Nucleation: what happens at the initial stage?

Wall-induced phase transition controlled by layering freezing

Molecular dynamics simulations of the Lennard-Jones model are used to study phase transitions at a smooth surface. Our motivation is the observation that the existence of an attractive wall facilitates crystallization. To investigate how this wall influences phase transitions, the strength of wall-particle interaction is varied in our studies. We find that the phase behavior depends on the strength parameter α, i.e., the ratio between wall-particle and the particle-particle attraction strength. Three critical values of the ratio, namely, αp, αw, and αc, are used to define the qualitative nature of the phase behaviors at a smooth surface. Some interesting phenomena due to the increase of α are observed. First, a set of close-packed planes, i.e., {111} planes in fcc structures or {0001} planes in hcp structures, are "rotated" from intersecting to parallel to the wall when α = αp; second, the layering phase transition close to the wall antecedes that of the bulk when α = αw. Finally, the first-order phase transition in the first two layers is supplanted by a continuous phase transition when α = αc, which to some extent can be treated as a quasi-two-dimensional process. We find that bulk freezing always discontinuously occurs through a first-order phase transition, and seems to be isolated from the freezing process occurring close to the attractive surfaces. Moreover, during the heating process, we observe minimal dependence at a strongly attractive surface.

Nucleation and particle growth with fluctuating rates at the intermediate stage of phase transitions in metastable systems

An exact analytical solution of an integro-differential model describing the transient nucleation of solid particles (nuclei) and their growth with fluctuating rates at the intermediate stage of bulk phase transitions in metastable systems is constructed. Two important cases of the Weber–Volmer–Frenkel–Zel'dovich and Mier nucleation kinetics are detailed for supercooled melts and supersaturated solutions.

Kinetic trapping of metastable amino acid polymorphs

Second harmonic generation (SHG) microscopy measurements indicate that inkjet-printed racemic solutions of amino acids can produce nanocrystals trapped in metastable polymorph forms upon rapid solvent evaporation. Polymorphism impacts the composition, distribution, and physico-kinetic properties of organic solids, with energetic arguments favoring the most stable polymorph. In this study, unfavored noncentrosymmetric crystal forms were observed by SHG microscopy. Polarization-dependent SHG measurement and synchrotron X-ray microdiffraction analysis of individual printed drops are consistent with formation of homochiral crystal production. Fundamentally, these results provide evidence supporting the ubiquity of Ostwald’s Rule of Stages, describing the hypothesized transitioning of crystals between metastable polymorphic forms in the early stages of crystal formation. Practically, the presence of homochiral metastable forms has implications on chiral resolution and on solid form preparations relying on rapid solvent evaporation.

Experimental modelling of single-particle dynamic processes in crystallization by controlled colloidal assembly

A comprehensive review of the experimental modeling of single particle dynamics in crystallization is presented.

On the mechanism of metal nanoparticle synthesis in the Brust-Schiffrin method

Brust-Schiffrin synthesis (BSS) of metal nanoparticles has emerged as a major breakthrough in the field for its ability to produce highly stable thiol functionalized nanoparticles. In this work, we use a detailed population balance model to conclude that particle formation in BSS is controlled by a new synthesis route: continuous nucleation, growth, and capping of particles throughout the synthesis process. The new mechanism, quite different from the others known in the literature (classical LaMer mechanism, sequential nucleation-growth-capping, and thermodynamic mechanism), successfully explains key features of BSS, including size tuning by varying the amount of capping agent instead of the widely used approach of varying the amount of reducing agent. The new mechanism captures a large body of experimental observations quantitatively, including size tuning and only a marginal effect of the parameters otherwise known to affect particle synthesis sensitively. The new mechanism predicts that, in a constant synthesis environment, continuous nucleation-growth-capping mechanism leads to complete capping of particles (no more growth) at the same size, while the new ones are born continuously, in principle leading to synthesis of more monodisperse particles. This prediction is validated through new experimental measurements.

Propagation of biochirality: crossovers and nonclassical crystallization kinetics of aspartic acid in water

All experimental procedures discussed could be treated as a screening tool for probing the existence of molecular association among the chiral molecules and the solvent system. The molecular association phases of a racemic conglomerate solution (CS) and a racemic compound solution (RCS), and the templating effect of aspartic acid solid surface were observed to minimize the chance of redissolving racemic conglomerate and racemic compound aspartic acid in water and reforming an RCS in crossovers experiments. Only 1 %wt% of l-aspartic acid was adequate enough to induce a transformation from a racemic compound aspartic acid to a racemic conglomerate aspartic acid. This would make the propagation of biochirality more feasible and sound. However, tetrapeptide, (l-aspartic acid)4 , failed to induce enantioseparation as templates purely by crystallization. Nonclassical crystallization theory was needed to take into account the existence of a CS. Fundamental parameters of the crystallization kinetics such as the induction time, interfacial energy, Gibbs energetic barrier, nucleation rate, and critical size of stable nuclei of: (i) racemic compound aspartic acid, (ii) racemic compound aspartic acid seeded with 1 %wt% l-aspartic acid, (iii) racemic conglomerate aspartic acid, and (iv) l-aspartic acid were evaluated and compared with different initial supersaturation ratios. Morphological studies of crystals grown from the crystallization kinetics were also carried out.

How Crystals Nucleate and Grow in Aqueous NaCl Solution

Large-scale molecular dynamics simulations (64 000 particles) are used to examine the microscopic mechanism of crystal nucleation and growth in a slightly supersaturated solution of NaCl in water at 300 K and 1 atm. Early-stage nucleation is observed, and the growth of a single crystal is followed for ∼140 ns. It is shown that the nucleation and growth process is better described by Ostwald's rule of stages than by classical nucleation theory. Crystal nucleation originates in a region where the local salt concentration exceeds that of the bulk solution. The early-stage nucleus is a loosely ordered arrangement of ions that retains a significant amount of water. The residual water is slowly removed as the crystal grows and evolves toward its stable anhydrous state.

New insights into the transformation of calcium sulfate hemihydrate to gypsum using time-resolved cryogenic transmission electron microscopy

We use time-resolved cryogenic transmission electron microscopy (TR-cryo-TEM) on a supersaturated solution of calcium sulfate hemihydrate to examine the early stages of particle formation during the hydration of the hemihydrate. As hydration proceeds, we observe nanoscale amorphous clusters that evolve to amorphous particles and then reorganize to crystalline gypsum within tens of seconds. Our results indicate that a multistep particle formation model, where an amorphous phase forms first, followed by the transformation into a crystalline product, is applicable even at time scales of the order of tens of seconds for this system. The addition of a small amount of citric acid significantly delays the reorganization to gypsum crystals. We hypothesize that available calcium ions form complexes with the acid by binding to the carboxylic groups. Their incorporation into a growing particle produces disorder and extends the time over which the amorphous phase exists. We see evidence of patches of "trapped" amorphous phase within the growing gypsum crystals at time scales of the order of 24 h. This is confirmed by complementary X-ray diffraction experiments. Direct imaging of nanoscale samples by TR-cryo-TEM is a powerful technique for a fundamental understanding of crystallization and many other evolving systems.

A generic crystallization-like model that describes the kinetics of amyloid fibril formation

Associated with neurodegenerative disorders such as Alzheimer, Parkinson, or prion diseases, the conversion of soluble proteins into amyloid fibrils remains poorly understood. Extensive "in vitro" measurements of protein aggregation kinetics have been reported, but no consensus mechanism has emerged until now. This contribution aims at overcoming this gap by proposing a theoretically consistent crystallization-like model (CLM) that is able to describe the classic types of amyloid fibrillization kinetics identified in our literature survey. Amyloid conversion represented as a function of time is shown to follow different curve shapes, ranging from sigmoidal to hyperbolic, according to the relative importance of the nucleation and growth steps. Using the CLM, apparently unrelated data are deconvoluted into generic mechanistic information integrating the combined influence of seeding, nucleation, growth, and fibril breakage events. It is notable that this complex assembly of interdependent events is ultimately reduced to a mathematically simple model, whose two parameters can be determined by little more than visual inspection. The good fitting results obtained for all cases confirm the CLM as a good approximation to the generalized underlying principle governing amyloid fibrillization. A perspective is presented on possible applications of the CLM during the development of new targets for amyloid disease therapeutics.

From nanoscale liquid spheres to anisotropic crystalline particles of tin: decomposition of decamethylstannocene in organic solvents

Routes are presented for synthesizing nano- and mesostructured β-tin particles in the form of monocrystalline spheres, cubes, and bars, as well as polycrystalline rods and needles, by the decomposition of decamethylstannocene in organic solvents under various conditions. The formation of the observed shapes is based on the presence of liquidlike and of partly crystalline droplets. These particle stages allow structure-determining processes such as entire coalescence, oriented superficial coalescence or superficial induced crystallization. Entire coalescence and oriented superficial coalescence take place in the absence of surfactants; the superficially induced crystallization occurs in the presence of ionic additives. The observed tin morphologies depend on the competition between droplet growth and crystallization behavior. The different tin particles are investigated by electron microscopy (SEM, TEM, HRTEM), selected area electron diffraction (SAED), and differential scanning calorimetry (DSC).

In situ XAFS experiments using a microfluidic cell: application to initial growth of CdSe nanocrystals

The design and performance of a compact fluorescense XAFS apparatus equipped with a microfluidic cell for in situ studies of nanoparticles are described. CdSe nanoparticles were prepared by solution reaction starting from trioctylphosphine-Se. Time-resolved experiments were performed by precisely controlling the reactor coordinates (x,y), allowing the synchrotron X-ray beam to travel along a reactor channel, covering nucleation and initial growth of nanoparticles. Detailed analysis of EXAFS data combined with UV-vis spectra allow reliable estimation of particle size and density in the initial growth that cannot be accessible by conventional optical techniques based on a long-range order. The Se K-XANES spectra are interpreted by multi-scattering calculations providing bond formation kinetics consistent with the EXAFS data.

Uncovering molecular processes in crystal nucleation and growth by using molecular simulation

Exploring nucleation processes by molecular simulation provides a mechanistic understanding at the atomic level and also enables kinetic and thermodynamic quantities to be estimated. However, whilst the potential for modeling crystal nucleation and growth processes is immense, there are specific technical challenges to modeling. In general, rare events, such as nucleation cannot be simulated using a direct "brute force" molecular dynamics approach. The limited time and length scales that are accessible by conventional molecular dynamics simulations have inspired a number of advances to tackle problems that were considered outside the scope of molecular simulation. While general insights and features could be explored from efficient generic models, new methods paved the way to realistic crystal nucleation scenarios. The association of single ions in solvent environments, the mechanisms of motif formation, ripening reactions, and the self-organization of nanocrystals can now be investigated at the molecular level. The analysis of interactions with growth-controlling additives gives a new understanding of functionalized nanocrystals and the precipitation of composite materials.

Direct observation of transient Ostwald crystallization ordering from racemic serine solutions

The Ostwald rule of stages describes the conjectured transitioning through metastable polymorphic crystal structures during crystallization. Direct observation of the Ostwald rule of stages using was performed using solutions of simple amino acids by second-order nonlinear optical imaging of chiral crystals (SONICC). SONICC, which is based on second-harmonic generation (SHG) imaging, enabled detection of homochiral microcrystals that survived only a few seconds before being converted to the more stable SHG-inactive polymorphic forms.