New insights on the crystalline forms in binary systems of n -alkanes: Characterization of the solid ordered phases in t
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New insights on the crystalline forms in binary systems of n-alkanes: Characterization of the solid ordered phases in the phase diagram tricosane 1 pentacosane F. Rajabalee,a) V. M´etivaud, D. Mondieig, and Y. Haget Centre de Physique Mol´eculaire Optique et Hertzienne, UMR 5798 au CNRS Universit´e Bordeaux I, F-33405 Talence Cedex, France
M. A. Cuevas-Diarte Departament de Cristal.lografia, Universitat de Barcelona, Marti i Franqu`es, E-08028 Barcelona, Spain (Received 26 January 1998; accepted 18 February 1999)
X-ray diffraction analyses of the pure components n-tricosane and n-pentacosane and of their binary mixed samples have enabled us to characterize the crystalline phases observed at “low temperature.” Contrary to what was announced in literature on the structural behavior of mixed samples in odd-odd binary systems with Dn 2, the three domains are not all orthorhombic. This work has enabled us to show that two of the domains are, in fact, monoclinic (Aa, Z 4), and the other one is orthorhombic (Pca21 , Z 4). The conclusions drawn in this work can easily be transposed to other binary systems of n-alkanes.
I. INTRODUCTION
Polymorphism of n-alkanes1–23 (Cn H2n12 ; hereafter denoted by Cn ) and determination of binary phase diagrams22–41 in the alkane family have been reported on in recent years in numerous publications. The polymorphism of n-alkanes is rather well known where many solid-solid phase transitions have been observed as a function of temperature. For the odd alkanes, unitary phase diagrams showing the transition temperatures as a function of chain length snd have been determined9,10 for the range going from C9 to C45 where four solid ordered phases (we call ordered phases those which are not accompanied with orientational disorders like the rotator phases) have been identified: two orthorhombic forms (space groups Pcam and Pnam ) and two monoclinic forms (space groups Aa and A2). All four phases possess four molecules per unit cell (Z 4). However, these unitary phase diagrams stress only on the low-temperature region, the rotator domains being given as a set. Five rotator forms are known to exist,19,20 denoted RI , RII , RIII , RIV , and RV , and are observed until C39 H80 . In the range 8 < n < 28, Robl`es et al.1,22 and Espeau et al.2 give the complete phase transitions in these alkanes incorporating the rotator forms. Odd alkanes always show at least one rotator phase before melting. For the even alkanes, the polymorphism is even more complex and the polymorphic behavior is often different, depending on the initial state of the alkane.
a)
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http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 6, Jun 1999
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A triclinic form Tp sP1, Z 13 d is observed in the range 8 < n < 24,1,2,22 while several monoclinic modifications M011 , M201 , M012 , and M101 are found in longer even alkanes as from C26 .23 All these forms ha
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