Denise Mondieig

Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid

The polymorphism of normal saturated even carboxylic acids from n-decanoic to n-eicosanoic acid is discussed. Seven crystal modifications, including polymorphs and polytypes, were identified and fully characterized by the combination of calorimetric measurements (DSC) at atmospheric and high pressures, X-ray powder diffraction, FT-IR spectroscopy and scanning electron microscopy (SEM). All seven crystal forms, including polymorphs and polytypes, are observed at room temperature. Forms A2 and Asuper are triclinic, form C is monoclinic and forms E and B show both a monoclinic and an orthorhombic polytype. The triclinic modifications A2 and Asuper predominate for acids up to n-tetradecanoic acid (C14H27O2H). The orthorhombic and the monoclinic forms predominate for acids from n-hexadecanoic (C16H31O2H) up to n-eicosanoic acid (C20H39O2H). When the temperature is increased, all the crystal modifications transform irreversibly to the C form. In the first part of this paper, cell parameters for the different forms are given, the observed temperatures and enthalpies of the transitions are reported and the stability of the different forms is discussed. In the second part, we state the main contribution of each technique for the identification and interpretation of the polymorphism of even numbered carboxylic acids.

Protection of temperature sensitive biomedical products using molecular alloys as phase change material.

In this paper we present an example of the application of molecular alloys for thermal protection of biomedical products during transport or storage. Particularly, thermal protection of blood elements have been considered at different temperatures. All steps from basic research to marketing have been addressed. The high latent heat of fusion of the components allows us to propose molecular alloys as materials for thermal energy storage and also for thermal protection over a large range of temperatures, which can be used in many industrial sectors.

Perfect families of mixed crystals: The rotator I N-alkane case

The experimental systems considered in this paper are isobaric binary mixed crystals, and their properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. More in particular, the excess behavior is examined for families of systems where the components of each of the member systems belong to a group of chemically coherent substances. The accent is on a group of 18 systems composed of n-alkanes in the range from C11H24 to C26H54, such that the components of an individual system are either neighbors or next nearest neighbors. In these systems mixed crystalline solid forms are stable in which the molecules have rotational freedom around their long axis. One of these forms is rotator I, and it is this form for which thermodynamic mixing properties have been determined. The magnitudes of the excess enthalpy and excess entropy are system-dependent and can be correlated to the geometric mismatch, i.e., the relative difference in number of carbon atoms of the molec...

Melting behaviour in the n-alkanol family. Enthalpy-entropy compensation

The melting behaviour was studied in ten systems: C15OH–C16OH, C16OH–C17OH, C17OH–C18OH, C18OH–C19OH, C19OH–C20OH with Δn = 1 (difference in chain length), C15OH–C17OH, C16OH–C18OH, C17OH–C19OH, C18OH–C20OH with Δn = 2, and C16OH–C20OH with Δn = 4. The phase that melts is either the monoclinic R′IV(C2/m, Z = 4) or the hexagonal R′II(Rm, Z = 6) rotator form. One of the most important issues in the melting of these systems is that when the two original compounds of the system are isostructural, the phase diagram does not always show total miscibility. In the systems studied here, only the C15OH–C16OH, C18OH–C19OH and C19OH-C20OH systems show total miscibility. In the other systems in which the two original compounds are isostructural, miscibility is partial, as in the systems where the two original compounds are not isostructural. In this family, as in other families of mixed crystals, there is an excess enthalpy-entropy compensation. This compensation has a temperature dimension, and is called the compensation temperature (θ) of the family and/or subfamily. In the case of the R′II and R′IV rotator forms of the n-alkanols family its value is 362 K. This value is in line with the trend show by a large group of organic and inorganic mixed crystalline materials.

Miscibility study in stable and metastable orientational disordered phases in a two-component system (CH3)CCl3+CCl4

The orientationally disordered stable and metastable mixed crystals of the two-component system methylchloroform ((CH3)CCl3)+carbon tetrachloride (CCl4) have been characterised from a crystallographic and thermodynamic point of view. The monotropic behaviour of the metastable phase in the pure components is maintained for the whole range of composition. The lattice symmetry of the stable orientationally disordered phase of methylchloroform has been found to be isostructural with that of the carbon tetrachloride compound. Continuous series of both stable and metastable mixed crystals give rise to a double isomorphism relationship, one for the stable state and another for the metastable stable of the pure components.

First experimental demonstration of crossed isodimorphism: (CH3)3CCl + CCl4 melting phase diagram

The melting phase diagram of the two-component system 2-methyl-2-chloro-propane ((CH3)3CCl) and carbon tetrachloride (CCl4) has been determined by combining X-ray powder diffraction and thermal analysis. The isomorphism relation between the orientationally disordered (OD) stable face-centered cubic (FCC) phase of (CH3)3CCl and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component. This equilibrium interferes with a rhombohedral plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant. A thermodynamic analysis, in terms of the crossed isodimorphism, has also been performed in order to obtain the excess properties of the FCC and R OD phases.

Polymorphism in 2-X-Adamantane Derivatives (X = Cl, Br)

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group P212121, Z = 4) and a high-temperature plastic phase (Fm3̅m, Z = 4) from 277.9 ± 1.0 K to the melting point at 413.4 ± 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (P1̅, Z = 2), which transforms to a monoclinic phase (C2/c, Z = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (P63/mcm, Z = 6) at ca. 241 K and the highest one, cubic (Fm3̅m, Z = 4), being stable from 244 ± 1.0 K up to the melting point at 467.5 ± 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br···Br as well as C-Br···H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks.

High-pressure properties inferred from normal-pressure properties

From the stable and metastable normal-pressure phase equilibria involved in two-component systems sharing compounds of the series CCl4−nBrn, n = 0,...,4, several thermodynamic properties concerning non-experimentally available phase transitions have been determined. To do so, the well-established concept of crossed isodimorphism has been considered to involve the isomorphism relationships between the low-temperature monoclinic phases as well as, for both rhombohedral and face-centred cubic, orientationally disordered phases appearing in the compounds of the series. On the basis of such relations, the thermodynamic properties of the two-phase equilibria are extrapolated as a function of mole fraction to the pure compounds for which the involved transitions do not exist at normal pressure. The obtained thermodynamic properties are used to build up the topological pressure–temperature phase diagrams of the compounds of the series. The results are compared with the experimental pressure–temperature phase diagrams obtained by means of density measurements as a function of pressure and temperature.

Perfect families of mixed crystals: the ‘‘ordered’’ crystalline forms of n-alkanes

The experimental systems considered in this paper are isobaric binary mixed crystals of n-alkanes in the crystalline forms Op, Mdci and Mdep, which, on the temperature scale, precede mixed crystals in the rotator forms. The properties studied are the thermodynamic mixing properties, actually the excess enthalpy and excess entropy. For 14 binary systems in the range C17 to C28, quantitative thermodynamic data have been obtained by studying the transition to the rotator forms, for which the thermodynamic mixing properties had been determined earlier. The most important outcome of the investigation is that the complete set of systems is characterised by (i) a uniform ratio between excess enthalpy and excess entropy, having the value of 335 K, and (ii) a uniform dependence of the excess enthalpy on relative difference between number of carbon atoms in the molecules of the components of the system.

Non Isomorphism and Miscibility in The Solid State: Determination of The Equilibrium Phase Diagram n-Octadecane C18H38 + n-Nonadecane C19H40

Abstract The phase diagram of the binary n-alkane system C18H38-C19H40 is determined by calorimetric and X-ray diffraction methods. The present experimental results and previous work on pure C18H38 and C19H40 paraffins allow us to identify all the phases of the binary system. The phase diagram exhibits no less than six distinct solid domains: three are one-phase regions, triclinic (T), orthorhombic (O) and face-centered orthorhombic, called rotator phase (RI), and three are two-phase equilibria [T + O], [T + RI] and [O + RI]. The two regions [O] and [RI] occur on large composition ranges (from the pure constituent C19H40). The solid-liquid equilibria are separated by a eutectic invariant. They can be explained in terms of crossed isodimorphism.