Layering of [BMIM]+-based ionic liquids at a charged sapphire interface

Area of Science:

• Materials Science
• Physical Chemistry
• Surface Science

Background:

• Room temperature ionic liquids (RTILs) are salts that are liquid at ambient temperatures.
• Understanding the structure of RTILs at interfaces is crucial for their application in various fields.
• Charged surfaces can significantly influence the organization of RTILs.

Purpose of the Study:

• To determine the interfacial structure of two model RTILs, [BMIM](+)[PF(6)](-) and [BMIM](+)[BF(4)](-), at a charged aluminum oxide (Al(2)O(3)(0001)) interface.
• To correlate interfacial structure with bulk properties and surface energies.
• To elucidate the dominant interactions governing interfacial ordering.

Main Methods:

• High energy (72.5 keV) X-ray reflectivity was employed to achieve subnanometer resolution of the interfacial structure.
• Bulk diffraction patterns of the RTILs were measured.
• Surface and interface energies were measured using complementary techniques.

Main Results:

• [BMIM](+)[PF(6)](-) exhibited alternately charged, exponentially decaying near-surface layering.
• [BMIM](+)[BF(4)](-) displayed only a single layer of enhanced electron density at the interface.
• Interfacial layering correlated with bulk diffraction patterns, while surface energies showed no significant differences between the two RTILs.

Conclusions:

• Interfacial ordering in these RTILs is primarily driven by electrostatic ion-ion interactions, mirroring bulk correlations.
• Hydrogen bonding and dispersion interactions play a secondary role in interfacial structure.
• The observed differences in layering are attributed to the distinct anions ([PF(6)](-) vs. [BF(4)](-)).