L. Ventolà

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.

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).

Selective Use of Stones in the Medieval Cathedral of Tarragona, Spain: Construction and Historical Reasons

Stone selection is not a random choice for builders and architects. From the Stonehenge Bluestones to Frank Lloyd Wrights “desert rubble masonry” in Taliesin West or Peter Zumthors thermal bath in Vals, the stones selected form a key part of the buildings message, providing special significance for those who are aware of the selection. A clear example of this significance is the Cathedral of Tarragona, Spain. This study details the stones used in the construction of the cathedral between the 12th and 14th centuries, combining Romanesque and Gothic styles. A petrologic description was conducted for all the stones and their physical properties were measured, including thermal expansion, compressive strength, open porosity, density, and hydric expansion. These findings lead to a discussion of the reasons why certain stones were chosen.

Thermal protection using molecular alloys as phase change materials

Addresses for Correspondence: M. Cuevas-Diarte, T. Calvet, L. Ventola, (e-mail: lourdes_ventola@yahoo.com; Tel+34 934021350 Fax+34 934021340; Departament de CristaUografia, Mineralogia i Diposits Minerals. Facultat de Geologia. Universitat de Barcelona. Mart{ i Franques sin, E-08028 Barcelona, Spain; D. Mondieig, Centre de Physique Moleculaire et Optique Hertzienne, UMR 5798 au CNRS Universite Bordeaux I. 351 Cours de la Liberation, 33405 Talence, France