Valeria Molinero of the University of Utah is supported by an award from the Chemical Structures, Dynamics and Methods program in the Chemistry Division for a computational study aimed at elucidating microscopic pathways by which clathrate hydrates nucleate and grow from aqueous solutions and from ice. The project combines the power of advanced simulation methods to sample rare nucleation events with atomistic and coarse-grained molecular dynamics simulations. The project tests a transformative hypothesis: that a monatomic model of water with very short-ranged interactions and without hydrogen atoms but able to form the characteristic tetrahedral ?hydrogen-bonded" configurations of water, produces nucleation and grow of clathrates following the same pathways than fully atomistic water models. If this method is validated, it will permit the study of molecular mechanisms at less than 1% of the computational cost. The study is the first to determine the molecular mechanisms of formation and structure of the nuclei for clathrates containing large guest and hydrophilic guests and two distinct guests (double clathrates). It is the first project to investigate the mechanisms of nucleation and growth of clathrates from ice and the structures of the ice/clathrate and ice/guest interfaces using simulations.
Clathrate hydrates hold high promise as an abundant energy source, for storage and transportation of natural gas and hydrogen, and for sequestration of carbon dioxide. A prominent challenge for these applications is the slow kinetics of formation of gas hydrate clathrates. On the other hand, clathrate plugs form in oil and gas pipelines, causing enormous economic losses and safety concerns. A key impediment to retard or hasten the crystallization of clathrates is the lack of understanding of the molecular mechanisms by which they nucleate and grow. A professor from Westminster College, an undergraduate institution, is participating in this project, as well as undergraduates and graduate students. The PI and her research group also participate in an outreach program to high schools called "The Leo on Wheels" of the Utah Science Center.
Gas clathrate hydrates are crystalline solids of water and small gas molecules, like methane, which typically do not dissolve in liquid water. Clathrate hydrate crystals resemble ice, but they contain a higher density of methane than compressed natural gas. The high density of gas molecules in clathrate hydrates makes them attractive for the storage and transportation of natural gas. Moreover, gas clathrate hydrates occur naturally in the permafrost and the ocean floor and are the most abundant source of fossil fuels on our planet. Clathrates can also form, unwanted, on oil and gas pipelines, resulting in high economic losses and safety issues. The failure of the first containment dome in the 2010 Deep Horizon oil spill in the Gulf of Mexico is a prominent example of damage induced by clathrate hydrate-related blockage. Therefore, methods to prevent the nucleation or growth of gas clathrates crystals under pipeline operating conditions are much sought after. On the other hand, the slow kinetics of formation of clathrate hydrates poses a challenge for the synthesis of clathrates for storage and transportation of gas. The aim of this project was to provide a molecular understanding of the mechanisms of hydrate formation and decomposition, which are of paramount importance for the ability to prevent as well as promote hydrate growth. Intellectual Merit A seminal contribution of this project has been the elucidation of a novel multistep mechanism for the homogeneous nucleation of clathrate hydrates. The mechanism explains how do clathrates crystals of hydrophobic guests form although water and hydrophobic molecules do not mix. The mechanism we unraveled for clathrate formation involves amorphous precursors and has direct analogies to the mechanisms of crystallization of proteins, colloids and nanoparticles. Our work suggests that the commonality with these systems resides in the existence of a high temperature metastable amorphous phase in the phase diagram of clathrates and these substances. A second, transformative contribution from the work performed under this award is the demonstration that a monatomic water model without hydrogen atoms or electrostatic interactions, and which is about 180 times computationally more efficient that atomistic models, correctly represents the structure and the thermodynamics of clathrate hydrates and hydrophobic hydration, and the mechanisms of nucleation and growth of clathrate hydrates. The models and algorithms developed in this project are now used by a wide community of researchers. Other important research achievements of the project include: Determined the size and crystallinity of critical clathrate nuclei in a wide range of temperatures. Developed the first set of order parameters that can distinguish among clathrate crystal polymorphs, amorphous clathrates and blob precursor, and the first algorithm that identifies hexagonal ice, cubic ice, clathrate hydrates, and liquid water. Determined that the guest occupancy of binary hydrates can be tuned by selecting the growth conditions. Our results have implications for storage of hydrogen, carbon dioxide and methane. Established that the nucleation of clathrates in the presence of a growing ice boundary layer occurs through a homogeneous mechanism: the increasing gas concentration at the ice boundary, and not the ice surface itself is responsible for the nucleation of the clathrates. Provided an explanation for the long-standing puzzling relationship between water activity and freezing temperature of water in solutions. Characterized the ice-clathrate interface and ice/gas interface under conditions of methane clathrate formation. We conclude that that the ice surface is not particularly effective at increasing the rate of nucleation of clathrate hydrates. Determined the conditions and mechanisms of cross-nucleation between clathrate polymorphs and the structure of the interface between hydrates. Demonstrated that quasicrystals derived from clathrate hydrates can have stability close to the most stable crystal phase. Broader Impacts This project contributed to the education and training of the scientific workforce and to establish collaborations between the University of Utah and Westminster College, an undergraduate college in Salt Lake City. Four graduate students, five undergraduate students (three of them from Westminster College), and one high school student performed research in this project under the supervision of PI Molinero. The results of this work were disseminated to the scientific community through two completed Ph.D. thesis, 16 peer-reviewed publications, and 73 conferences, workshops and seminars presentations (32 by students). Results of this project have been incorporated into classes taught by PI Molinero at the University of Utah, and disseminated to high school students and to the general public through public lectures, through videos that illustrate molecular processes in water, ice and clathrates for The Leo on Wheels, a program of The Leonardo (a contemporary museum of art and science in Salt Lake City) that reaches annually over 14000 students in Utah, through the development of materials for the Water Exhibit at The Leonardo, and through a workshop on the nucleation and growth of crystals using 3D visualization that was presented annually to high school students at the University of Utah.
