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

30 August 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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) lead to bilayer formation, Nd3+ (82 C/mm3) and Sr2+ (33 C/mm3) lead 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.


liquid-liquid extraction
Langmuir monolayers
inverted bilayers
lanthanide separation
ion-specific effects
amphiphile aggregation
vibrational sum frequency generation
ion adsorption

Supplementary materials

Supporting Information for Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface
Supplemental information for the main article. Additional VSFG, XFNTR, and XR results are described.


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.