Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface*

Amphiphilic lipid-ion interactions at aqueous interfaces drive the assisted ion transport in various biological and industrial systems. In chemical separations of heavy elements, lipids coordinate metal ions and solubilize them in an organic phase. Direct observation of lipid-metal interactions is highly difficult at the buried oil/water interface, and is accessible with limited experiments. Here, we demonstrate that inverted bilayer structures previously observed at oil/aqueous interfaces can also be formed at the air/aqueous interface. This facilitates the easier study of lipid-ion interactions over a wide range of parameters with multiple probes, including synchrotron X-ray reflectivity (XR), X-ray fluorescence near total reflection (XFNTR), and vibrational sum-frequency generation spectroscopy (VSFG). The formation of bilayers is highly sensitive to the metal ion charge density. While Lu³⁺ (115 C/mm³) led to bilayer formation, Nd³⁺ (82 C/mm³) and Sr²⁺ (33 C/mm³) led to monolayers. By introducing Lu³⁺ ions to preformed lipid monolayers, we extract kinetic parameters corresponding to monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the presence of background salts where thiocyanate accelerates the conversion more than nitrate does. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods. Finally, this model system can be used to study a wide variety of lipid-ion interactions, opening a new avenue in molecular-scale understanding of these important systems.