Ionic Specificity in pH Regulated Charged Interfaces: Fe3+ versus La3+

We determine the distribution of two trivalent ions Fe3+ and La3+ next to two different amphiphilic charged interfaces as ions or complexes, consisting of the phosphate lipid dihexadecyl phosphate (DHDP) and the fatty acid arachidic acid (AA). These amphiphiles provide a wide range of plc values, from 2.1 (DHDP) to 5.1 (AA), thus allowing manipulation of the surface charge over extremely low pH (pH similar to 1 or larger), and the two ions provide two limiting cases of specificity for the amphiphiles. We find that La3+ distribution is mostly sensitive to the surface charge, whereas the Fe3+ binding depend:: on its character in the solution and is highly specific, as indicated by the crucial role played by iron complexes (Fe(OH)(3) or Fe(OH)(2+)) forming covalent bonds even for an uncharged interface. The implications of the results to other ions and/or amphiphilic interfaces are also discussed.