(5j) Molecular Simulations of Nucleation and Adsorption

In this poster, I will be presenting my work that I had done during my stint as a graduate student as well as a post-doctoral fellow. My research work has centered on the use of molecular simulations to study the phenomena of nucleation and adsorption.

Nucleation is one of the primary mechanisms of a first order phase transition. Examples include condensation, vaporization, crystallization etc. In spite of the importance of nucleation, our understanding of nucleation remains incomplete. Although we have a qualitative understanding of the mechanism of nucleation, no satisfactory theory exists for accurately predicting the nucleation rate. This is because nucleation is an activated process and its rate depends on the nature and properties of the critical nucleus. Direct observation of the critical nucleus is very difficult since (i) it is microscopic is size consisting of few hundreds of particles, (ii) its formation is very rare (typically 10¹ to 10⁶ nuclei per cm³ per second) and (iii) its lifetime is very short. However, these characteristics of the critical nucleus make it an interesting problem to study via molecular simulations. I have studied nucleation via Monte Carlo simulations for the case of liquid to vapor transition (bubble nucleation) and liquid to solid transition (crystallization). The chief aim of molecular simulations is to study the thermodynamic properties of the critical nucleus.

One of the main challenges of studying bubble nucleation is coming up with a molecular level definition of a bubble inside a liquid. Unlike the well studied case of vapor to liquid nucleation where there are several generally agreed upon criteria for definition of a liquid cluster, there is no readily apparent physical microscopic definition of the bubbles that participate in the liquid to vapor transition. My study of superheated liquids, which are metastable, using Monte Carlo simulations have demonstrated the existence of a cavity with a certain maximum size such that presence of larger cavities creates an instability leading to spontaneous phase transition to vapor phase[1]. This cavity is called a critical cavity. I then confirmed their existence through density functional theory (DFT) studies of cavities in superheated liquid[2]. I have shown that critical cavity represents a true thermodynamic instability. Also more importantly, work of critical cavity formation was shown to be a tight upper bound to work of forming critical bubble and radius of critical cavity is a lower bound to radius of critical bubble. This relationship provides a new insight into the molecular mechanism of bubble nucleation and represents a step forward in coming up with a definition of the critical bubble.

My study of crystallization focuses on nucleation during crystallization of binary mixtures of supercooled liquids[3]. Liquid mixtures are interesting because they are generally harder to crystallize than a pure liquid. They also show much more variety in phase behavior as compared to pure fluids. Their phase diagrams vary from simple spindle shaped structure to complex ones showing azeotropes, eutectic points, peritectic points, compound solid formation etc. Thus under varying compositions a liquid mixture can form crystals with that not only vary in composition but also in structure. I have performed Monte Carlos simulations of crystal nucleation to study the effect of these complex phenomena on the nucleation barrier and structure of critical nucleus. Our calculations indicate that fractionation of species upon crystallization increases the difficulty of crystallization of fluid mixtures and in absence of fractionation (azeotropic conditions) the nucleation barrier is comparable to pure fluids. We have also studied crystal nucleation in liquid mixtures that can form substitutionally ordered solids. In such systems which also show solid-solid phase separation, we find that the phase that nucleates is the one whose equilibrium composition is closer to the composition of the fluid phase.

My work on adsorption involves the study of liquid phase adsorption of linear alkanes in zeolites via molecular simulations. Molecular simulations have become an important tool for understanding adsorption in zeolites. However, bulk of molecular simulations of adsorption have focused mainly on adsorption from the gas phase. Liquid phase adsorption differs from gas phase adsorption due to the high density of the adsorbates inside the zeolites which results in the intermolecular forces among adsorbate molecules playing a significant role in the adsorption mechanism. Although in principle, grand canonical Monte Carlo (GCMC) simulation of an adsorbed phase in contact with a liquid or gas phase is similar, there are several challenges in practice. As mentioned above, liquid phase adsorption is characterized by high density of adsorbates inside the zeolite. Hence insertions and deletions of molecules become inefficient via convention GCMC techniques resulting in poor sampling averages. Recently, De Meyer et al[4] have shown that through the use of proper techniques such as configurational biasing and identity swaps which increase the sampling efficiency, GCMC simulations can be applied to liquid phase adsorption of n-alkanes in silicalite. The n-alkanes were modeled as chain molecules using a united atom forcefield. I have applied these techniques to study adsorption of alkanes in "cage-like" Zeolite A (LTA-5A), Zeolite X (NaX) and Zeolite Y (NaY). The results such as molecular comformations and ordering have been compared and contrasted with the "channel-like" structure of silicalite. In addition, I have also studied the adsorption of linear alpha-olefins in silicalite and compared the results with alkanes. Energy efficient separation of alkane/olefin mixtures is a holy grail of separations. Here we analyze the molecular-level conformations, siting and packing of the chains to understand how these factors affect the macroscopic adsorption and selectivity.

1. S. Punnathanam and D. S. Corti, "Homogeneous bubble nucleation in stretched fluids: Cavity formation in the superheated Lennard-Jones liquid", Ind. Eng. Chem. Res., 41, 1113-1121, 2002

2. S. Punnathanam and D. S. Corti, "Cavity formation in the superheated Lennard-Jones liquid and its connection to homogeneous bubble nucleation: A density-functional theory study", J. Chem. Phys., 119, 10224-10236, 2003

3. S. Punnathanam and P. A. Monson, "Crystal Nucleation in Binary Hard Sphere Mixtures: A Monte Carlo Simulation Study", J. Chem. Phys., (in press)

4. K. M. A. DeMeyer, S. Chempath, J. F. M. Denayer, J. A. Martens, R. Q. Snurr and G. V. Baron, "Packing effects in the liquid-phase adsorption of C5-C22 n-alkanes on ZSM-5.", J. Phys. Chem. B, 107, 10760-10766, 2003