Adsorption Modes of Na⁺, Li⁺, and Mg²⁺ to a Model Zwitterionic Lipid Bilayer

The adsorption of ions to soft-porous interfaces plays a critical role in many physical and biological processes, such as the function of electrochemical energy storage devices or the attachment of membrane proteins to cell surfaces. In this work, we characterize different adsorption modes and describe the adsorption behavior of Na⁺, Li⁺, and Mg²⁺ onto a porous substrate. We identify three categories of adsorption based on the degree of dehydration of the ion, viz., steric adsorption corresponding to a lack of dehydration, imperfect adsorption with partial dehydration, and perfect adsorption representing total dehydration. Using 1-palmitoyl-2-oleoyl-<i>sn</i>-glycero-3-phosphatidylcholine (POPC) in salt solution as a generic model system for salt at a soft and porous interface, based on the simulation model used, we find that anions, Cl^-, always adsorb sterically. Among cations, the divalent Mg²⁺ does not dehydrate and is also adsorbed sterically. On the other hand, Na⁺ adsorbed to a large fraction perfectly, and Li⁺ exhibits a significant fraction of imperfectly adsorbed ions, We demonstrate that, with everything else held fixed, the adsorption mode of a cation is determined solely by the strength of the electric field produced by the ion at the distance of the hydration shell.