Entropy or Enthalpy? Second Harmonic Generation from Liquid Sheets Reveals Different Ion Adsorption Mechanisms for Air–Water and Oil–Water Interfaces

The mechanism by which ions adsorb to immiscible liquid-liquid interfaces is central to our understanding of heterogeneous aerosol chemistry, "on-water" catalysis, and biological systems. However, the microscopic details regarding ion adsorption to such interfaces have primarily come from theoretical efforts, owing to several experimental difficulties, viz., reliable preparation of experimentally accessible and contamination-free liquid-liquid interfaces and the absence of spectroscopic tools to unambiguously probe buried surfaces. To overcome these challenges and reveal vital details of ion adsorption to a prototypical hydrophobic liquid-liquid interface, we combine free-flowing planar liquid sheets with deep UV second harmonic generation spectroscopy. The micrometer-thick, layered free-flowing sheets enable the simultaneous measurement of ion adsorption to both the air-water and water-heptane interfaces, and interference of the signal between multiple interfaces permits the retrieval of valuable phase information that is normally not accessible with this technique. Specifically, we measure temperature-dependent Langmuir isotherms of the thiocyanate anion at both interfaces and disentangle the entropic and enthalpic contributions to the Gibbs free energy of adsorption. We find that anion adsorption to these two surfaces has opposite thermodynamic driving forces, with ions being stabilized at the air-water interface by a favorable enthalpy change, whereas ions are stabilized at the water-heptane interface by a favorable entropy change.