CDI-Type II: From Simulation Data to Mechanistic Understanding: Applications to Clathrate Hydrate Nucleation Mechanisms

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Amadeu Sum and David Wu, Colorado School of Mines (CSM), Gregg Beckham, CSM and National Renewable Energy Laboratory (NREL), Baron Peters , University of California, Santa Barbara and Valeria Molinero, University of Utah are supported by a CDI-TYPE II award to develop novel path sampling methods, based in statistical mechanics and information theory in order to collect and analyze data from large scale molecular simulations. These new techniques are being developed with the ultimate goal of elucidating the mechanism of clathrate hydrate nucleation. The specific aims are 1) to develop order parameters to identify ice and clathrate hydrate structures; 2) develop the Packet Evolution Path Sampling (PEPS) method; and 3) apply PEPS to simpler systems to demonstrate the utility of the method. The order parameters are a key component this effort as they provide physical insight and quantify the mechanisms of nucleation. The new sampling method (PEPS) provides a more efficient method of sampling and extracting mechanistic details on nucleation by harvesting the evolution along a 'reaction coordinate.' Lennard-Jones liquid-to-solid nucleation and water-to-ice nucleation are the testing systems to demonstrate the first application of the methods which will then be applied to clathrate hydrates nucleation. The methods themselves, however, are generally and may be used to gain understanding from molecular simulations of a variety of complex phenomena.Understanding transition mechanisms in real systems is a daunting problem for molecular simulation, both in terms of efficient data collection and analysis. There is a need for transformative new methods that can be applied to realistic systems and huge data sets that are now commonplace with increasing computing power. One specific motivating problem where new path sampling methods can be applied is to the mechanism of clathrate hydrate nucleation. Clathrate hydrates, ice-like compounds formed from hydrogen-bonded water cages surrounding small non-polar molecules such as methane or carbon dioxide, have gained interest for their importance in energy (hydrates are present at the ocean floor in quantities estimated to be at least twice that of known oil and gas reserves; conversely, plugging of pipelines by gas hydrate formation poses the primary flow assurance problem to the oil and gas industry, as well as an impedance in the containment of a deepwater oil/gas blowout) and the environment (carbon sequestration and methane release from natural hydrate deposits). In a broader context for the application to hydrate nucleation, fundamental knowledge will be gained in nucleation theory, understanding arising from hydrophobic and hydrophilic interactions, and new approaches for structural quantification. Insight into the molecular mechanism for hydrate nucleation may be transformative because it will be constructing knowledge from microscopic interactions to macroscopic behavior, as opposed to current approaches where the reverse is predominant.