title: Ionic specificity in pH regulated charged interfaces: Fe3+ versus La3+.
abstract: We determine the distribution of two trivalent ions Fe(3+) and La(3+) 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 pK(a) values, from 2.1 (DHDP) to 5.1 (AA), thus allowing manipulation of the surface charge over extremely low pH (pH ∼1 or larger), and the two ions provide two limiting cases of specificity for the amphiphiles. We find that La(3+) distribution is mostly sensitive to the surface charge, whereas the Fe(3+) binding depends 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.
authors.affiliation: Ames Laboratory, and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Wenjie
authors.initials: W
authors.lastname: Wang
authors.forename: Rebecca Y
authors.initials: RY
authors.lastname: Park
authors.forename: David H
authors.initials: DH
authors.lastname: Meyer
authors.forename: Alex
authors.initials: A
authors.lastname: Travesset
authors.forename: David
authors.initials: D
authors.lastname: Vaknin