The phase transition via homogeneous nucleation is a fundamental process and plays important roles in many areas of science and technology, however, serious unrealiability remains in model predictions for nucleation rates. We recently performed direct, large molecular dynamics (MD) simulations of some homogeneous nucleation processes: vapor-to-liquid nucleation with (1-8)×109 Lennard-Jones (LJ) atoms (or 4×106 water molecules), and liquid-to-vapor nucleation with 5×108 LJ atoms. These large system sizes allow us to measure extremely low and accurate nucleation rates. Our MD simulations of argon vapor-to-liquid nucleation succeeded in quantitatively reproducing the nucleation rates obtained in recent laboratory experiments at the same pressures and temperatures. It is also possible to determine the formation free energy of clusters over a wide range of cluster sizes from measurements of the cluster size distribution and to test the nucleation theory from the precise comparisons. Our results indicate that the classical nucleation theory needs updates in the surface energy of nano-sized clusters, the sticking probability, and the prefactor in the nucleation formula.
We believe that elucidation of the role of hydrated layer on the surface of crystals and growth units, and incorporation of significances of nanoscale materials are the keys to understand nucleation processes. We are trying to observe at the moments of nucleation in nanoscale using advanced transmission electron microscopies (TEM) for long years. Here, we report results of nucleation of sodium chlorate as an example from an ionic liquid instead of water as a solvent. Ionic liquid has negligible vapor pressure and is not charged up by electron-beam irradiation due to its relatively higher electrical conductivity and, therefore, has a great potential to study crystallization using TEM. Nevertheless, observation of nucleation processes is still very difficult even used ionic liquids. To overcome the difficulties, the nucleation processes have been expected from the results of the opposite process, i.e., by observation of dissolution processes. We successfully found the concurrent formation of two polymorphic crystals without the contribution of an amorphous intermediate stages at the equilibrium concentration.
Recent studies for the early stages of crystal formation from aqueous solution have shown that crystals grow not only by the monomer-by-monomer addition mechanisms described in the classical nucleation theory, but also by the aggregation of particles, ranging from multi-ion complexes to fully formed nanocrytals. These particle-based growth processes have been considered as “non-classical” crystallization pathways and attracted research especially in the field of biomineralization for the last 20 years, however physical and chemical understanding for these processes is still lacking. Here we review literature reports on this mechanism to provide a general view for this new concept.
This review paper describes the structure of amorphous calcium carbonate (ACC), which we recently investigated by means of molecular dynamics simulation. Our simulation suggested that the structure of ACC resembled that of vaterite. However, adding Mg2+ ions disrupted the formation of the vaterite-like structure in ACC. Moreover, the structure of ACC in the presence of Mg2+ ions approached that of monohydrocalcite when the concentration of H2O molecules was high. The relationship between the simulated structure of ACC and the structure of calcium carbonate crystals nucleated through the formation of ACC particles in real systems is discussed. In addition, the relationship between the present simulation results and the formation of aragonite thin-film on the matrices of polyvinyl alcohol in the presence of Mg2+ ions is also discussed.
Dynamical observations for microscopic supersaturated solutions are an innovative attempt for nucleation or aggregation process from solutions. In order to observe molecular behaviors in solutions, time resolved and localized observation is required for presumably highly molecular dynamic entities. This paper aims to introduce commentary for the methodology developments as a novel approach for time-resolved dynamical observations of solute molecular behaviors in supersaturated solutions. As part of our efforts of studying the epoch-making method, we present the nano-scale rotational dynamics with micro-second time scale resolution for inorganic supersaturated solution by Diffracted X-ray Tracking (DXT) and translational dynamics with milli-second time scale observations by Dark-field Microscopy (DFM). Furthermore, we introduce X-ray Scattering experiments results in small and wide angle region for supersaturated solution to discuss the relationship between local dynamics and solution structures.
Chiral asymmetry induced by circularly polarized light(CPL)-chiral matter interaction has attracted many researchers on homochirality. Although sodium chlorate (NaClO3) chiral crystallization, in which achiral solutes acquire chirality during its crystallization, has been used as a model compound representing chiral phenomena, statistically-significant chiral asymmetric state in the chiral crystallization has not been achieved by CPL irradiation. Here we show that the chiral asymmetric state can be provoked by inducing chiral nucleation via laser trapping of plasmonic Ag nano-aggregates using a continuous-wave visible circularly polarized laser. In addition, we reveal a pathway indicating “newborn crystals are ambidextrous.” by in-situ observation of early stage of the laser-induced chiral crystallization in contrast with a stereotypical view that “the crystal handedness is already determined at nucleation.”, which is based on the picture of the classical nucleation theory.