Hofmeister effects in micromolar electrolyte solutions

Area of Science:

• Physical Chemistry
• Surface Science
• Ion Hydration

Background:

• Ions influence aqueous interfaces via specific (Hofmeister) and non-specific (Coulomb) effects.
• The origins of specific ion effects (SIE) remain unclear due to complex interactions.
• Coulomb effects diminish significantly at concentrations below 10 μM (>50 nm ion separation).

Purpose of the Study:

• To investigate the existence and nature of SIE at previously unexplored low concentrations (<1 μM).
• To determine if SIE persist beyond the range of electrostatic interactions.

Main Methods:

• Utilized online electrospray mass spectrometry to measure relative iodide/bromide surface populations on aqueous jets.
• Varied concentrations of electrolytes (XCl and NaY) to probe ion interactions.

Main Results:

• Detected specific ion effects (SIE) at concentrations as low as ~0.1 μM for all tested electrolytes.
• Observed preferential suppression of iodide/bromide by co-ions with matching interfacial affinities.
• Found that these effects occur below the typical range of electrostatic interactions.

Conclusions:

• Specific ion effects (SIE) operate at significantly lower concentrations than previously thought.
• The observed phenomena suggest dynamical processes are responsible for SIE at low concentrations.
• These findings necessitate a re-evaluation of the mechanisms underlying ion-interface interactions.