Aqueous interfaces, electrolytes and microdroplet chemistry

6 problems

• Molecular mechanism of the Hofmeister series

  Contested

  Since 1888 it has been known that salts fall in a fixed order, the Hofmeister series, in how strongly they make proteins precipitate. Why ions of the same charge rank this way is still debated.

• Magnitude of the electric field at water-hydrophobic interfaces

  Open

  Some explanations of fast droplet chemistry need very strong electric fields right at the water surface. Measuring how strong these fields really are would test those explanations.

• Mechanism of reaction acceleration in microdroplets

  Contested

  Reactions in sprayed droplets are often reported to run much faster than in a flask. Proposed reasons include strong electric fields at the surface, partial drying of reactants and evaporation that concentrates them.

• Spontaneous hydrogen peroxide formation in pure water microdroplets

  Contested

  Some experiments report that spraying pure water into tiny droplets makes hydrogen peroxide with no added chemicals. Other experiments trace the peroxide to ozone from air or to contact with surfaces.

• First-principles prediction of salt solubility and activity in water

  Open

  How much table salt dissolves in water, and how strongly the dissolved ions interact, are basic measured numbers. Computer models of salt water still fail to reproduce them without being fitted to them.

• Is the neat water surface acidic or basic?

  Contested

  Air bubbles and oil drops in pure water move in an electric field as if negatively charged, which suggests hydroxide ions gather at the surface. Spectroscopy and simulations instead point to hydronium ions there.