Squeezing Oil into Water under Pressure: Inverting the Hydrophobic Effect

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Physics

Background:

• Understanding fluid mixtures is crucial for various chemical and geological processes.
• The behavior of water-methane mixtures under pressure is not fully understood.
• Hydrophobicity is a key concept in chemistry, but its behavior in complex mixtures needs further investigation.

Purpose of the Study:

• To determine the molecular structure of dense homogeneous fluid water-methane mixtures.
• To investigate the pressure-dependent hydration of methane in water.
• To elucidate the microscopic mechanisms of mixing in water-methane systems.

Main Methods:

• High-pressure neutron-scattering techniques were employed to study water-methane mixtures.
• Ab initio molecular dynamics simulations were performed to complement experimental data.
• Structural properties and phase behavior were analyzed at varying pressures (1.7 and 2.2 GPa).

Main Results:

• A fully hydrogen-bonded water network was observed in the mixed state.
• Methane's hydration shell showed pressure dependence, with increased water coordination at higher pressures.
• Simulations reproduced the phase transition from separation to mixing and observed structural properties.
• Mixing was accompanied by an unexpected enhancement in methane polarization.

Conclusions:

• High pressure induces mixing in water-methane fluids, altering methane's hydration shell.
• Electronic effects, specifically methane polarization, play a significant role in miscibility.
• The fundamental understanding of hydrophobicity needs to be revised to include these electronic effects in complex mixtures.