Interactions of different anions with phospholipid membranes in aqueous salt solutions were
investigated by molecular dynamics simulations and fluorescence solvent relaxation measurements.
Both approaches indicate that the anion-membrane interaction increases with the size and softness of the
anion. Calculations show that iodide exhibits a genuine affinity for the membrane, which is due to its
pairing with the choline group and its propensity for the non-polar region of the acyl chains, the latter
being enhanced in polarizable calculations showing that the iodide number density profile is expanded
towards the glycerol level. Solvent relaxation measurements using Laurdan confirm the influence of
large soft ions on the membrane organization at the glycerol level. In contrast, chloride exhibits a peak
at the membrane surface only in the presence of a surface-attracted cation, such as sodium but not
potassium, suggesting that this behavior is merely a counter-ion effect.