Solute Mole Fraction Research Articles

Is Passynski's Approach to Hydration Numbers Consistent with Thermodynamics?

Hydrophilic and hydrophobic phenomena occur in aqueous solutions. Despite the complex nature of the molecular interactions, the propensity of molecules and ions to hydration is sometimes characterized by a single "hydration number". Passynski's method for determining the hydration numbers in dilute aqueous solutions belongs to the group of methods based on the analysis of the isentropic compressibility of a mixture. Isentropic compressibility is a thermodynamic material constant; thus, the paper deals with Passynski's approach discussed in terms of thermodynamics. First, Passynski's assumptions were applied to the volume of the mixture. Subsequent strict thermodynamic derivation led to a formula for the hydration number which resembled that of Onori rather than the original one. Passynski's number turned out to be inconsistent with the thermodynamics and mechanics of fluids. This is a rather purely empirical measure of the slope of the dependence of isentropic compressibility on the solute mole fraction in a dilute aqueous solution. Being the quotient of the slope and the isentropic compressibility of pure water, Pasynski's numbers are more convenient to analyze and discuss than the slopes themselves. Conclusions about molecular interactions based on these numbers must be treated with considerable caution.

Protein Cryoprotectant Ability of the Aqueous Zwitterionic Solution.

Protein cryopreservation is important for the long-term storage of unstable proteins. Recently, we found that N-acetylglucosaminyltransferase-V (GnT-V) can be cryopreserved in a deep freezer without temperature control using a dilute binary aqueous solution of 3-(1-(2-(2-methoxyethoxy)ethyl)imidazol-3-io)butane-1-carboxylate (OE2imC3C) [10 wt %, mole fraction of solute (x) = 7.75 × 10-3], an artificial zwitterion. However, it is unclear which solvent properties are required in these media to preserve unstable proteins, such as GnT-V. In this study, we investigated the melting phenomena and solution structure of dilute binary aqueous OE2imC3C solutions [x = 0-2.96 × 10-2 (0-30 wt %)] using differential scanning calorimetry (DSC) and Raman and Fourier transform infrared (FTIR) spectroscopies combined with molecular dynamics (MD) simulation to compare the cryoprotectant ability of OE2imC3C with two general cryoprotectants (CPAs), glycerol and dimethyl sulfoxide. DSC results indicated that aqueous OE2imC3C solutions can be melted at lower temperatures with less energy than the control CPA solution, with increasing x, primarily due to OE2imC3C having a higher content of unfrozen water molecules. Moreover, Raman and FTIR results showed that the high content of unfrozen water molecules in aqueous OE2imC3C solutions was due to the hydration around the ionic parts (the COO- group and imidazolium ring) and the OCH2CH2O segment. In addition, the MD simulation results showed that there were fewer structured water molecules around the OCH2CH2O segment than the hydration water molecules around the ionic parts. These solvent properties suggest that dilute aqueous OE2imC3C solutions are effective in preventing freezing, even in a deep freezer. Therefore, this medium has the potential to act as a novel cryoprotectant for proteins in biotechnology and biomedical fields.

Tuning the low-temperature phase behavior of aqueous ionic liquids.

Water's anomalous behavior is often explained using a two-liquid model, where two types of water, high-density liquid (HDL) and low-density liquid (LDL), can be separated via a liquid-liquid phase transition (LLPT) at low temperature. Mixtures of water and the ionic liquid hydrazinium trifluoroacetate were suggested to also show an LLPT but with the advantage that there is no rapid ice crystallization hampering its observation. It remains controversial whether these solutions exhibit an LLPT or are instead associated with complex phase separation phenomena. We here show detailed low-temperature calorimetry and diffraction experiments on aqueous solutions containing hydrazinium trifluoroacetate and other similar ionic liquids, all at a solute mole fraction of x = 0.175. Hydrazinium trifluoroacetate, ammonium trifluoroacetate, ethylammonium trifluoroacetate and hydrazinium pentafluoropropionate all boast exothermic transitions unrelated to crystallization as well as remarkable structural changes upon cooling into the glassy state. We propose a model inspired by micelle formation and decomposition in surfactant solutions, which is complemented by MD simulations and allows rationalizing the rich phase behavior of our mixtures during cooling. The fundamental aspect of the model is the hydrophobic nature of fluorinated anions that enables aggregation, which is reversed upon cooling and culminates in the remarkable exothermic first-order transition observed at low temperature. That is, we assign the first-order transition not to an LLPT but to phase-separations similar to the ones when falling below the Krafft temperature. All other solutions merely show simple vitrification behavior. Still, they exhibit distinct differences in liquid fragility, which is decreased continuously with decreasing hydrophobicity of the anions. This might enable the systematic tuning of ionic liquids with the goal of designing aqueous solutions of specific fragility.

Diffusion coefficients in binary electrolyte mixtures by dynamic light scattering and molecular dynamics simulations

In this work, three binary electrolyte mixtures represented by different molecular solvents and dissolved lithium bis(trifluoromethylsulfonyl)imide ([Li][NTf2]) are characterized by determining their Fick diffusion coefficient D11. For this, dynamic light scattering (DLS) and molecular dynamics (MD) simulations were applied in the temperature range between (293.15 and 398.15) K at a solute mole fraction x[Li][NTf2] of 0.05. DLS is a contactless measurement technique that requires no calibration and allows to investigate multiple thermophysical properties, including D11, in macroscopic thermodynamic equilibrium. To get reliable thermophysical properties for the binary mixtures from MD simulations, force fields (FFs), describing inter- and intramolecular interactions between atoms, for the pure solvents were taken from the literature and reparametrized. The objective for the reparameterization was to accurately predict the static dielectric constant ε, density ρ, and viscosity η of the solvents. Using the optimized FFs, D11 for the binary electrolyte mixtures could be predicted via MD simulations and the results agree with those from DLS within an average absolute relative deviation of 23%. Most data points agree within combined uncertainties, considering an average expanded statistical uncertainty (coverage factor k = 2) of 18% for D11 from MD simulations and an average expanded experimental uncertainty (k = 2) of 10% for D11 from DLS. η and ε of the solvent were found to correlate with D11 and, thus, the diffusive mass transport in electrolyte mixtures. A large η leads to small D11, due to the slower dynamics in the system. Also a small solvent ε leads to small D11, which is due to the formation of ion pairs or clusters, which hinders the free movement.

Properties of water solutions. The effective molality

Textbooks and research articles referring to properties of water solutions commonly state that Raoult’s law requires the diminution of vapor pressure of solutions of non-volatile solutes to be proportional to the mole fraction of solutes and that most solutions do not obey the law. This is wrong; twice. First: Raoult expressed concentrations in moles of solute per 100 mol of solvent, not in molar fractions; the two quantities are not related linearly. Second: The decrease in vapor pressure is linearly related to the effective molality of solute particles in adherence with the law as originally stated by Raoult and with the same slope for all solutes. In water solvent, molality is currently defined as moles of solute-derived particles in 55.509 mol (1.000 kg) of solvent. If a solute binds strongly with some water molecules, the bound waters become part of the solute and move as one unit with it, as noted by Einstein in 1906. Bound waters are no longer serving as solvent and are not available for binding with any additional solute. Hence, there are fewer than 55.509 mol of solvent and the effective molality of solutes changes accordingly. The importance of this was pointed out in 1912 by Arrhenius, in 1921 by Scatchard and in 1947 by Robinson. Neither they nor anyone else yet provided measures of these changes. The present work does so. Linearity is maintained up to the point where one half of the total water is bound. Past this, additional solute must remove water equally from free and from bound waters and linearity with the effective molality is no longer possible.

SAMs on Gold Derived from Adsorbates Having Phenyl and Cyclohexyl Tail Groups Mixed with Their Phase-Incompatible Fluorinated Analogues.

This article investigates two types of mixed self-assembled monolayers (SAMs) derived from adsorbates having cyclohexyl and phenyl tail groups mixed with their perfluorinated analogues. The mixed SAMs were characterized using ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization-modulation infrared reflection absorption spectroscopy, and contact angle measurements. The XPS results show preferential adsorption of the nonfluorinated adsorbate in the perfluorocyclohexyl-terminated/cyclohexyl-terminated pair due to the steric bulk of the tail groups. On the other hand, mixed surfaces with a precisely controlled surface composition were achieved with the phenyl-terminated/perfluorophenyl-terminated mixed SAMs, exhibiting a linear relationship between the mole fraction on the surface and the mole fraction in solution. The results suggest that the relative solubility, steric bulk of the tail group moiety, and the interaction between two different adsorbates are the key factors driving the phase phenomena observed in the SAMs. More importantly, this study suggests that the interfacial properties can be controlled with a minimal loss of packing densities with the phenyl-terminated/perfluorophenyl-terminated mixed SAMs.

Study on the Phase Equilibria of the Ternary Mixed Solvent System NaCl–NaBr–CH3OH–H2O at 298 and 323 K

Aiming to improve the utilization of bromine resources in bittern, the solubilities of the ternary NaCl–NaBr–CH3OH–H2O mixed solvent system at 298 and 323 K were measured by the method of isothermal dissolution equilibrium, and the corresponding phase diagrams were constructed. The results show that the phase diagrams at the two temperatures are similar, and both have only one univariant curve and one crystallization region corresponding to a solid solution Na(Cl, Br) without invariant points. Based on the extended Pitzer model, Pitzer parameters were calculated, and the activity product of the solid solution was regressed into a polynomial of Br– mole fraction in the solid solution. The solubilities of the ternary NaCl–NaBr–CH3OH–H2O mixed solvent system at 298 K were calculated. The computed results are in accordance with the experimental values.

Viscosity and Interfacial Tension of Binary Mixtures Consisting of an n-Alkane, Branched Alkane, Primary Alcohol, or Branched Alcohol and a Dissolved Gas Using Equilibrium Molecular Dynamics Simulations

This study aims to characterize binary mixtures consisting of a liquid with a dissolved gas by determining their dynamic viscosity and interfacial tension using equilibrium molecular dynamics (EMD) simulations in the temperature range between (298 and 573) K and for solute mole fractions up to 0.20. With the help of a systematic variation of solvent and solute molecules, the influence of their molecular characteristics, e.g., in form of size, shape, or polarity, on the thermophysical properties of the mixtures is discussed. For this, eight different alkanes and alcohols with a carbon number between 12 and 40 as solvents and seven solutes in form of hydrogen, helium, methane, water, nitrogen, carbon monoxide, or carbon dioxide are studied. Using EMD simulations, the liquid dynamic viscosity is determined in the slightly compressed liquid phase close to saturation conditions. Simulations at vapor–liquid-equilibrium (VLE) are performed to determine the interfacial tension and to calculate the solute molecules at the vapor–liquid interface. To check the applicability of the EMD simulations, data for the dynamic viscosity and interfacial tension from this work are compared to experimental data of binary mixtures with the same solutes and similar solvents. The results from this work show that the impact of the dissolved gas on the thermophysical properties is strongly depending on its molecular characteristics. For example, the properties of mixtures containing dissolved He are usually within combined uncertainties with the ones of the pure solvent. In contrast, dissolving CO2 leads to a pronounced reduction in both properties at comparable solute mole fractions. For the molecular characteristics of the solvent, the carbon chain length is shown to influence mainly the interfacial tension and the polarity mainly influences the viscosity.

Viscosity and Interfacial Tension of Binary Mixtures Consisting of Linear, Branched, Cyclic, or Oxygenated Hydrocarbons with Dissolved Gases Using Surface Light Scattering and Equilibrium Molecular Dynamics Simulations

In the present study, the influence of the molecular characteristics of the solvent and solute on the dynamic viscosity and interfacial tension of binary mixtures consisting of a liquid with a dissolved gas is investigated using surface light scattering (SLS) and equilibrium molecular dynamics (EMD) simulations. In detail, binary mixtures consisting of linear, branched, cyclic, or oxygenated hydrocarbons and the solutes hydrogen, helium, methane, water, carbon monoxide, or carbon dioxide are studied in the temperature range between (298 and 573) K and for solute mole fractions up to 0.2. With SLS, the liquid dynamic viscosity and interfacial tension of the binary mixtures could be accessed in macroscopic thermodynamic equilibrium with average expanded uncertainties (coverage factor k = 2) of (2.4 and 2.3)%, respectively. While EMD simulations were able to predict the influence of the dissolved gases on the interfacial tension of the binary mixtures, the simulations fail to represent the influence of the dissolved gas on the viscosity. Due to the systematic variation of the solvent and solute molecules, the influence of the molecular characteristics, e.g., in the form of size, shape, or polarity, on the thermophysical properties of the mixtures is discussed. Dissolving carbon dioxide, e.g., leads to a reduction of both properties by up to 60% compared to the properties of the pure solvent. Dissolved helium, on the other hand, has only a small influence on the properties of the pure solvent. The influence of dissolved water was found to be negligible in mixtures with an alkane but strongly increases both properties when dissolved in an alcohol, which may be explained by the formation of hydrogen bonds.

Absence of the liquid-liquid phase transition in aqueous ionic liquids.

The anomalies of supercooled water may be explained by an underlying liquid-liquid phase transition (LLPT) between high- and low-density states. Recently, its observation at 185 K was inferred using solutions containing aqueous ionic liquids at a solute mole fraction of x=0.156 [Woutersen et al., Science 359, 1127 (2018)10.1126/science.aao7049]. We employ x-ray diffraction, calorimetry, and dilatometry on these hydrazinium trifluoroacetate solutions at x=0.00-0.40 to show that the transition at 185 K is not related to a genuine LLPT of water. Continuous densification upon compression, continuous changes of halo position, and absence of thermal signatures for a high- to low-density transition rule out the possibility of an LLPT for x≥0.13. The data show that employing sophisticated solutions adds a layer of complexity that hampers extrapolation of the LLPT concept from one- to two-component systems. The possibility of an LLPT can only be probed for pure water or sufficiently dilute aqueous solutions.

Measurement of refractive index and viscosity for aqueous solution of sodium acetate, sodium carbonate, trisodium citrate, (glycerol + sodium acetate), (glycerol + sodium carbonate), and (glycerol + trisodium citrate) at T = 293.15 to 303.15 K and atmospheric pressure

The Refractive index and viscosity are important physico-chemical properties of solutions that have many applications in research and industrial works. In this study, we measured the refractive index and viscosity for binary solutions of (water + sodium acetate), (water + sodium carbonate), and (water + trisodium citrate), as well as ternary solutions of (water + sodium acetate + glycerol), (water + sodium carbonate + glycerol), and (water + trisodium citrate + glycerol) in a wide range of mole fractions of solutes at T = (293.15, 298.15 and 303.15) K and atmospheric pressure. These measurements were done in order to study on the interactions between solute and solvent species in our studied solutions at different temperatures. For binary mixtures, the measured refractive indices were correlated with a linear equation. The values of constants of this linear equation, K r ,s, can help us to better understand the power of interaction between solute and solvent species in binary solutions of this study. Also, for ternary solutions of this study, the values of refractive index were calculated using a multiple equation that was suggested by Koohyar in 2018. Jones-Dole equation and Kumar equation were used for fitting viscosities of all solutions at different temperatures. In addition, the values of change of refractive index on mixing, Δ n D , and activation energy for viscous flow, E a(vis) , were calculated using refractive index and viscosity data, respectively. For binary solutions of this study the values of Δ n D were fitted by Redlich-Kister polynomial equation. For ternary solutions of this study the values of Δ n D were fitted by Singe et al., Calvo et al., and also, Nagata and Sakura equations. • The refractive indices of some binary and ternary aqueous solutions were studied. • Components were water, glycerol, sodium acetate, sodium carbonate, sodium citrate. • Two equations were applied to fit obtained refractive index data. • The fitting parameters were used to investigate on interactions between species. • The obtained data of this study can be used in food industries.

Modeling and Simulation of the Absorption of CO2 and NO2 from a Gas Mixture in a Membrane Contactor

The removal of undesirable compounds such as CO2 and NO2 from incineration and natural gas is essential because of their harmful influence on the atmosphere and on the reduction of natural gas heating value. The use of membrane contactor for the capture of the post-combustion NO2 and CO2 had been widely considered in the past decades. In this study, membrane contactor was used for the simultaneous absorption of CO2 and NO2 from a mixture of gas (5% CO2, 300 ppm NO2, balance N2) with aqueous sodium hydroxide solution. For the first time, a mathematical model was established for the simultaneous removal of the two undesired gas solutes (CO2, NO2) from flue gas using membrane contactor. The model considers the reaction rate, and radial and axial diffusion of both compounds. The model was verified and validated with experimental data and found to be in good agreement. The model was used to examine the effect of the flow rate of liquid, gas, and inlet solute mole fraction on the percent removal and molar flux of both impurity species. The results revealed that the effect of the liquid flow rate improves the percent removal of both compounds. A high inlet gas flow rate decreases the percent removal. It was possible to obtain the complete removal of both undesired compounds. The model was confirmed to be a dependable tool for the optimization of such process, and for similar systems.

Thermodynamic Stability Conditions of Clathrate Hydrates in Methane/Carbon Dioxide + Tetrahydrofuran + Cyclopentane + Water Systems: Experimental Measurement and Modeling

In this communication, experimental hydrate dissociation data for methane + tetrahydrofuran (THF) (0.03 and 0.0555 mole fractions in aqueous solution) + cyclopentane (CP) + water systems are reported in the temperature range from 294.8 to 301.3 K and pressure range from 1.91 to 4.95 MPa, which are measured using an isochoric pressure-search method. Other experimental data were collected from the literature. Then a thermodynamic model for four-phase equilibria, hydrate (H)–aqueous liquid (Lw)–organic liquid (La)–vapor/gas (V), systems composed of water, methane (CH4), carbon dioxide (CO2), and/or CP was developed. The thermodynamic model is based on the solid solution theory of van der Waals–Platteeuw (vdW-P) for the hydrate phase and Peng–Robinson equation of state (P-R EOS) for vapor/gas phase. For calculating the activity coefficient of water and THF in the aqueous phase, the universal quasichemical (UNIQUAC) functional-group activity coefficient (UNIFAC) model is also used. The Langmuir constants of TH...

Octahedral distortion due to oxygen vacancy reduction in La3+ modified BNT-BTO solid solutions near morphotropic phase boundary

Currently, there is great interest in perovskite compounds (ABO3) due to their various applications as well as fundamental research that has taken place. However, oxygen vacancy in the perovskite is a very important parameter to control its physical properties. Hence, the polycrystalline ((Bi1−xLax)0.5Na0.5)0.94Ba0.06TiO3 (BLNBTO) with x = 0.00, 0.02, 0.04, 0.06, 0.08 and 0.10 solid solutions was synthesized via a conventional solid-state reaction process to study the oxygen vacancies and its influence on dielectric property. Structural phase formation of the solid solutions has been studied by x-ray diffraction (XRD) technique. Rietveld analysis of the XRD pattern reveals the decrease of octahedral distortion with the increase of La3+ concentration in BNBTO (x = 0.00) solid solution. The reduction of octahedral distortion in the solid solutions was also supported by Raman spectra analysis. The presence of oxygen vacancy has been observed from the analysis of x-ray photoelectron spectroscopy spectra corresponding to the different elements of the solid solutions. Furthermore, the distinct peak at high-temperature range (250 °C–350 °C) in impedance plots is ascribed to the presence of oxygen vacancy related dielectric relaxation phenomenon. The activation energy increases from 0.99 to 1.53 eV with the increase of La3+ mole fraction in BNBTO solid solution, which is obtained from impedance data using the Arrhenius equation. The increase of activation energy has been correlated with the reduction of oxygen vacancy.

Dielectric Relaxation and Thermodynamic Studies of Binary Mixtures of 2-Nitrotoluene with Primary and Secondary Alcohols at Different Temperatures

The dielectric complex spectra of 2-nitrotoluene with primary or secondary alcohol binary mixtures were studied over the frequency range of 10 MHz to 20 GHz for the whole solute mole fraction range at four different temperatures. An unusual suppression phenomenon was observed in the real and imaginary parts of the mixture complex spectrum, which are smaller than those for the pure alcohols, at low solute concentrations. The dielectric constant and dielectric relaxation time values were obtained by fitting the complex dielectric spectrum data to the single Debye model using a non-linear least squares method. The dielectric constant of mixtures decrease with the increasing mole fraction of 2NT in both the primary alcohols and secondary alcohols; the dielectric relaxation time decreases for all the five binary systems. Using the dielectric data, derived dielectric parameters, namely: the excess dielectric constant, excess inverse relaxation time, effective Kirkwood correlation factor, molar activation enthalpy and molar activation entropy, were calculated. The non-linear variation of permittivity (ɛ0) reveals the change in size and shape of hetero-molecular complex due to intermolecular H-bond interaction. The negative variation of the excess permittivity constant confirms that the dipoles form multimer structures with anti-parallel ordering of unlike dipoles. The molar activation enthalpy was found to be higher at 0.2 mol fraction of 2NT for primary alcohol binary system. To confirm the molecular function group interaction, a FT-IR spectroscopy study was carried out at 298 K. The FT-IR analysis confirmed the formation of hydrogen bonds between the hydrogen atom of hydroxyl groups of the alcohols and the oxygen atom of nitro groups of 2NT in the binary mixtures.

Mole fraction measurement of methane and ethane in aqueous solutions of NaCl and MgCl2 during hydrate growth

In this paper, the mole fraction of methane and ethane in aqueous solutions of NaCl and MgCl2 during hydrate growth in a semi-batch stirred tank reactor is reported. Methane mole fraction measurements in aqueous solutions of NaCl were conducted at molalities of 0.529 and 1.092, temperatures of 275.15–290.15 K, and pressures of 40–68 bar. The same conditions were provided to measure the mole fraction in the aqueous solution of MgCl2 at molalities of 0.325 and 0.670 mol kg−1. We also report the mole fraction of ethane in aqueous solution of NaCl at molalities of 1.901 and 3.020 mol kg−1, temperatures of 273.15–278.15 K, and pressures of 11–24 bar; however, molalities of 0.325 and 0.791 mol kg−1, temperatures of 279.15–283.15 K, and pressures of 14–29 bar were investigated for the C2H6-MgCl2 system. Furthermore, the effects of pressure and salt concentrations on the mole fraction of these gases in aqueous phase in the presence of hydrate were investigated. It was observed the mole fraction of hydrate formers in the aqueous phase are close to solubility of this gases at experimental temperature and corresponding equilibrium pressure. In addition, it was found the mole fraction of methane and ethane in bulk liquid phase remain constant during the hydrate growth period, whereby the fugacity of the aqueous phase was constant throughout this period.

Raoult's law revisited: accurately predicting equilibrium relative humidity points for humidity control experiments.

The humidity surrounding a sample is an important variable in scientific experiments. Biological samples in particular require not just a humid atmosphere but often a relative humidity (RH) that is in equilibrium with a stabilizing solution required to maintain the sample in the same state during measurements. The controlled dehydration of macromolecular crystals can lead to significant increases in crystal order, leading to higher diffraction quality. Devices that can accurately control the humidity surrounding crystals while monitoring diffraction have led to this technique being increasingly adopted, as the experiments become easier and more reproducible. Matching the RH to the mother liquor is the first step in allowing the stable mounting of a crystal. In previous work [Wheeler, Russi, Bowler & Bowler (2012). Acta Cryst. F68, 111-114], the equilibrium RHs were measured for a range of concentrations of the most commonly used precipitants in macromolecular crystallography and it was shown how these related to Raoult's law for the equilibrium vapour pressure of water above a solution. However, a discrepancy between the measured values and those predicted by theory could not be explained. Here, a more precise humidity control device has been used to determine equilibrium RH points. The new results are in agreement with Raoult's law. A simple argument in statistical mechanics is also presented, demonstrating that the equilibrium vapour pressure of a solvent is proportional to its mole fraction in an ideal solution: Raoult's law. The same argument can be extended to the case where the solvent and solute molecules are of different sizes, as is the case with polymers. The results provide a framework for the correct maintenance of the RH surrounding a sample.

A molecular study of the wastewater contaminants atenolol and atrazine in 1-n-butyl-3-methylimidazolium based ionic liquids for potential treatment applications

ABSTRACTThe use of room temperature ionic liquids (ILs) as novel solvents in a range of industrial separation processes has received tremendous attention and research. Here, we investigate the potential application of ILs to remove the wastewater contaminants atenolol and atrazine. Specifically, using molecular simulation we predict IL/water partition coefficients of atenolol and atrazine in seven common 1-n-butyl-3-methylimidazolium based ILs and investigate the underlying structural details. We find that atenolol and atrazine have a large affinity for the ILs over water. With a solute mole fraction of 0.1, the IL characteristic structure is virtually unchanged, suggesting ILs are able to accommodate the solutes well. Additionally, we demonstrate that within an IL, the solute affinity for the IL can be greatly improved by selecting an anion and/or cation that interacts strongly with the solute.

Determination of melting temperatures in hydrocarbon mixtures by differential scanning calorimetry

There is a lack of consistency in the literature about how to determine the melting (liquidus) temperature in a hydrocarbon mixture from thermograms recorded by differential scanning calorimetry (DSC). This paper establishes a robust technique for determining liquidus temperatures by DSC by testing two methods detailed in the literature and assessing the potential for de-mixing to preclude repeatable measurements. Liquidus temperatures determined via the end set scanning method were found to be consistent with literature measurements of the same mixture obtained visually, and with a liquidus temperature measured for a fresh sample using the step method. In contrast, use of the thermogram’s peak temperature produced inconsistent results that often could not be reasonably extrapolated to zero scan rate. The impact of any sample de-mixing that may have occurred over multiple freeze-melt cycles was negligible, as demonstrated by the consistency of the thermograms repeated at the same scan rate, and the consistency of liquidus temperatures obtained with different sample loadings into the DSC. New (solid+liquid) equilibrium results are reported for {heptane+hexadecane (C16)} and (hexane+hexadecane) binaries as well as a (hexane+para-xylene+hexadecane) ternary over a temperature range from (260.80 to 279.17)K at atmospheric pressure. Comparisons of the binary measurements against both literature data and the calculations with a property package implemented in commercial software showed deviations of less than 1K for mixtures with C16 solute mole fractions around 0.3, and −3K for the mixture with a C16 solute mole fraction around 0.1, due to the increasing sensitivity of the liquidus temperature on composition as the solute fraction decreases. The ternary mixture, with a C16 solute mole fraction of around 0.1, showed a deviation of −5K, suggesting the property package does not adequately capture the interactions associated with the presence of an aromatic component.

Solubility, structure, and morphology in the co-precipitation of cadmium and zinc with calcium-oxalate

Calcium-oxalates (Ca-Ox), which are widely produced by microorganisms and plants, are ubiquitous and persistent biominerals in the biosphere. We investigated the potential trapping of two phytotoxic metals, cadmium (Cd) and zinc (Zn) by isomorphous substitution into the crystalline structure of Ca-Ox precipitated over a wide range of Cd2+/Ca2+ or Zn2+/Ca2+ ratio in solution. We employed atomic absorption spectroscopy, X-ray diffraction (XRD), and optical microscopy to evaluate our hypotheses that favorable solid-solution conditions and structural framework of crystal habits promote selective metal trapping within Ca-Ox precipitates. Chemical analysis demonstrated more effective Cd-Ox/Ca-Ox than Zn-Ox/Ca-Ox co-precipitate formation at the same trace metal mole fraction in solution. The XRD results revealed sequestration of Cd, but not Zn, within Ca-Ox monohydrate (whewellite). Comparative chemical analysis with Cd-Ox formation in the absence of Ca-Ox showed that the whewellite solid-solution formation lowered the solubility of Cd2+ below that of pure Cd-Ox. The XRD patterns indicated that Zn2+ precipitated as a separate pure Zn-Ox crystal that is largely excluded from the Ca-Ox structure. Furthermore, the presence of Zn2+ in solution favored the formation of the less stable Ca-Ox dihydrate (weddellite) over whewellite. In agreement with the XRD data, visualization of the co-precipitates by optical microscopy illustrated combined mineral phases of Cd-Ox with Ca-Ox whereas Zn-Ox and Ca-Ox exhibited two distinct mineral morphologies. Our findings shed light into the structural factors that are most critical in facilitating the trapping of toxic trace metals within Ca-Ox crystals.