Measured depletion of ions at the biomembrane interface

Expected from theory and simulations, depletion of ions at fuzzy biomembrane interfaces has long eluded experiments. Here, we show how salt exclusion can be accurately measured by surprisingly simple yet accurate benchtop measurements. Multilamellar aggregates of common phospholipids sink in low salt but float in salt solutions that are much less dense than the lipid itself. By manipulating bath and lipid densities, using heavy water and varied lipid chain length, we obtain accurate exclusion curves over a wide range of KCl and KBr concentrations. While maintaining a constant width at low salt, the exclusion layer decreases in high salt, following the Debye screening length. Consistent with interfacial accumulation of polarizable ions, bromide salts are less excluded than chloride, with an attraction of ∼2k_BT per Br^- ion. So far neglected in theoretical descriptions, the competition between salt exclusion and binding is critical to understanding membrane interactions and specific ionic effects.