M. A. Cuevas-Diarte

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.

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.

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.

Thermal cycling of molecular alloys and eutectics containing alkanes for energy storage

It is shown that molecular alloys of alkanes can provide suitable materials to work as MAPCMs (Molecular Alloys Phase Change Materials) in the field of energy storage and/or thermal protection, if two conditions are respected. The first is to choose the right composition(s) in the right alkanes system(s) to obtain the melting of the alloy at the required level of temperature. The second one is to choose alloys having a narrow thermal window. Reliability tests have been performed on different kinds of these sorts of MAPCMs. Their melting and crystallization behavior during thermal cyclings is excellent either at the mg scale or at the several grams scale.

Solid-solid and solid-liquid equilibria in the n-alkanols family: C18H37OH-C20H41OH system

C18H37OH–C20H41OH is an example of a binary system showing isopolymorphism. The two alkanols display the same polymorphic behaviour. At low temperatures, they crystallize into the same ordered form γ (C2/c, Z = 8). On heating, γ transforms into the rotationally disordered form R′IV (C2/m, Z = 4), at a few degrees below the melting point of the latter. However, in most mixed samples of this system a β form (P21/c, Z = 8), metastable in the two pure components, has also been observed at low temperatures. At high temperatures, the β form transforms into the R′II form (Rm, Z = 3). This R′II form is also metastable in the two pure components. The β form presents conformational defects, and molecules with all-trans conformation co-exist with molecules with CO-gt-conformation, in contrast, all the molecules in the γ form present all-trans conformation. In the R′II form the rotational disorder is more accentuated than in the R′IV form. The disorder of composition (molecular alloys) stabilizes over wide ranges of compositions the β (disorder of conformation) and R′II (disorder of rotation) forms. Five solid–solid domains ([γ + β], [β + R′II], [γ + R′II], [γ + R′IV] and [R′II + R′IV]) related by two peritectoid and eutectoid invariants, and two solid–liquid domains ([R′IV + L] and [R′II + L]) related by a eutectic and a peritectic invariant, are present. The [β + R′II] domain has a minimum. All these domains are observed for compositions rich in the two pure components. The experimental phase diagram data are fully supported by the thermodynamically calculated phase diagram. The R′II + liquid domain has a width of less than 1 K; therefore, and due to the large heat effect, the systems alloys are good candidates for the storage of thermal energy.

The C19H39OH-C20H41OH system: Experimental phase diagram and thermodynamic modelling.

The experimental phase diagram of the C19H39OH–C20H41OH system has been determined and, subsequently, subjected to thermodynamic modelling. The pure components of the system are polymorphic. At low temperature they have monoclinic phases that are different: a γ phase (C2/c, Z = 8) for C20H41OH and a β phase (P21/c, Z = 8) for C19H39OH. At high temperature, a few degrees before melting, the two components have the same monoclinic phase R′IV (C2/m, Z = 4). The solid–liquid equilibrium can be explained by simple isomorphism, and the solid–solid equilibria by crossed isodimorphism. The solid–liquid domain ([R′IV + L]) is very narrow (less than 0.2 K), and the enthalpy of fusion is high enough to consider the alloys of the system as promising candidates for thermal energy storage. Three solid–solid domains were observed ([β + R′IV], [γ + R′IV] and [β + γ]), these are related by a peritectoid invariant at ≈323 K, with compositions from 55 and 80 mol% in C20H41OH. One of the domains ([β + R′IV]) has a minimum at ≈321 K and about 40 mol% in C20H41OH. The calculated phase diagram, obtained by thermodynamic analysis, is in full agreement with the experimental one.

Solid state equilibrium in the n-alkanols family: the stability of binary mixed samples.

The stability of binary mixed samples in the normal alkanols family is studied here via the C18H37OH–C20H41OH and C19H39OH–C20H41OH systems. The stability of mixed samples depends on the method used for their preparation. In the samples obtained by the dissolution–evaporation (D + E) method (with diethyl ether), the phases in the solid–solid and solid–liquid equilibria are stable after preparation. However samples obtained by the melting–quenching (M + Q) method (quenching in liquid nitrogen) are only stable for phases in the solid–liquid equilibria. The phases observed in the solid–solid equilibria evolve over time, even after two years’ storage at low temperature (279 K).

Application of the “shape-factors method” to purity analysis of compounds by thermal methods

Abstract The shape-factors method may be applied to the determination of phase diagrams by thermal methods. In this paper, its applicability to purity evaluation of compounds (organic and inorganic) is discussed. It is a rapid and easy method which can be applied either to impurities which form a eutectic phase diagram with the main component or to impurities miscible in the solid state with the compound to be purified.

Crystal structure determination of 1-pentanol from low-temperature powder diffraction data by Patterson search methods

In the course of our research on normal alkanols, the crystal structure of 1-pentanol has been solved by applying Patterson-search methods to laboratory powder X-ray diffraction data recorded on a curved position-sensitive detector (CPS120) at 183 K. The crystal structure was refined with the rigid-body Rietveld least-squares method. The cell is monoclinic, space group P 2 1 ∕ c , Z =4, and the cell parameters are a =15.592(9) A, b =4.349(1) A, c =9.157(1) A, β =104.7(7)°, V =600.6(3) A 3 . There is one molecule in the asymmetric unit with the O–H bond in gauche conformation with respect to the alkyl skeleton. Packing is defined by the hydrogen bonds linking the 1-pentanol molecules along zigzag chains parallel to b .