Adsorption\nModes of Na<sup>+</sup>, Li<sup>+</sup>, and Mg<sup>2+</sup> to a\nModel Zwitterionic Lipid Bilayer

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