Abstract
The presence of a metastable fluid-fluid critical point is thought to dramatically influence the crystallization pathway, increasing the nucleation rate by many orders of magnitude over the predictions of classical nucleation theory. We use molecular dynamics simulations to study the kinetics of crystallization in the vicinity of this metastable critical point and throughout the metastable fluid-fluid phase diagram. To quantitatively understand how the fluid-fluid phase separation affects the crystal nucleation, we evaluate accurately the kinetics and reconstruct the thermodynamic free-energy landscape of crystal formation. Contrary to expectations, we find no special advantage of the proximity of the metastable critical point on the crystallization rates. However, we find that the ultrafast formation of a dense liquid phase causes the crystallization to accelerate both near the metastable critical point and almost everywhere below the fluid-fluid spinodal line. These results unveil three different scenarios for crystallization that could guide the optimization of the process in experiments
Highlights
The presence of a metastable fluid-fluid critical point is thought to dramatically influence the crystallization pathway, increasing the nucleation rate by many orders of magnitude over the predictions of classical nucleation theory
The ratio b/a determines whether the system has a metastable liquid phase[8,21,22], and for this choice of parameters the metastable critical point is at Tc = (0.3916 ± 0.0005)U0/kB, ρc = (0.523 ± 0.005)(1/a3), and Pc = (0.0519 ± 0.0005)U0/a3, with Tc/Tm 0.64 at ρc, where Tm is the temperature of sublimation[16]
As a further proof that within the spinodal region the nucleation process is regulated by the appearance of the liquid phase, we find that the crystal nucleation rate I is proportional to the number of liquid-like molecules —as one would expect if crystallization is taking place in the liquid phase—rather than to the total number of molecules in the system, as would be the case for crystallization directly from the vapor phase
Summary
The presence of a metastable fluid-fluid critical point is thought to dramatically influence the crystallization pathway, increasing the nucleation rate by many orders of magnitude over the predictions of classical nucleation theory. We find that the ultrafast formation of a dense liquid phase causes the crystallization to accelerate both near the metastable critical point and almost everywhere below the fluid-fluid spinodal line. These results unveil three different scenarios for crystallization that could guide the optimization of the process in experiments. Crystallization is a very important phenomenon that usually proceeds via the nucleation and subsequent growth of nanometer-sized crystallites that form spontaneously out of a supersaturated solution or undercooled melt[1,2,3,4,5] This process does not occur immediately below the melting line.
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