Binding of trivalent ions on fatty acid Langmuir monolayer: Fe versus La.

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