Binding of trivalent ions on fatty acid Langmuir monolayer: Fe3+ versus La3.

Langmuir monolayers consisting of fatty acid molecules were prepared on solutions of FeCl3 and LaCl3 to investigate adsorption of trivalent metal ions on carboxylic headgroups by using sum-frequency vibrational spectroscopy. Fe3+ ions bound to the fatty acid headgroups only in the form of hydroxide complexes (Fe(OH)x +3-x), and sum-frequency intensity of water stretch modes increased markedly upon adsorption of ion hydroxide. On the other hand, La3+ ions bound to the charged anionic headgroup as bare trivalent ions. Upon Fe(OH)x +3-x adsorption, the sum-frequency spectrum of carboxyl headgroups showed significant redshift which is opposite to the case of La3+ as well as those for alkali (Na+, K+) and alkali earth metal (Ca2+, Mg2+) ions, which also supports that Fe3+ binding is by covalent metal-ligand bonding, while La3+ binding is by Coulomb attraction.