The crystallization of the n-alkane C294H590 from solution:: Inversion of crystallization rates, crystal thickening, and effects of supersaturation

Single crystals of the uniform n-alkane C294H590 were grown from dilute solution in toluene in the temperature range 75 degrees 0 to 85 degrees C; the subsequent dissolution behavior during reheating was determined using calorimetry. Two crystal forms were recorded corresponding to once- and twice-folded chains. During isothermal crystallization, when primary crystallization was in the twice-folded form, transformation from the twice-folded to the once-folded state was observed. The processes of growth and transformation were followed as a function of the concentration (as derived from endotherm areas of material crystallized) of the alkane remaining in solution. These data were used, along with dissolution onset temperatures, to plot an equilibrium phase diagram for the dissolution temperature of the twice-folded crystal form as a function of solution concentration. The importance of supersaturation, as well as supercooling, in the behavior of long n-alkanes is highlighted, in contrast to the behavior of polyethylene. Crystallization and nucleation rates were measured using calorimetric methods. At the lower crystallization temperatures, it was possible to monitor crystallization directly from the crystallization exotherms; at the higher crystallization temperatures, the degree to which crystallization had proceeded as a function of time was determined from dissolution endotherms. A distinct minimum in the crystallization rate was observed at 80.5 degrees, near the transition temperature from the primary growth of twice-folded crystals (found at lower temperatures) and once-folded crystals (found at higher temperatures). This is in continuity with the previously observed minimum associated with the transition between the once-folded and extended conformation in shorter n-alkanes. Here, for the first time, the transition applies to two-folded conformations, but the same explanation for the rate minimum is invoked.