Packing of Paraffin Chains in 4 Stable Modifications of n-Tritriacontane

Abstract

The n-alkane C33H68 exhibits three phase transitions between room temperature and the melting point Tm= 71.8°C The modes of chain packing in the four modifications (A, B, C, D) were determined from Weissenberg photographs. Modification A, existing at temperatures <54.5°C, is the known orthorhombic form (space group Pcam) generally found at low temperatures for odd-numbered paraffins. The modifications B and C, which are stable in the temperature ranges 54.5°C< T< 65.5°C and 65.5°C < T< 68°C, possess monoclinic unit cells (with space groups Aa and A2 respectively), the crystals being twins. In the high-temperature form D, the so-called 'rotator-phase', the crystalline order is strongly disturbed. Only a few low-order reflexions exist. Their positions in reciprocal space show that the chain axis is tilted from the normal to the end group planes by approximately 19.5° . The lateral packing closely approaches a hexagonal form but does not reach it.