Surface structure evolution in a homologous series of ionic liquids

Area of Science:

• Physical Chemistry
• Materials Science
• Surface Science

Background:

• Interfaces of room temperature ionic liquids (RTILs) are crucial for applications and fundamental science.
• Understanding how RTIL interface structure changes with cation alkyl chain length, transitioning from Coulomb to van der Waals interactions, remains largely unexplored for homologous series.

Purpose of the Study:

• To investigate the evolution of the liquid-air interface structure in a homologous series of RTILs as a function of cation alkyl chain length.
• To elucidate the transition from simple liquid-like interfaces to nanostructured interfaces driven by molecular self-segregation.

Main Methods:

• Utilized angstrom-resolution X-ray methods to probe the surface structure of RTILs.
• Analyzed surface-normal electron density profiles to characterize interface morphology.
• Investigated the decay of surface structures into the bulk liquid.

Main Results:

• For short alkyl chains ([Formula: see text]), RTILs exhibit monotonic electron density profiles, similar to conventional liquids.
• Increasing alkyl chain length ([Formula: see text]) induces nanoscale self-segregation, leading to layered structures with alternating charged headgroups and apolar chains.
• Layering periodicity and decay lengths show a linear dependence on alkyl chain length, explained by models involving chain interdigitation or liquid-like chain behavior.
• A distinct surface phase was observed for [Formula: see text] above its melting point.
• No long-range order was detected within the surface layer.

Conclusions:

• The cation's alkyl chain length dictates the interfacial behavior of RTILs, driving a transition from simple to complex nanostructured interfaces.
• The observed layering and its characteristics provide insights into supramolecular self-aggregation and bulk-surface structure relationships in ionic liquids.
• Findings advance the understanding of RTIL interfaces, with implications for their design and application in various fields.