Miquel Àngel Cuevas-Diarte

In situ synchrotron radiation X-ray diffraction study of crystallization kinetics of polymorphs of 1,3-dioleoyl-2-palmitoyl glycerol (OPO)

We examined the influence of kinetics on the polymorphic behavior of 1,3-dioleoyl-2-palmitoyl glycerol (OPO), a triacylglycerol (TAG) present in natural oils such as olive oil and human breast milk. Pure OPO was heated at 15 °C min−1 in all cases. The polymorphic crystallization of OPO was studied at different cooling rates (15, 2, 1, and 0.5 °C min−1), and the dynamic polymorphic transformations were characterized on heating by simultaneously using Differential Scanning Calorimetry (DSC) and Synchrotron Radiation X-Ray Diffraction (SR-XRD) with small-angle X-ray diffraction (SAXD) and wide-angle X-ray diffraction (WAXD). Thermo-Optical Microscopy (TOM) was also used in order to observe the polymorphic transitions. Polymorphic forms of OPO were identified and characterized. As the cooling rate decreased, more stable forms crystallized, not following the Ostwald step rule, although in most cases concurrent crystallization occurred and the whole process was highly complex.

In situ observation of transformation pathways of polymorphic forms of 1,3-dipalmitoyl-2-oleoyl glycerol (POP) examined with synchrotron radiation X-ray diffraction and DSC

POP (1,3-dipalmitoyl-2-oleoyl glycerol), a major triacylglycerol component of vegetable and animal fats found in chocolate, fat spread, and other edible fats, crystallizes in seven polymorphic forms: α, γ, δ, β′2, β′1, β2, and β1. The crystallization and transformation pathways of the polymorphs of POP are influenced by many factors. In the present work, we studied the influence of the rates of cooling and heating on the polymorphic crystallization and transformation of POP. The rate of cooling (heating) was changed from 15 to 2, 1, and 0.5 °C min−1 (15 to 2, 1, 0.5, and 0.1 °C min−1). The crystallization and transformation processes were examined with differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (SR-XRD). The results indicated that more stable forms were formed in a higher quantity when POP was slowly cooled and heated, whereas less stable forms occurred at higher rates of cooling and heating. Specifically, less stable α and γ forms were directly obtained at cooling rates of 15 to 0.5 °C min−1. The more stable forms of β′ or β did not occur even at a rate of cooling of 0.5 °C min−1, whereas α or γ transformed to β′ or β at heating rates of 0.5 and 0.1 °C min−1. The polymorphic transformations occurred either in solid-state or melt-mediation and were largely influenced by the heating rates. We discuss the present results by considering the differences in activation free energies for the transformations from α or γ to more stable forms, which may determine the heating-rate-dependent transformation pathways.

Heterogeneous microstructures of spherulites of lipid mixtures characterized with synchrotron radiation microbeam X-ray diffraction

We observed microstructures of spherulites made of two triacylglycerols (TAGs) (POP (1,3-dipalmitoyl-2-oleoyl-sn-glycerol) and OPO (1,3-dioleoyl-2-palmitoyl-sn-glycerol)), which exhibit the mixing behavior of a molecular compound (MC) formation at a ratio of POP : OPO = 50 : 50, using synchrotron radiation microbeam (beam area of 5 × 5 μm2) X-ray diffraction (SR-μ-XRD). The spherulites were grown from neat liquid, and from solution containing 50% n-dodecane and 50% POP + OPO. SR-μ-XRD analysis revealed heterogeneous distributions of MCPOP:OPO and component TAGs in every spherulite; TAG compositions in the inner and outer areas differed when the relative ratios of POP and OPO were changed. In the 75POP : 25OPO spherulites, MCPOP:OPO always occupied the inner areas and POP dominated the outer areas as a result of different rates of crystallization of MCPOP:OPO and POP in the spherulite. In contrast, in the 25POP : 75OPO spherulites, the entire area was homogeneously occupied either with almost all OPO or with coexisting MCPOP:OPO and OPO. The microstructures of the spherulites grown from n-dodecane solutions demonstrated basically the same characteristics as those grown from the neat liquid both for 75POP : 25OPO and 25POP : 75OPO. SR-μ-XRD also clarified that the lamellar planes of the crystals are directed parallel to the radial direction from the central to the outer regions of the spherulites.

Competing Intermolecular Interactions in the High‐Temperature Solid Phases of Even Saturated Carboxylic Acids (C10H19O2H to C20H39O2H)

Structural knowledge of the high-temperature phases of saturated carboxylic acids (C(n)H(2n-1)O(2)H) from C(6)H(11)O(2)H to C(23)H(45)O(2)H is now complete. Crystal structures of the high-temperature phases of even acids from decanoic (C(10)H(19)O(2)H) to eicosanoic (C(20)H(39)O(2)H) are reported. The crystal structures of the six compounds were determined from powder X-ray diffraction data following direct space methods and refined by the Rietveld method combined with force field geometry optimization. The combination proved to be a valuable approach to obtain structures that are chemically sensible and in close agreement with the powder pattern. At the end of the process solid-state DFT calculations were applied to improve the overall accuracy of the system but in this case DFT did not render better structures. The high-temperature solid phases of even carboxylic acids are all P2(1)/c with Z=4, the molecules are united into dimers via strong hydrogen bonds. Two major types of interactions govern the crystal packing of carboxylic acids, hydrogen bonds and van der Waals interactions. A survey of the intermolecular interactions has revealed that hydrogen bonds are the dominant interaction for acids with less than 23 carbon atoms in the alkyl chain while van der Waals interactions dominate the packing for acids with more than 23 carbon atoms.

Polymorphism and solid-state miscibility in the pentadecanoic acid-heptadecanoic acid binary system.

The pentadecanoic acid-heptadecanoic acid (C(15)H(29)OOH-C(17)H(33)OOH) binary system is dealt with in this article. Combined thermal analysis and X-ray powder diffraction experiments are performed to characterize the polymorphism of the pure compounds and of their mixed samples. In particular, modern methods of crystal structure resolution from powder data (direct space methods) are applied in order to investigate and compare the molecular arrangement within the solid phases of the fatty acids considered. A proposal of the binary phase diagram is given. It exhibits no less than eight distinct solid phases stabilized on relatively narrow domains of composition which shows the reduced miscibility of the constituents. Finally, a structural model of one of the intermediate solid solutions is developed which well accounts for the mixing behaviour of the two fatty acids and permits to propose an explanation about their low solid-state miscibility.

Phase behavior of binary mixture systems of saturated-unsaturated mixed-acid triacylglycerols: effects of glycerol structures and chain-chain interactions.

We systematically examined the phase behavior of binary mixtures of mixed-acid triacylglycerols (TAGs) containing palmitic and oleic acid moieties 1,3-dioleoyl-2-palmitoyl-glycerol (OPO), 1,2-dipalmitoyl-3-oleoyl-rac-glycerol (PPO), and 1,2-dioleoyl-3-palmitoyl-rac-glycerol (OOP), which are widely present in natural fats and are employed in the food, pharmaceutical, and cosmetic industries. Differential scanning calorimetry and X-ray diffraction methods were applied to observe the mixing behavior of PPO/OPO, OOP/OPO, and PPO/OOP under metastable and stable conditions. The results led to three conclusions: (1) Eutectic behavior was observed in PPO/OPO. (2) Molecular compound (MC) crystals were formed in the mixtures of OOP/OPO and PPO/OOP. (3) However, the MC crystals occurred only under metastable conditions and tended to separate into component TAGs to form eutectic mixture systems after 17 months of incubation. These results were contrary to those of previous studies on 1,3-dipalmitoyl-2-oleoyl glycerol (POP)/OPO and POP/PPO in which the MC crystals were thermodynamically stable. We determined that specific molecular interactions may cause this different phase behavior (stability of POP/OPO and POP/PPO MC crystals and metastability of OOP/OPO and PPO/OOP MC crystals). All results confirm the significant effects of molecular structures of glycerol groups, interactions of fatty acid chains, and polymorphism of the component TAGs on the mixing behavior of mixed-acid TAGs.

Polymorphism, crystal growth, crystal morphology and solid-state miscibility of alkali nitrates

In this review, we recollect and discuss the most relevant advancement in crystallographic aspects of alkali nitrates compounds published since 1975, the year of publication of the last review by Rao et al. about some topics discussed here. This review is focused in five nitrate compounds of the first periodic table column: LiNO3, NaNO3, KNO3, RbNO3 and CsNO3. For each one, we have compiled the information about crystal structure, polymorphism and phase transition, crystal growth and crystal morphology, and the phase diagram and solid-state miscibility results. Considerable numbers of papers have been published in the last 40 years, in particular in relation to binary phase diagrams and solid-state miscibility of alkali nitrates. A variety of phase diagram descriptions appears in the 10 possible binary combinations for the five compounds. To finish, we discuss and propose a geometric model in order to explain the different binary phase diagrams observed between these compounds.

pVT Measurements and Related Studies on the Binary System nC16H34 -nC17H36 and on nC18H38 at High Pressures

Abstract The phase diagram of the binary system nC16H34 -nC17H36 has been established at ambient pressure using DSC and crystallographic measurements. At low temperatures below the rotator phase RI there exist two crystal forms Op (about x(C17) = 0.25) and Mdci (about x(C17) = 0.67) which are different from the crystal structures of the pure compounds (Tp for C16 and Oi for C17). Furthermore two compositions: (a) C16/C17 = 3:1 and (b) = 1:2, which correspond to the coexistence range of Op and Mdci, were chosen for high pressure DTA and pVT measurements, yielding the following findings: The specific volume of the rotator phase of C17 is distinctly lower than those of the binary systems at the same state point. Assuming the existence of a metastable rotator phase for C16, an excess volume of Δ VE/V ≈ 0.01 can be estimated. Due to the very enlarged coexistence range of RI, the mixtures reach their lower transition point at considerably lower temperatures (in isobaric measurements) or higher pressures (in isothermal measurements), where the specific volume is lower than that of C17 at its transition point. Furthermore, the volume and enthalpy changes of the Φord -RI transition is distinctly smaller for the binary systems than for pure C17. Thus the specific volumes of the phases Op and Mdci are appreciably larger than ν(spec.) of C17. Op and Mdci have practically the same specific volume in accordance with the crystallographic results. Enthalpy values are obtained with the aid of the Clausius-Clapeyron equation which agree well with enthalpies derived from the DSC measurements. Furthermore, pVT data have been established for the liquid and solid phases of nC18H38 in the neighbourhood of the melting curve, allowing to determine volume and enthalpy changes of melting as a function of pressure.

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