The ion–lipid battle for hydration water and interfacial sites at soft-matter interfaces

• Competition of large chaotropic ions with lipids for interfacial sites.
• Disruption of soft matter interfaces by ions.
• Ion–Lipid competition for water of hydration.
• Modification of ion–surface interactions by coordinating ligands.

At charged surfaces “bound” ions reduce the repulsive electrostatic forces, while dissociated ions control the osmotic pressure in colloidal systems. For systems charged through ionic adsorption on the other hand, the adsorbed ions determine the charging boundary condition and colloidal interactions. Soft-matter interfaces have considerable flexibility and compressibility, hence ionic adsorption at such interfaces may generate new phenomena when (a) the ions compete with the lipid or polymeric components for water of hydration, or (b) position themselves at the polar–nonpolar interface and modify its structure. We review some recent advances on the understanding of specific ion effects from this perspective, and provide some unpublished illustrative examples involving soft flexible interfaces. We propose an extension of the chaotropic series to include disruptors of soft matter, which may act as cosurfactants or even as hydrotropes. We also examine the effects of coordinating ligands on specific ion adsorption at soft interfaces, using lanthanides as test cations, and discuss how such effects may be used to change the affinities between ions and interfaces in controlled ways.

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