Characterization of nucleation of methane hydrate crystals: Interfacial theory and molecular simulation.

HYPOTHESIS: Solutions of water and methane gas at favorable thermodynamic conditions lead to the formation of crystalline methane hydrates. In natural and industrial environments, the nucleation process might occur in the solution's bulk or at the solid-liquid and liquid-gas interfaces, which evolve into distinct morphologies. A complete molecular level understanding and material characterization of preferred nucleation sites and morphologies is required to inhibit or promote crystallization, as required. METHODOLOGY: Computational simulations are utilized in this work in combination with analytical theory to calculate the supersaturation and interfacial tension as the driving force and suppressor, respectively, in the hydrate crystal formation process. We employ accurate molecular dynamics (MD) techniques to obtain critical thermodynamic and mechanical properties, and subsequently, analyze the formation using the classical nucleation theory (CNT). FINDINGS: We report the interfacial tension at all possible interfaces in water-methane gas solutions. We apply both our direct numerical simulation method and Antonow's rule to find the tension at the methane hydrate and gas interface, and importantly conclude that Antonow's rule overestimates the values. We calculate the work of formation and nucleation rate of the methane hydrate with and without additives. The nucleation probabilistically forms in the ranked order of film-shaped, cap-shaped, lens-shaped, and homogeneous. We postulate that the premelting of hydrate crystals at the hydrate-gas interface creates an intermediate quasi-liquid layer, which works in favor of the lens-shaped formation compared to homogeneous cases. However, the subtle difference in surface energy indicates high concentration of water and gas molecules at the interface is the main reason behind lens-shaped clustering. We lastly show that ice properties cannot be used to approximate the hydrate formation work.

Confined semiflexible polymers suppress fluctuations of soft membrane tubes.

We use Monte Carlo computer simulations to investigate tubular membrane structures with and without semiflexible polymers confined inside. At small values of membrane bending rigidity, empty fluid and non-fluid membrane tubes exhibit markedly different behavior, with fluid membranes adopting irregular, highly fluctuating shapes and non-fluid membranes maintaining extended tube-like structures. Fluid membranes, unlike non-fluid membranes, exhibit a local maximum in specific heat as their bending rigidity increases. The peak is coincident with a transition to extended tube-like structures. We further find that confining a semiflexible polymer within a fluid membrane tube reduces the specific heat of the membrane, which is a consequence of suppressed membrane shape fluctuations. Polymers with a sufficiently large persistence length can significantly deform the membrane tube, with long polymers leading to localized bulges in the membrane that accommodate regions in which the polymer forms loops. Analytical calculations of the energies of idealized polymer-membrane configurations provide additional insight into the formation of polymer-induced membrane deformations.