Different Adsorption Behavior of Rare Earth and Metallic Ion Complexes on Langmuir Monolayers Probed by Sum-Frequency Generation Spectroscopy

Abstract

Adsorption behavior of counterions under a Langmuir monolayer was investigated by sum-frequency generation (SFG) spectroscopy. By comparing SFG spectra of arachidic acid (AA) Langmuir monolayer/water interface with and without added salt, it was found that the simple trivalent cation La 3+ adsorbed on AA monolayer only when the carboxylic headgroups are charged (deprotonated), implying that counterion adsorption is induced by Coulomb interaction. On the other hand, metal hydroxide complex Fe(OH)3 adsorbed even on a charge-neutral AA monolayer, indicating that the adsorption of iron hydroxide is due to chemical interaction such as covalent or hydrogen bonding to the headgroup of the molecules at the monolayer.

Interaction of Arachidic Acid Langmuir Monolayers with Trivalent Ions La³⁺ and Fe³⁺ Studied by Vibrational Sum-Frequency Spectroscopy

“…For the pH = 2.86 of the 10 ¹3 M FeCl 3 solution, with the pK a = 5.1 of the AA, the headgroups of the AA monolayer are mostly neutral, 10,12,14) so unlike for the La 3+ ions, there is no Coulombic binding for the Fe 3+ ions at the interfacial layers. Woongmo Sung et al 13) have performed a SFG study on these systems with the surface area of the AA monolayer maintained at ³20 ¡ 2 /molecule. As this surface area value is nearly the collapsed limit of the liquid-condensed (LC) phase, they observed a relatively weak SF intensity from the monolayer in the presence of 1 mM LaCl 3 in the subphase water.…”

“…Previous studies 10,12,13) on AA monolayers on water often used a molecular area value of ³20 ¡ 2 . In this paper, we will discuss why using a molecular area of 22.6 ¡ 2 leads to more sensitive effects on the AA monolayer, which help distinguish the roles of iron complexes Fe(OH) 2+ and Fe(OH) 3 that the previous studies did not point out.…”

Antagonistic Role of Aqueous Complexation in the Solvent Extraction and Separation of Rare Earth Ions

“…[21][22] Model systems of water-insoluble surfactant monolayers at the liquid-vapor interface have also been studied by interface-sensitive methods to explore the interactions between ions and surfactants whose headgroups mimic those of more weakly amphiphilic extractants. [26][27][28][29][30][31][32][33][34][35][36][37] Although studies at the liquid-vapor interface cannot model ion transport through the interface or structures that may form uniquely at a liquid-liquid interface, they may be able to model many aspects of the nanoscale variation of density, dielectric properties, and water structuring near a liquid interface that can influence ion-extractant complexation. However, studies of water-insoluble surfactants or extractants cannot address the issue of the site of complexation of water-soluble extractants.…”

Three-Dimensional Analysis of the Collapse of a Fatty Acid at Various Compression Rates using In Situ Imaging Ellipsometry

“…Investigations of Langmuir monolayers have uncovered a rich variety of physical phenomena [1][2][3][4]. When a 2D film is compressed to its stability limit, it explores the third dimension by collapse.…”