Anion Competition at Positively Charged Surfactant Monolayers.

Area of Science:

• Physical Chemistry
• Surface Science
• Computational Chemistry

Background:

• Anion interactions with hydrophobic protein and polymer surfaces are hydration-dependent.
• Interactions at positively charged surfactant monolayers are complex due to electrostatic forces, hydration, and ion-ion effects.

Purpose of the Study:

• To investigate the preferential interactions of chloride and bromide anions with cationic surfactant monolayers.
• To elucidate the microscopic mechanisms governing anion adsorption at charged interfaces.

Main Methods:

• Molecular dynamics simulations were employed to model anion behavior.
• Three surface-sensitive X-ray scattering techniques were used for experimental validation.

Main Results:

• Bromide ions showed preferential interaction over chloride ions with dihexadecyldimethylammonium (DHDA+) and dioctadecyldimethylammonium (DODA+) monolayers.
• Simulations accurately reproduced experimental interfacial electron density profiles.
• Ion pairing was observed for both anions, but bromide formed more stable pairs due to reduced hydration.

Conclusions:

• Ion hydration is the primary factor controlling interfacial electrolyte structure.
• Bromide's lower hydration energy drives its preferential adsorption and formation of stable ion pairs.
• Long-range lateral correlations of bromide ions were observed on the DODA+ monolayer.