Abstract:
It is by now well known that monodisoerse n-alkanes, long
enough to exhibit chain folding, show a minimum in crystal growth rate
with increasing supercooling ΔT. The minimum occurs near the
transition from extended-chain (E) and once-folded chain growth (F),1
or from once-folded to twice-folded growth.2,3 The anomaly is attributed
to the self-poisoning or pinning effect whereby, as transition
temperature is approached from above, the unstable once-folded
overgrowth diverges and virtually stops the growth of extended-chain
crystals.4 Preliminary solution-crystallization experiments have shown a
similar kinetic anomaly as a function of increasing concentration.5 Here
we present a more detailed study of concentration dependence of
crystal growth rate, resolved into step initiation and step propagation
rates. The study confirms the strongly negative reaction order in the
range of strong poisoning, and an order of 2 to 3 for folded-chain
secondary nucleation in the range of weak poisoning, giving support to
the idea of a 2-dimensional secondary nucleus.
