THERMAL-BEHAVIOR OF POLYMORPHIC N-ALKANES - EFFECT OF COOLING RATE ONTHE MAJOR TRANSITION-TEMPERATURES

The thermal behaviour of selected polymorphic even-numbered and odd-nu mbered n-alkanes, namely n-C23H48 n-C25H52, n-C28H58, n-C30H62, n-C33H 68, n-C34H70 and n-C37H76 was studied using differential scanning calo rimetry (d.s.c.). Both the melting and the crystallization d.s.c. ther mograms for these paraffins exhibit two major peaks, alpha and beta, c orresponding to the liquid-solid and solid-solid transitions. At the s ame rate of temperature change, the shape and peak height of the melti ng endotherms differ considerably from those of the cooling exotherms. The experimental results are explained in terms of the established cr ystallographic descriptions of the crystalline rotator phases for thes e alkanes. More importantly, the peak-to-peak temperature difference ( Delta T-peak = T-alpha- T-beta) depends on the carbon number but not o n the cooling rate. The following linear relation, which is independen t of cooling rate, is proposed for the variation of Delta T-peak with the carbon number (C): Delta T-peak=17 - 0.42C.