Correlation of Crystal Structures and Phase Transitions with the Number of Carbon Atoms in Ultra-pure Higher n-Alkanes

摘要

The crystal structures of n-alkanes (abbr. Cn, n: the number o fcarbon atoms) stable at room temperature, are orthorhobic for odd-numbered Cn and monoclinic for even-numbered Cn. It is known that crystal structure is apt to be affected by homologous impurities. X-ray long periods L provide a convenient check for the structure of n-alkanes, so we examine the correlation with the number of carbon atoms in n-alkanes up to ultra-pure synthesized Cl160. From the equations. L_odd=0.1267n＋0.2005, L_even=0.1131n＋0.1768(nm), the angle between the chain axes and the end group planes are 90deg and 63.2deg respectively. Besides, in order to obtain the equilibrium melting temperature Tmdeg, as the convergence temperature, we used Broadhurst's equation, for C39～C160. The equation was fitted by the least squares to the melting point determined by capillary tube method, The result of calculation is Tm(n_=414.7 (n＋0.4)/(n＋6.3) (K). The value of Tmdeg is 0.4K higher and total error became smaller about 2K, compared with Broadhurst's values.