Concentrated electrolytes, ion correlations and confined water

5 problems

• Origin of the anisotropic dielectric response of nanoconfined water

  Open

  Water squeezed into a gap about a nanometer wide barely responds to an electric field across the gap, as if it were oil, yet it responds enormously to a field along the gap. A single microscopic explanation of both facts is missing.

• Why like-charged particles attract in monovalent salt solutions

  Contested

  Objects with the same electric charge should repel, and standard theory of salt solutions forbids attraction when the salt ions carry a single charge. Yet experiments repeatedly report like-charged colloids (microscopic suspended particles) attracting in such solutions.

• Do ions cross angstrom-scale slits as field-dissociated ion pairs?

  Open

  In channels only a few atoms high, the electric attraction between positive and negative ions becomes much stronger because the surrounding water and walls shield it poorly. Theory predicts ions then travel in bound pairs that an applied voltage separates, giving unusual memory-like electrical behavior.

• Is the underscreening scaling exponent 3 universal across electrolytes?

  Contested

  Experiments suggest one simple power law describes how the shielding distance grows for many different concentrated salts. Whether the power is really the same number everywhere, and what that number is, would strongly constrain any explanation.

• Why measured screening lengths grow in concentrated electrolytes

  Contested

  In dilute salt water, ions shield electric charges over a distance that shrinks as more salt is added. Force measurements show that in very concentrated salt solutions and liquid salts this shielding distance grows again and becomes surprisingly long, which theory and simulations mostly fail to reproduce.