Dissolving oil in water

Researchers have used a combination of calculations and experimental techniques to prove that at high pressure oil can dissolve in water, but water cannot dissolve in oil.

Everyone knows that water and oil are chemically insoluble, they don’t mix. That's true at the molecular level, but it’s always possible to mix them, as in milk or French Dressing, as an emulsion of small droplets. Even in an emulsion, the droplets contain many millions of a single type of molecule. To determine whether a homogeneous sample is an emulsion or a solution requires information about which molecules are next to which.

Information about this can be obtained by neutron scattering - a method where a beam of neutron particles is deflected in characteristic ways by different atoms and their neighbours. But, for a molecular mixture, the neutrons scattered from the different atom types are all mixed together.  This is an example of a so-called "inverse problem", where the only way to proceed is to make a model for the system, and see if it fits the data. Although there isn't enough information in the data to solve the structure, there is enough data to reject most wrong models.

Even in an emulsion, molecules on the surface of a droplet are next to molecules on the other type e.g. in a cube made of one thousand small cubes almost half are on the surface. In real emulsion droplets it is less than a millionth. So the model structure must be made and analysed very carefully to distinguish between a fine emulsion and a true solution. We solved the problem of how to do this, and applied it to a mixture of methane (the simplest oil) and water, with remarkable results.

Under normal conditions, there is demixing - distinct water and methane droplets.  But as pressure increases, the methane dissolves in the water droplets, but the water does not dissolve in the methane. The model demonstrates that at the molecular level this can be understood in terms of chemical bonding. Water is held together by a network of hydrogen bonds, so each water molecule has only four nearest-neighbours. This forms an open network which can restructure to accommodate methane molecules creating a solution denser than either water on methane - high density is favoured at high pressure. By contrast, fluid methane forms many, weaker, bonds between molecules: there is no way to create space for dissolved water without breaking bonds.