Ordering of ionic liquids at a charged sapphire interface: Evolution with cationic chain length

Area of Science:

• Materials Science
• Physical Chemistry
• Surface Science

Background:

• Room Temperature Ionic Liquids (RTILs) exhibit molecular layering in bulk.
• This layering extends to the RTIL/sapphire interface, influenced by alkyl chain length.
• The negatively charged sapphire surface is known to compress these layers and alter interfacial structure.

Purpose of the Study:

• To investigate the molecular layering and structural changes of RTILs at the sapphire interface.
• To understand the impact of the negatively charged sapphire surface on RTIL interfacial structure.
• To elucidate the role of cationic alkyl chain length and interfacial forces.

Main Methods:

• Utilized X-ray reflectivity (XR) to study the RTIL/sapphire interface.
• Examined a broad homologous series of 1-alkyl-3-methylimidazolium bis(trifluoromethansulfonyl)imide RTILs.
• Compared interfacial layering with bulk and RTIL/air interface data.

Main Results:

• Observed two distinct interfacial spacings (d_a and d_b).
• d_a spacing showed compression and significant polar slab thinning, suggesting anion exclusion.
• Layering range at the interface exceeded that of the bulk.

Conclusions:

• The charged sapphire surface significantly alters RTIL interfacial structure, inducing compression and anion exclusion.
• Interfacial structure is governed by a balance of Coulomb and dispersion interactions, modified by the charged interface.
• Further research is needed to clarify the coexistence of different layering modes.