Solid-solid Transition Temperature Research Articles

Correlation between Structure and Thermal Properties of N-vinyl-3-alkylimidazolium Magnetic Ionic Liquids

Influence of structural variations of N-vinylimidazolium tetrahalogenidoferrate (III) magnetic ionic liquids (MILs) on thermal properties was investigated. Using Gaussian09/B3PW91/6-311G (2d, p) density functional methods, the microstructure of the MILs were analyzed. With differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA), the thermal properties of the MILs were performed. The results showed that the interaction energies of ion pairs decreased, the crystallization temperature increased first and then decreased, and the surfusion first decreased and then increased with the elongation of the alkyl chain length on cation; with the increase of Br atom, the interaction energies of ion pairs increased; the interaction energies of ion pairs increased in the MILs with the same cation or anion, the nature of polarity of MILs increased and the melting point rose; as the cation or anion in MILs had a smaller size, it could have the solidsolid transition temperature. The results indicated that the decomposition temperature with the same type of MILs increased with the interaction energies of ion pairs. The interaction energy of ion pairs can be used to illuminate the correlation between the thermal properties and the structure of MILs. ILs possesses the properties of macromolecular.

Thermal conductivity of heavy, even-carbon number n-alkanes (C22 to C32)

The thermal conductivity of even carbon number n-alkanes from n-dodecane to n-dotriacontane (C22H46 to C32H66) was measured in both the liquid and solid phases, in the temperature range from 297.15 to 353.15 K at 0.1 MPa. To assure the purity and map the solid phase of the different samples during the thermal conductivity measurements of the solid samples, the melting point, solid-solid transition temperature and their respective enthalpies were also evaluated. The thermal conductivity measurements for the liquid and solid samples were obtained within uncertainty levels better than 3% and 5%, respectively. To the best of our knowledge, this is the first report on the thermal conductivity of liquid n-hexacontane, n-octacontane, n-triacontane and n-dotriacontane, and, for solid samples, from n-docosane to n-dotriacontane.

Formation of a solid solution between [N(C2H5)4][BF4] and [N(C2H5)4][PF6] in crystal and plastic crystal phases.

The phase behavior of [N2222][BF4] and [N2222][PF6] (N2222+ = tetraethylammonium cation) binary systems has been investigated in the present study. Differential scanning calorimetry revealed that the crystal-to-plastic-crystal transition temperature decreases upon mixing the two salts, with a minimum at x([N2222][PF6]) = 0.4, where x([N2222][PF6]) denotes the molar fraction of [N2222][PF6]. Powder X-ray diffraction analysis indicated the formation of a solid solution with a rock-salt type structure in the plastic crystal phase at all ratios and the lattice parameter a changes according to Vegard's law. In the crystal phase, two solid solution phases based on the structures of the single salts are observed. Raman spectroscopy confirmed the changes in the solid-solid transition temperature as observed by differential scanning calorimetry. Consequently, in the resulting phase diagram, the solid solution is formed in a wide x([N2222][PF6]) range for both the crystal and plastic crystal phases.

Impurity effects on solid-solid transitions in atomic clusters.

We use the harmonic superposition approach to examine how a single atom substitution affects low-temperature anomalies in the vibrational heat capacity (CV) of model nanoclusters. Each anomaly is linked to competing solidlike "phases", where crossover of the corresponding free energies defines a solid-solid transition temperature (Ts). For selected Lennard-Jones clusters we show that Ts and the corresponding CV peak can be tuned over a wide range by varying the relative atomic size and binding strength of the impurity, but excessive atom-size mismatch can destroy a transition and may produce another. In some tunable cases we find up to two additional CV peaks emerging below Ts, signalling one- or two-step delocalisation of the impurity within the ground-state geometry. Results for Ni74X and Au54X clusters (X = Au, Ag, Al, Cu, Ni, Pd, Pt, Pb), modelled by the many-body Gupta potential, further corroborate the possibility of tuning, engineering, and suppressing finite-system analogues of a solid-solid transition in nanoalloys.

Vibrational and thermodynamic properties of metal clusters with up to 150 atoms calculated by the embedded-atom method

The full vibrational spectrum of Ni${}_{N}$ and Cu${}_{N}$ clusters for clusters with $N$ from 2 to 150 has been determined. The vibrational frequencies show a highly irregular behavior as function of cluster size and geometry. Furthermore, using the obtained results, we determine quantum mechanically the thermodynamics of the clusters using the superposition approximation. The obtained heat capacity shows clear cluster-size effects. In addition, the solid-solid transition temperature for several structural changes in nickel and copper clusters has been calculated.

Phase Equilibria Study of {N-Hexylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide + Aromatic Hydrocarbons or an Alcohol} Binary Systems

Isoquinolinium ionic liquid (IL) has been synthesized from N-hexylisoquinolinium bromide as a substrate. Specific basic characterization of the synthesized compound is included, which includes NMR spectra, elementary analysis, and water content. The basic thermal properties of the pure IL, that is, melting and solid-solid transition temperatures, as well as the enthalpy of fusion, or solid-solid transition have been measured using a differential scanning microcalorimetry technique. The density and viscosity as a function of temperature have been measured for the pure IL at temperatures higher than the melting temperature and were extrapolated to T = 298.15 K. The temperature-composition phase diagrams of 8 binary mixtures composed of the IL N-hexylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide, ([HiQuin][NTf(2)]) and an aromatic hydrocarbon (benzene, or toluene, or ethylbenzene, n-propylbenzene) or an alcohol (1-butanol, or 1-hexanol, or 1-octanol, or 1-decanol) have been determined from ambient temperature to the boiling-point temperature of the solvent at ambient pressure. A dynamic method was used over a broad range of mole fractions and temperatures from 270 to 330 K. For the binary systems, the eutectic diagrams were observed with immiscibility in the liquid phase with an upper critical solution temperature (UCST). In the case of the mixture {IL + benzene, or alkylbenzene} the eutectic systems with mutual immiscibility in the liquid phase with very high UCSTs were observed. These points were not detectable with our method and were observed at low IL mole fraction. For mixtures with alcohols, it was observed that with an increasing chain length of an alcohol, the solubility decreases and the UCST increases. The coexistence curves corresponding to liquid-liquid phase equilibrium boundaries and the solid-liquid phase equilibrium has been correlated using the well-known nonrandom two-liquid (NRTL) model.

A Quantitative Model to Evaluate Solubility Relationship of Polymorphs from Their Thermal Properties

The objective of the study is to develop a model to estimate the solubility ratio of two polymorphic forms based on the calculation of the free energy difference of two forms at any temperature. This model can be used for compounds with low solubility (a few mole percent) in which infinite dilution can be approximated. The model is derived using the melting temperature and heat of fusion for apparent monotropic systems, and the solid-solid transition temperature and heat of transition for apparent enantiotropic systems. A rigorous derivation also requires heat capacity (Cp) measurement of liquid and two solid forms. This model is validated by collecting thermal properties of polymorphs for several drugs using conventional or modulated differential scanning calorimetry. From these properties the solubility ratio of two polymorphs is evaluated using the model and compared with the experimental value at different temperatures. The predicted values using the full model agree well with the experimental ones. For the purpose of easy measurement, working equations without Cp terms are also applied. Ignoring Cp may result in an error of 10% or less, suggesting that the working equation is applicable in practice. Additional error may be generated for the apparent enantiotropic systems due to the inconsistency between the observed solid-solid transition temperature and the true thermodynamic transition temperature. This inconsistency allows the predicted solubility ratios (low melt/high melt) to be smaller. Therefore, a correction factor of 1.1 is recommended to reduce the error when the working equation is used to estimate the solubility ratio of an enantiotropic system. The study of the free energy changes of two crystalline forms of a drug allows for the development of a model that successfully predicts the solubility ratio at any temperature from their thermal properties. This model provides a thermodynamic foundation as to how the free energy difference of two polymorphs is reflected by their equilibrium solubilities. It also provides a quick and practical way of evaluating the relative solubility of two polymorphs from single differential scanning calorimetry runs.

Chain-Length-Dependent Conformational Transformation and Melting Behavior of Alkyl/Oligo(oxyethylene)/Alkyl Triblock Compounds: α-Hexyl-ω-hexyloxyoligo(oxyethylene)s

The chain-length-dependent conformational transformation and melting behavior of triblock compounds α-hexyl-ω-hexyloxyoligo(oxyethylene)s, H(CH 2 ) 6 (OCH 2 CH 2 ) m O(CH 2 ) 6 H (abbreviated as C 6 E m C 6 s) (m = 1-7), have been studied by differential scanning calorimetry and infrared spectroscopy. With an increase in the number of oxyethylene units (m), the molecular form of the triblock compounds in the solid state changes from an all-trans planar form (y form) to a planar/helical/planar triblock form (β form) at m = 5. The melting points of the y form C 6 E m C 6 s are much lower than the melting points of n-alkanes with similar molecular masses. This result is interpreted as due to the higher Gibbs energy of crystal for C 6 E m C 6 s. The observed thermodynamic quantities indicate that the planar structure of the oligo(oxyethylene) chain is stabilized by the force of the magnitude that maintains the rotator phase of n-alkanes. The /3-form C 6 E m C 6 s melt stepwise through a solid-solid transition, at which the melting of only the alkyl blocks of the molecule occurs. The alkyl chains in the β-form C 6 E m C 6 s are loosely packed in the crystal even below the solid-solid transition temperature, while the crystallinity of the oligo(oxyethylene) block is significantly high. The molecular form of the alkyl/oligo(oxyethylene)/alkyl triblock compounds in the solid state is determined by the balance of the intramolecular conformational restoring force in the central oligo(oxyethylene) block and the intermolecular packing force in the end alkyl blocks. The conformational restoring force results from a tendency of the oligo(oxyethylene) chain to resume its intrinsic helical structure.

The temperature-size phase diagram of large SF 6 clusters by computer simulation

We report the full temperature-size phase diagram for closed-shell sulfur hexafluoride (SF 6) clusters by molecular dynamics simulation. A bulk-like phase behaviour is observed in the size range N=19 to the thermodynamic limit. The solid-solid transition temperature is roughly unchanged from the thermodynamic limit to a size of ≈ 1000 molecules and decreases as clusters get smaller. A triple point is observed at T ≈ 60 K for N ≈ 30. The main features of the diagram can be explained in terms of the simple thermodynamic model of transition temperature depression based on surface tension difference between the phases.

A simple differential thermal analysis technique for determining the pressure dependence of solid-liquid and solid-solid transition temperatures

A simple differential thermal analysis technique for determining solid-liquid and solid-solid transition temperatures as functions of pressure is described. The important feature of the technique is the use of a thin-walled ptfe tube both to contain the sample and to transmit pressure to it. The method has been tested at temperatures in the range 200–310 K at pressures of up to 350 MPa. Transition temperatures as functions of pressure are reported for benzene, carbon tetrachloride, and acetonitrile and compared with literature data.

Solid-solid transition temperatures in SrCl 2 from differential thermal analyses to 0.6 GPa

The reversible phase transition between the high-temperature, cubic C1 and the low-temperature, orthorhombic C23 polymorphs of SrCl 2 has been investigated by differential thermal analysis under hydrostatic pressure to 0.63 GPa. The C23→C1 transition temperature varies linearly with pressure at the rate of 0.42 4 μK(Pa) −1 from the highest pressures down to ca. 0.34 GPa, and also linearly with slope 1.73 μK(Pa) −1 at pressures ≲0.26 GPa; the reverse, C1→C23 transition is not observed or deduced ≲0.21 GPa. The observed curvature for the C1–C23 phase boundary over the range ≲0.26–0.34 GPa, 1000–1050 K can be attributed to intersection with the “diffuse” transition in C1; the latter transition, however, could not be observed unambiguously. Linear extrapolation to 0.1 MPa places the C23→C1 transition near 553 K, which implies that C23 - not C1 - is the stable low-temperature polymorph. The recently-investigated transitions in PbF 2 closely parallel these in in SrCl 2.

19F nuclear magnetic resonance and relaxation study of hexafluoride solid solutions (UF 6) x(MoF 6) (1− x) and molecular motion rates near the phase transitions in molecular crystals

Solid solutions of (UF 6) x (MoF 6) 1− x are studied by 19F NMR. Separate signals are observed for the MoF 6 and UF 6 fluorine, in the plastic phase as well as in the low temperature rigid phase. The good homogeneity of the solutions is checked by line-width analysis; measurement and theoretical analysis of cross-relaxation effects are performed in the plastic phase of a (UF 6) 0.08(MoF 6) 0.92 solution. Translational motion is the same as in pure MoF 6, but slightly slower. In the ordered phase, T 1 and T D are measured; a model for describing the molecular motion and relaxation rates in the solution from the pure compound values is given. It is shown that, more generally, for globular compounds, the translational correlation time at the melting point in the plastic phase is about 2 × 10 −7 sec and the reorientational correlation time in the rigid phase at the solid-solid transition temperature (rotational melting) is about 5 × 10 −9 sec, almost independently of the compound or the phase transition temperature. It is also stated that observation of such correlation between transition temperatures and motional rates is not restricted to globular compounds.

Study of crystal transformations by an evolved gas analysis technique

Small organic molecules can be entrapped within inorganic crystalline materials during precipitation. The material has no apparent influence on solid-solid transition temperatures, but some of it is released during the transition.Detection of the escape is reproducible within 1°C and may be used for temperature calibration. More generally, the escape of the organic during chemical reactions could be used for many of the applications of emanation thermal analysis.In coupled systems such as DTA or TG with mass spectrometry, or other systems involving gas transport, the time delay of the system can be measured accurately.