Isothermal refolding in crystals of long alkanes in solution. I. Effect of surface ‘self‐poisoning’

Lamellar thickening in folded‐chain crystals of the n‐alkane C₁₉₈H₃₉₈ in solution has been shown to occur at the crystallization temperature. The transformation is from crystals with their chains folded in half to extended‐chain crystals, with no observable intermediate forms. This conforms with the extensive existing evidence that integer‐folded forms are thermodynamically preferable. Measurements of the kinetics of lamellar thickening have revealed an unexpected maximum in the thickening rate as a function of temperature: above 75°C, thickening slows down with increasing temperature. This unprecedented result is an indirect consequence of another, equally puzzling, anomaly: the rate of extended‐chain nucleation at the side faces of folded‐chain crystals decreases with increasing solution concentration. This unique observation is attributed to “self‐poisoning” of the lateral surface of the growing nucleus by frequent folded‐chain depositions which inhibit the process of molecular extension. The same effect has been invoked previously to explain the anomalous crystallization rate minima in long alkanes and its present manifestation under different circumstances is regarded as an important independent validation of the concept of “self‐poisoning.”