Molecular simulation approaches to study crystal nucleation from solutions: Theoretical considerations and computational challenges

Abstract

Nucleation is the initial step in the formation of crystalline materials from solutions. Various factors, such as environmental conditions, composition, and external fields, can influence its outcomes and rates. Indeed, controlling this rate‐determining step toward phase separation is critical, as it can significantly impact the resulting material's structure and properties. Atomistic simulations can be exploited to gain insight into nucleation mechanisms—an aspect difficult to ascertain in experiments—and estimate nucleation rates. However, the microscopic nature of simulations can influence the phase behavior of nucleating solutions when compared to macroscale counterparts. An additional challenge arises from the inadequate timescales accessible to standard molecular simulations to simulate nucleation directly; this is due to the inherent rareness of nucleation events, which may be apparent in silico at even high supersaturations. In recent decades, molecular simulation methods have emerged to circumvent length‐ and timescale limitations. However, it is not always clear which simulation method is most suitable to study crystal nucleation from solution. This review surveys recent advances in this field, shedding light on typical nucleation mechanisms and the appropriateness of various simulation techniques for their study. Our goal is to provide a deeper understanding of the complexities associated with modeling crystal nucleation from solution and identify areas for further research. This review targets researchers across various scientific domains, including materials science, chemistry, physics and engineering, and aims to foster collaborative efforts to develop new strategies to understand and control nucleation.This article is categorized under: Molecular and Statistical Mechanics > Molecular Dynamics and Monte‐Carlo Methods Molecular and Statistical Mechanics > Free Energy Methods Theoretical and Physical Chemistry > Statistical Mechanics

Is the Local Ion Density Sufficient to Drive NaCl Nucleation from the Melt and Aqueous Solution?

“…43, 77 The nucleation of NaCl in the aqueous solution is concentration-dependent. 21 Simulations with high concentration leads to a 2-step nucleation mechanism, forming an amorphous aggregation blob and then an ordered solid. 21,35 In this work, the NaCl concentration is low (1.5c s ) and the onestep mechanism is consistent with previous reports.…”

“…CNT assumes a single free energy barrier given as a function of the radius r of spherically shaped nuclei clusters. It describes nucleation as a competition between the free energy cost for forming a surface (scaling as r 2 ) and the free energy benefit from a new crystalline phase (scaling as r 3 ) . CNT is an oversimplified model and completely overlooks the “two-step” mechanism in addition to other assumptions, often leading to significant errors in the nucleation rate even for simple systems. , As a result, it is not surprising that CNT is not able to describe the nucleation of other molecules with more complicated interactions or shapes, such as urea and glycine. , …”