System-density fluctuations and electro-dissociation of methane clathrate hydrates in externally-applied static electric fields

Abstract

Non-equilibrium molecular-dynamics (NEMD) simulations of bulk methane clathrate hydrates have been conducted in a range of externally-applied static electric fields of up to 2.0V·nm−1 in intensity, at 250K and 60atm. Studies into frequencies of system-mass-density fluctuations showed that these clathrates have two major modes: the dominant one is attributable to water molecules’ librations and occurs at 720cm−1, regardless of applied fields. A more minor global density fluctuation arises at 10–12cm−1, due to the propagation of local-density fluctuations; again, this is independent of applied fields. A threshold intensity of 1.2V·nm−1 was necessary to overcome the strong clathrate hydrogen-bonding network to produce any appreciable structural changes. Aside from variations in hydrate system density per se, a key interest in this study was (electro-) dissociation; a number of analysis methods were used to gauge this, including system-density and configurational-energy studies (and their respective autocorrelation functions and corresponding Fourier transforms), as well as radial distribution functions (RDFs) and density of states (DOS). In terms of electro-dissociation itself, a ‘plateau’ intensity of 1.6V·nm−1 led to outright dissociation over the 0.5ns timescales probed in the current study, where any field intensity above this level produced essentially identical dissociation outcomes: a marked loss of hydrate structure and increase in system configurational energy. RDF analysis of electro-dissociation indicates the collapse of the host lattice towards an amorphous structure and concomitant release of methane molecules from their now-collapsed cages to form a ‘nano-bubble’. Upon post-dissociation field removal, it was found that this process was irreversible: the system transitions to an entirely new, less dense structure, featuring a phase-segregated methane nano-bubble within a liquid-like aqueous phase.