Surface structure of a "non-amphiphilic" protic ionic liquid

Area of Science:

• Physical Chemistry
• Surface Science
• Materials Science

Background:

• Understanding the nanostructure of ionic liquid surfaces is crucial for predicting their behavior in various applications.
• Ethanolammonium nitrate (EtAN) is an ionic liquid with potential applications where surface properties are critical.

Purpose of the Study:

• To elucidate the nanostructure and molecular orientation at the ethanolammonium nitrate (EtAN)-air interface.
• To investigate the formation of interfacial clusters and their impact on surface properties.

Main Methods:

• X-ray reflectometry (XRR) to probe electron density profiles.
• Vibrational sum frequency spectroscopy (VSFS) for molecular orientation and functional group analysis.
• Neutral impact collision ion scattering spectroscopy (NICISS) for elemental surface composition.

Main Results:

• XRR data indicates a diffuse electron density profile, suggesting interfacial clustering rather than a sharp interface.
• VSFS reveals surface -CH(2)- moiety coverage, with -NH(3)(+) and -OH groups oriented deeper within the interface.
• NICISS elemental analysis shows surface carbon enrichment, consistent with cation orientation and aggregate formation.

Conclusions:

• The EtAN-air interface is characterized by diffuse nanostructure and the presence of EtAN clusters.
• Molecular orientation at the interface is specific, with nonpolar groups exposed and charged groups potentially shielded.
• The findings provide fundamental insights into ionic liquid surface behavior and nanostructure.