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The α-relaxation dynamics of 1-cyano-adamantane (CNA) and its mixtures with 1-chloro-adamantane (ClA) has been studied by means of broadband dielectric spectroscopy. The existence of orientationally disordered (OD) face centered cubic mixed crystals (ClA1-XCNAX) for 0.5≤X≤1 has been put in evidence by thermodynamics and structural analyses. In addition to the OD phase of CNA, mixed crystals with compositions higher than the equimolar one exhibit a freezing of the orientational degrees of freedom into a glassy state, which involves also a strong increase of the antiferroelectric order at temperatures higher than the dielectric glass transition temperature. This experimental evidence is revealed by a stairlike effect in the variation of the Kirkwood factor with the temperature as a consequence of a twin effect in the dielectric strength without any anomaly in the temperature-density curves. The characteristic relaxation times are analyzed as a function of temperature and mole fraction. By setting a common temporal origin (“isochronal origin”) at τ(Tg)=100 s for each mole fraction, it emerges that the substitution of ClA molecules by those of CNA (diminution of X) gives rise to a slow down in the dynamics, despite that the molecular volume of ClA molecules are smaller than those of CNA. This fact goes along and is accompanied by a diminution of the lattice packing with the decrease of composition. It is also shown that the heterogeneities produced by the concentration fluctuations due to the chemical disorder are the main contribution to the non-exponential character of the α-relaxation peaks. [ABSTRACT FROM AUTHOR]

Understanding the phase behavior of active pharmaceutical ingredients is important for formulations of dosage forms and regulatory reasons. Nimesulide is an anti-inflammatory drug that is known to exhibit dimorphism; however up to now its stability behavior was not clear, as few thermodynamic data were available. Therefore, calorimetric melting data have been obtained, which were found to be T I-L = 422.4 ± 1.0 K, Δ I → L H = 117.5 ± 5.2 J g −1 , T II-L = 419.8 ± 1.0 K and Δ II → L H = 108.6 ± 3.3 J g −1 . In addition, vapor-pressure data, high-pressure melting data, and specific volumes have been obtained. It is demonstrated that form II is intrinsically monotropic in relation to form I and the latter would thus be the best polymorph to use for drug formulations. This result has been obtained by experimental means, involving high-pressure measurements. Furthermore, it has been shown that with very limited experimental and statistical data, the same conclusion can be obtained, demonstrating that in first instance topological pressure-temperature phase diagrams can be obtained without necessarily measuring any high-pressure data. It provides a quick method to verify the phase behavior of the known phases of an active pharmaceutical ingredient under different pressure and temperature conditions. [ABSTRACT FROM AUTHOR]

We use calorimetry and dilatometry under hydrostatic pressure, X-ray powder diffraction and available literature data in a series of composition-related orientationally disordered (plastic) crystals to characterize both the plastic and melting transitions and investigate relationships between associated thermodynamic properties. First, general common trends are identified: (i) The temperature range of stability of the plastic phase T - T (where T and T are the plastic and melting transition temperatures, respectively) increases with increasing pressure and (ii) both the rate of this increase, d( T - T )/ dp, and the entropy change across the plastic transition analyzed as function of the ratio T / T are quite independent of the particular compound. However, the dependence of the entropy change at the melting transition on T / T at high pressures deviates from the behavior observed at normal pressure for these and other plastic crystals. Second, we find that the usual errors associated with the estimations of second-order contributions in the Clausius-Clapeyron equation are high and thus these terms can be disregarded in practice. Instead, we successfully test the validity of the Clausius-Clapeyron equation at high pressure from direct measurements. [ABSTRACT FROM AUTHOR]

The experimental determination by means of DSC together with a thermodynamic analysis of the two-component phase diagram of octylcyanobiphenyl (8CB)+ octyloxycyanobiphenyl (8OCB) has been performed. The thermodynamic consistency of the two-component system has been proved and similar excess Gibbs energies for mixtures in the isotropic, nematic and smectic phases have been obtained. In order to study the order of the nematic to smectic A phase transition, specific heat measurements obtained using modulated DSC have been made on three mixtures of 8CB+ 8OCB as well as on the pure components. An evolution of the α-critical exponent has been found when the composition of the mixture changes between pure 8CB and pure 8OCB showing crossover behaviour between 3D-XY universal class and the tricritical point. As the 8OCB content in the mixture is further increased, the α-value decreases with the decrease in the McMillan ratio (T[sub NA]/T[sub NI]) giving rise to a value of about 0.96 at which the 3D-XY mode would be hypothetically reached. The tricritical point would be hypothetically reached at a McMillan ratio of about 0.99 and the extrapolated tricritical amplitude ratio A[sup -]/A[sup +] is estimated to be 1.6, a value very different from that obtained for the [sup 3]He-[sup 4]He tricritical transition, but comparable with other experimental results reported in the literature. [ABSTRACT FROM AUTHOR]

Abstract: At room temperature, the hexagonal C60. 2(CH3)CCl3 solvate (a=10.13(1) Å, c=10.84(1) Å), made of alternating layers of C60 and solvent molecules, forms with a negative excess volume, and its desolvation enthalpy is virtually the same as the sublimation enthalpy of the pure solvent. Crystallographic and calorimetric studies vs temperature indicate that hexagonal C60. 2(CH3)CCl3 changes at 211.7K (1.3kJmol−1 of solvate) into an intermediate triclinic phase which transforms at 189.7K (4.1kJmol−1 of solvate) into another triclinic phase. A crystallographic analysis in the series of hexagonal C60. 2 YCCl3 solvates (Y=H, Cl, Br, CH3) reveals that: (i) the change in the unit-cell volume values is due to a change in axis c whose value depends on the size of Y, (ii) the molar volume of the solvates depends linearly on the molar volume of the solvents. Ageing studies at room temperature show that C60. 2(CH3)CCl3 loses its solvent molecules within a few days or a few months, depending on storage conditions. [Copyright &y& Elsevier]

Abstract: Crystals grown by evaporating C60 solutions in n-nonane were found to be crystals of the C60·2/3 n-nonane solvate, after they were stored for eight years in the dark at room temperature. The lattice is an I-centered orthorhombic lattice (a=10.10(1) Å, b=10.19(1) Å, c=48.71(5) Å), that forms with a negative excess volume of about −3.1%, and loses its solvent from about 399 K with an enthalpy change of about 49 J per gram of solvate. Thermodynamic and crystallographic data for available C60–n-alcane solvates are discussed and a van der Waals volume of ≈603 Å3 per C60 isolated molecule is proposed. [Copyright &y& Elsevier]

[Display omitted] • Pressure–temperature phase diagram of the dimorphism of prednisolone. • Transition temperature between the two polymorphs of prednisolone estimated between 400 and 463 K. • Prednisolone solid form II is the more stable form under ambient conditions. The dimorphism of the corticosteroid anti-inflammatory drug prednisolone has been investigated by the construction of a topological pressure–temperature phase diagram, using crystallographic and calorimetric data. The system is enantiotropic, because the temperature of the I-II equilibrium under atmospheric conditions (400–463 K) is lower than that of the two melting equilibria (518.7 K for form II and 526.3 K for form I). The slope of the I-II equilibrium in the pressure–temperature phase diagram is negative and relatively steep; therefore, form II, which is the stable form at room temperature, will not easily encounter conditions where form I will become stable even under industrial processing conditions. On the other hand, extreme small amounts of form I have been observed to spontaneously transform into form II in a time interval of about six years at room temperature and it can be concluded that although form I is very persistent under ambient conditions, it does slowly convert into form II. Moreover, the system does not obey the density rule. [ABSTRACT FROM AUTHOR]

Abstract: Orientationally disordered (OD) mixed crystals formed between cyanoadamantane (CN-A) and chloroadamantane (Cl-A) have been thermodynamically characterized by means of thermal analysis and X-ray diffraction. The dynamics of these OD mixed crystals has been studied by means of broad band dielectric spectroscopy. Results evidence that dynamics of the α-relaxation process associated with the overall molecular tumbling slows down when molecules of CN-A are substituted by molecules of Cl-A. [Copyright &y& Elsevier]