Nonclassical Nucleation—Role of Metastable Intermediate Phase on Crystal Nucleation

Dear Colleagues,

Classical nucleation theory (CNT), which was established about 90 years ago, has been very successful in many research fields, and continues to be the most commonly used theory in describing the nucleation process. For a fluid-to-solid phase transition, CNT states that the solute molecules in a supersaturated solution reversibly form small clusters. Once the cluster size reaches a critical value, it becomes thermodynamically stable and is favored for further growth. One of the most important assumptions of CNT is that the nucleation process is described by one reaction coordinate and all order parameters proceed simultaneously.

Recent studies in experiments, computer simulations and theory have revealed nonclassical features in the early stage of nucleation. In particular, the decoupling of order parameters involved during a fluid-to-solid transition leads to the so-called two-step nucleation mechanism, in which a metastable intermediate phase (MIP) exists between the initial supersaturated solution and the final crystals. Depending on the exact free energy landscapes, the MIPs can be a high-density liquid phase, mesoscopic clusters, or a pre-ordered state.

In this Topic, we focus on the role of the various MIPs on the early stage of crystal nucleation. Such MIPs have been observed in organic materials, minerals, colloids and proteins, resulting in various scenarios of nonclassical pathways of crystallization. Efforts are required on the characterization of the structural and dynamical properties of MIPs, the growth kinetics of both MIPs and crystals, as well as the theoretical understanding of the decoupling of order parameters.

Dr. Fajun Zhang
Dr. José Gavira
Dr. Geun Woo Lee
Prof. Dr. Dirk Zahn
Guest Editors

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