Adsorption of different trivalent ions on fatty acid Langmuir monolayer

Counterion binding to Langmuir monolayer is an important process in all areas of soft matter phenomena, ranging from biology, suspensions, colloids, and atmospheric processes. 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 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 headgroups (at high pH subphase) as bare trivalent ions, and reached saturation adsorption at 10 mM bulk concentration.