Abstract
It has recently been discovered that, when subjected to moderate amounts of pressure, methane dissolves in water to form binary mixtures of up to 40% molar methane. No significant solubility of water in methane is known. In these mixtures, the water hydrogen-bond network is largely complete and surrounds the methane molecules. The discovery of this dense mixture has once again highlighted the technical difficulties involved in accurately describing and sampling mixing phenomena both computationally and experimentally. Here, we present a systematic and critical study of the methods employed to characterize binary mixtures and their robustness. This study highlights the requirements needed to develop a quantitative understanding, and it proposes new and more accessible measures of miscibility to investigators, particularly for in silico analysis.
Highlights
The study of binary fluid mixtures has a long history dating back to the beginning of thermodynamics and chemistry.1 The initial development of both fields was mainly driven by technological motivations.2 Among binary mixtures, the methane–water system is important as its constituents are abundant in nature3,4 and as a model system for the study of the interplay between hydrophobic and hydrophilic interactions
We present a systematic and critical investigation of the molecular dynamics (MD) and empirical potential structure refinement (EPSR) methods previously deployed for the study of methane–water mixtures and assess their shortcomings and robustness
To gauge the effect that the system size has on the obtained results and if they permit a determination of whether a system is in a mixed or demixed state, we employed classical MD simulations, which demix the liquid system at this pressure range with standard potentials
Summary
The study of binary fluid mixtures has a long history dating back to the beginning of thermodynamics and chemistry. The initial development of both fields was mainly driven by technological motivations. Among binary mixtures, the methane–water system is important as its constituents are abundant in nature and as a model system for the study of the interplay between hydrophobic and hydrophilic interactions. At higher pressures, this behavior changes so that the solubility of methane in water begins to increase at 1.3 GPa and reaches a plateau at ∼40 mol. This behavior, though completely unexpected, may be regarded as mirroring the behavior of the solids: At these pressures, methane and water readily form solid hydrates.. This behavior, though completely unexpected, may be regarded as mirroring the behavior of the solids: At these pressures, methane and water readily form solid hydrates.12,13 Experiments have found these hydrates to be stable up to 150 GPa and 300 K.14 At higher pressures, this behavior changes so that the solubility of methane in water begins to increase at 1.3 GPa and reaches a plateau at ∼40 mol. % at 2 GPa. This behavior, though completely unexpected, may be regarded as mirroring the behavior of the solids: At these pressures, methane and water readily form solid hydrates. Experiments have found these hydrates to be stable up to 150 GPa and 300 K.14
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