The non-classical nucleation of crystals: microscopic mechanisms and applications to molecular crystals, ice and calcium carbonate

Abstract

Crystals form via nucleation followed by growth. Often nucleation data is interpreted using the classical theory of nucleation, which is essentially a simple theory for the nucleation of a fluid phase. I characterise this classical theory as making six assumptions; I discuss each assumption in turn. I then review experiments and simulations that find nucleation behaviour that cannot be described by the classical theory. The experiments are on the crystallisation from solution of molecules such as drugs and related molecules, ice and calcium carbonate. The review also covers work on non-classical nucleation in solutions of the protein lysozyme, and work on the fascinating phenomenon of nucleation induced by laser pulses. I hope this review will be of interest to those studying the crystallisation of both molecules and ions from solution. The review aims to advance our understanding of the crucial first step in crystallisation, and to enable researchers studying crystallisation in one system to learn from what others have done in studying analogous phenomena in different systems.

Nucleation of crystals that are mixed composites of all three polymorphs in the Gaussian core model

“…First, ifq 6 (i) < 0.3 (top subplot), particle i is classified as liquid-like. Otherwise, ifw 6 (i) > 0, it is classified as bcclike (middle subplot). Finally, if the particle is neither liquidlike nor bcc-like, we look atw 4 (bottom subplot).…”

“…Despite its importance, the mechanism by which one polymorph is selected preferentially during crystallization is currently poorly understood, and the subject of active research. 1,[5][6][7][8][9]12 Here, our focus is on nucleation, the first stage of crystallization, during which a microscopically small nucleus of the crystal phase is formed in the metastable fluid phase. It is well known that what happens at nucleation can determine important physical properties of much larger crystals.…”

Computer simulation of epitaxial nucleation of a crystal on a crystalline surface

“…[5][6][7][8][9][10][11][12] However, in most cases of experimental interest nucleation is not homogeneous, instead it occurs on heterogeneities such as container walls or impurity particles. 13 The reason for this is that on these surfaces part of the free energy cost of creating an interface surrounding the nucleating phase has already been paid. Heterogeneous nucleation is therefore typically many orders of magnitude faster than homogeneous nucleation, and the dominant mechanism.…”

On the reaction coordinate for seeded crystallisation

“…The crystallisation of undercooled liquids is a widely studied topic in recent research with many technological implications [1][2][3][4][5][6][7][8][9] . Due to its fundamental and practical importance, this process has been studied extensively in simple models, which can be realized experimentally in colloidal suspensions.…”