo binary system, largely improved by the contributions of atomic relaxation and positive excess volume of mixing in the Cr–Mo bcc alloys, also favors the approaches that extrapolate the Gibbs energy of the bcc phase retrieved at high temperatures, as in the CALPHAD method.

Thermophobicity of liquids: heats of transport in mixtures as pure component properties--the case of arbitrary concentration.

We have measured Soret coefficients of a large number of binary mixtures of 23 different organic solvents. The present analysis is based on 77 equimolar mixtures and strongly supports the thermophobicity concept previously developed for the heats of transport of originally 10 different substances [S. Hartmann, G. Wittko, W. Köhler, K. I. Morozov, K. Albers, and G. Sadowski, Phys. Rev. Lett. 109, 065901 (2012)]. Among the investigated compounds, cis-decalin is the most thermophobic, hexane the most thermophilic one. In addition to the equimolar mixtures, we have also analyzed the composition dependence of the Soret coefficients and the heats of transport for 22 selected binary mixtures. Both the interpretation of the heats of transport in equimolar mixtures as pure component thermophobicities and the composition dependence of the Soret coefficient can be understood on the basis of the thermodiffusion theory developed by Morozov [Phys. Rev. E 79, 031204 (2009)], according to which the composition dependence is determined by the excess volume of mixing.

Molecular dynamics simulations of tri-n-butyl-phosphate/n-dodecane mixture: thermophysical properties and molecular structure.

Molecular dynamics simulations of tri-n-butyl-phosphate (TBP)/n-dodecane mixture in the liquid phase have been carried out using two recently developed TBP force field models (J. Phys. Chem. B 2012, 116, 305) in combination with the all-atom optimized potentials for liquid simulations (OPLS-AA) force field model for n-dodecane. Specifically, the electric dipole moment of TBP, mass density of the mixture, and the excess volume of mixing were computed with TBP mole fraction ranging from 0 to 1. It is found that the aforementioned force field models accurately predict the mass density of the mixture in the entire mole fraction range. Commensurate with experimental measurements, the electric dipole moment of the TBP was found to slightly increase with the mole fraction of TBP in the mixture. Also, in accord with experimental data, the excess volume of mixing is positive in the entire mole fraction range, peaking at TBP mole fraction range 0.3-0.5. Finally, a close examination of the spatial pair correlation functions between TBP molecules, and between TBP and n-dodecane molecules, revealed formation of TBP dimers through self-association at close distance, a phenomenon with ample experimental evidence.

Water uptake, ionic conductivity and swelling properties of anion-exchange membrane

Water uptake, ionic conductivity and dimensional change of the anion-exchange membrane made by Tokuyama Corporation (A201 membrane) are investigated at different temperatures and water activities. Specifically, the amount of water taken up by membranes exposed to water vapor and membranes soaked in liquid water is determined. The water uptake of the A201 membrane increases with water content as well as temperature. In addition, water sorption data shows Schroeder's paradox for the AEMs investigated. The swelling properties of the A201 membrane exhibit improved dimensional stability compared with Nafion membrane. Water sorption of the A201 membrane occurs with a substantial negative excess volume of mixing. The threshold value of hydrophilic fraction in the A201 membrane for ionic conductivity is around 0.34, above which, the conductivity begins to rise quickly. This indicates that a change in the connectivity of the hydrophilic domains occurs when hydrophilic fraction approaches 0.34.

Solution Properties of the System ZrSiO4–HfSiO4: A Computational and Experimental Study

ZrSiO4 and HfSiO4 are of considerable interest because of their low thermal expansions, thermal conductivities, and the optical properties of HfSiO4. In addition, silicate phases of both are studied as model radioactive waste disposal materials. Previous first principles calculations reported near ideal mixing in the Zr1–xHfxSiO4 system, with a very weak propensity for phase separation. Density functional theory (DFT)/cluster-expansion first principles calculations presented in this work indicate near ideal mixing with a very weak propensity for ordering. Zr1–xHfxSiO4 samples (x = 0, 0.25, 0.5, 0.75, and 1.0) were synthesized from intimate stoichiometric mixtures of constituent-oxides and annealing at 1823 K for 20 days in a platinum crucible. Samples were characterized by X-ray diffraction (XRD; Rietveld analysis) and 29Si MAS NMR. The XRD data exhibited a pronounced negative deviation from Vegard’s law in the excess volume of mixing, and the 29Si MAS NMR spectra also suggest nonideal mixing. Given the v...

Atomistic computer modeling of the local structure and mixing properties of a grossular-uvarovite solid solution

An original methodology for the atomistic computer modeling of solid solutions was applied for the study of the mixing properties and local structure of the grossular-uvarovite, i.e., Ca3Al2[SiO4]3 Ca3Cr2][SiO4]3, garnet series. The parameters of the interatomic potentials for end members of this series were optimized using experimental data on their structural, elastic, and thermodynamic characteristics. The optimized model of the potentials allowed us to describe the elastic, structural, and thermodynamic characteristics of grossular and uvarovite and estimate the energy of point defects in these crystal structures. Calculations of the mixing properties and local structure for seven different compositions of the solid solution were carried out on a “Chebyshev” supercomputer (Moscow State University) in a 2 × 2 × 4 supercell of the garnet-type structure containing 2560 atoms. Mixing properties, such as the enthalpy of mixing, parameters of interaction, excess mixing volume, deviation of bulk modulus from additivity, and the vibrational and configuration contribution to the entropy of mixing, were determined. This allowed us to estimate the stability field for the grossular-uvarovite solid solution. Histograms of the interatomic distances M-O (M = Ca, Al, Cr, Si) and O-O in supercells were plotted and the parameters of relaxation and changes of the CrO6 and AlO6 octahedron volumes were estimated. The data of the simulation are quite consistent with the experimental data on this system and supplement it significantly.