Cation-induced monolayer collapse at lower surface pressure follows specific headgroup percolation

Area of Science:

• Surface Science
• Materials Chemistry
• Nanotechnology

Background:

• Langmuir monolayers are 2D structures that deform under mechanical compression.
• Structural deformations beyond the elastic limit are termed collapsed structures.
• Understanding monolayer collapse mechanisms and ion effects is crucial for materials design.

Purpose of the Study:

• To investigate monolayer collapse at lower surface pressures.
• To determine the effect of various divalent metal ions (Mg^{2+}, Ca^{2+}, Zn^{2+}, Cd^{2+}, Ba^{2+}) on stearic acid Langmuir monolayer collapse.
• To explore the influence of subphase pH on these ion-specific collapse behaviors.

Main Methods:

• Studied stearic acid Langmuir monolayers at lower (pH ≈ 6.8) and higher (pH ≈ 9.5) subphase pH.
• Investigated the effect of Mg^{2+}, Ca^{2+}, Zn^{2+}, Cd^{2+}, and Ba^{2+} ions.
• Utilized X-ray reflectometry, atomic force microscopy, and Fourier transform infrared spectroscopy.

Main Results:

• At lower pH, monolayers remain intact until higher surface pressures (>50 mN/m) with all tested cations.
• At higher pH, ion-specific structural changes occur at lower surface pressures.
• Ba^{2+} induced monolayer to multilayer transitions at lower pressures, while Zn^{2+} and Cd^{2+} formed bilayer structures; Fourier transform infrared spectroscopy confirmed coexisting conformations and transformations based on coverage ratios.

Conclusions:

• Divalent metal ions significantly alter stearic acid Langmuir monolayer collapse behavior, particularly at higher subphase pH.
• The observed ion-specific collapses are linked to metal-headgroup coordination and molecular conformation changes.
• The study correlates these phenomena with the 2D lattice percolation model.