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

Espeau, Philippe; Robles, L.; Cuevas‐Diarte, M. A.; Mondieig, D.; Haget, Y.

doi:10.1016/0025-5408(96)00123-7

Cited by 25 publications

(18 citation statements)

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“…In this picture, the phase transitions of long chains, including melting, are driven by conformational defects [16]; because of the exponential T dependence of p 0 , they occur in a narrow temperature range, in agreement with experiment [2,4,5].…”

Section: Alois Würgersupporting

confidence: 72%

“…The small difference of the two critical temperatures is in agreement with experiment where all transitions occur in a narrow temperature range just below the melting point. The calculated values are t 4.18 meV and t 3.92 meV, respectively; the data are indicated by circles and triangles [4,5]. The inset gives a schematic view of our defect model; a conformational defect changes the geometry of the molecule such that its pair interaction vanishes in the average and, e.g., the central grey molecule interacts with five neighbors only.…”

Section: Alois Würgermentioning

confidence: 99%

“…. , R V and several crystalline phases X have been characterized, depending on temperature, pressure, and the chain length [4][5][6][7]12]. Both phase transitions R II R I X have been observed for alkanes [1,6,10] and Langmuir monolayers [2,12].…”

Section: Alois Würgermentioning

confidence: 99%

“…PACS numbers: 68.35.Rh, 81.15.Lm Bulk crystals of normal alkanes C n H 2n12 and Langmuir monolayers of alkane derivatives show rich and surprisingly similar phase diagrams [1,2]. In a narrow temperature range below the 2D-liquid state, various rotator and crystalline phases have been characterized in terms of the distortion parameter with respect to the hexagonal lattice, the tilt angle, the rotational orientation about the molecular axis, and the stacking of adjacent layers [3][4][5][6][7][8][9][10][11].With respect to the molecular order within a single layer, these many phases reduce to three types: a hexagonal phase R II , an orthorhombic phase R I , and a phase with long-range herringbone order, X. The latter one has been observed for both bulk alkanes and Langmuir monolayers [1,12].…”

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Molecular Ordering in Amphiphile Monolayers and Alkanes

Würger

1999

Phys. Rev. Lett.

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We study the rotational order of a 2D array of hydrocarbon chains. Starting from known atomatom potentials we derive the molecular pair interaction and evaluate the thermodynamic behavior in terms of a molecular-field approximation. We find a hexagonal high-temperature phase, an intermediate orthorhombic phase, and at lower T a herringbone structure, in accordance with experiments on alkane crystals and Langmuir monolayers. The transitions are of first order, due to the strong distance dependence of the anisotropic part of the molecular pair potential. As to the variation of the critical temperatures with the chain length, we propose a simple model for conformational defects. PACS numbers: 68.35.Rh, 81.15.Lm Bulk crystals of normal alkanes C n H 2n12 and Langmuir monolayers of alkane derivatives show rich and surprisingly similar phase diagrams [1,2]. In a narrow temperature range below the 2D-liquid state, various rotator and crystalline phases have been characterized in terms of the distortion parameter with respect to the hexagonal lattice, the tilt angle, the rotational orientation about the molecular axis, and the stacking of adjacent layers [3][4][5][6][7][8][9][10][11].With respect to the molecular order within a single layer, these many phases reduce to three types: a hexagonal phase R II , an orthorhombic phase R I , and a phase with long-range herringbone order, X. The latter one has been observed for both bulk alkanes and Langmuir monolayers [1,12]. The two types of rotator phases have been confirmed by a molecular dynamics simulation [13]. These three phases and the corresponding molecular order are shown schematically in Fig. 1. Backbone ordering in amphiphile monolayers has been treated in Landau theory, including 3D aspects such as tilting [2,14].The purpose of the present paper is twofold: First, in a 2D approach where the hydrocarbon chains are considered as rigid rods, we point out the crucial role of the lattice distortion in driving the phase transitions and its relation to molecular ordering. Second, we propose a simple model for conformational defects that accounts for the observed variation of the critical temperatures with the chain length. (For rigid molecules one expects a linear dependence, whereas experiment reveals a much weaker variation.)We deal with a single layer of molecules and retain the rotational degrees of freedom only. Starting from the Buckingham atom-atom potential, b V ͑r͒ 2Ar 26 1 Be 2Cr ,with parameters A, B, C given by Williams [15], we determine the rotational pair potential of alkane molecules as a function of their distance and angles. (2) where r ij is the distance vector of the molecular centers and d p i are the relative coordinates of atom p i on molecule i. We expand this pair potential in a double Fourier series V ij P nm y ij nm e in͑w i 2a ij ͒1im͑w j 2a ij ͒ , where w i and w j give the molecular orientation in the x-y plane, and a ij is the angle of the distance vector r ij . When retaining the most relevant coefficients only, the potential energy read...

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Section: Alois Würgersupporting

confidence: 72%

Section: Alois Würgermentioning

confidence: 99%

Section: Alois Würgermentioning

confidence: 99%

mentioning

confidence: 99%

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Molecular Ordering in Amphiphile Monolayers and Alkanes

Würger

1999

Phys. Rev. Lett.

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“…The research topics of this group have both fundamental and applied purposes and concerns the investigation of the polymorphism and solid-state miscibility of coherent families of compounds and also the application of molecular alloys as tuneable phase change materials (MAPCM) for thermal protection or energy storage. In the past 10 years, numerous works have been undertaken on alkanes (C n H 2n+2 ) [1,2] and alkanols (C n H 2n+2 O) [3,4] which demonstrated in particular the feasibility of molecular alloys of alkanes for the applications [5][6][7]. Our interest is focused here on fatty acids (C n H 2n O 2 ), which are alkanes derivatives and are also promising candidates for the applications.…”

Section: Introductionmentioning

confidence: 99%