Long-range specific ion-ion interactions in hydrogen-bonded liquid films

Area of Science:

• Physical Chemistry
• Surface Science
• Ion Spectroscopy

Background:

• Anions are known to specifically populate fluid interfaces.
• Understanding these interfacial anion behaviors is crucial for various chemical and physical processes.

Purpose of the Study:

• To investigate the specific interactions of anions at hydrogen-bonded interfaces.
• To determine the distance over which these interactions occur and the factors influencing them.

Main Methods:

• Experiments were conducted on free-standing binary electrolyte films (Na(+), X(-)∕Y(-)) produced by gas-blown drops.
• Film composition was analyzed as a function of solvent, thickness, and third ion additions.
• In situ detection of excess anions in fragmented sub-micrometer droplets was performed using online electrospray ionization mass spectrometry.

Main Results:

• Larger anions were enriched in thinner films across all tested solvents (water, methanol, 2-propanol, acetonitrile).
• Third ions specifically perturbed anion ratios in water and methanol at sub-micromolar concentrations.
• No such perturbation was observed in acetonitrile or 2-propanol, suggesting solvent-specific interactions.

Conclusions:

• Anions interact specifically over unexpectedly long distances at hydrogen-bonded interfaces, particularly in water and methanol.
• The extended hydrogen-bond networks in water and methanol appear to facilitate these long-range anion interactions.
• Solvent properties like surface tension and dielectric permittivity play a role in the observed anion behavior at interfaces.