TY - JOUR

T1 - Size-Dependent Phase Stability of a Molecular Nanocrystal: a Proxy for Investigating the Early Stages of Crystallization

AU - Zahn, Dirk

AU - Anwar, Jamshed

PY - 2011/9

Y1 - 2011/9

N2 - We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of DL-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.

AB - We make the link between the size-dependent phase stability of a nanocrystal and the phase-transition behavior of emerging crystallites during the earliest stages of crystallization, by using the former as a proxy for the latter. We outline an extension of the classical nucleation theory to describe crystal nucleation and subsequent transformations of competing polymorphic phases that characterize Ostwald's rule of stages. The theoretical framework reveals that the relative stability of the competing phases is a function of cluster size, which in turn varies with time, and therefore explains the complex transformation behavior observed for some systems. We investigated the stability of a nanocrystal of DL-norleucine by means of molecular simulation as a proxy for post-nucleation phase-transformation behavior in emerging crystallites. The simulations reveal that, for nanocrystals, the surface energy of the transition state of a transformation can dominate the barrier to phase change, thus causing metastable phases to be stabilized, not because they are thermodynamically stable, but rather due to kinetic hindering. Therefore, in the context of the earliest stages of crystal growth, not only does phase stability vary as a function of cluster size, and hence time, but thermodynamically feasible transformations are also prone to kinetic hindering.

KW - phase transformations

KW - nanocrystal stability

KW - molecular simulations

KW - CRYSTAL POLYMORPHISM

KW - BETA-FORM

KW - TRANSITIONS

KW - SIMULATION

KW - crystal nucleation

KW - Ostwald's rule

KW - TRANSFORMATION

KW - TETROLIC ACID

KW - ENERGY

KW - CROSS-NUCLEATION

KW - ENERGETICS

KW - DL-NORLEUCINE

U2 - 10.1002/chem.201100710

DO - 10.1002/chem.201100710

M3 - Journal article

VL - 17

SP - 11186

EP - 11192

JO - Chemistry - A European Journal

JF - Chemistry - A European Journal

SN - 0947-6539

IS - 40

ER -