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Submonolayer Ionic Liquids Boundary Lubricants for Hard Disk Drives
Hantao Zhou1, Ao Fan1, Daniel Maksuta1
1Western Digital Corporation, 5601 Great Oaks Parkway, San Jose, California 95119-1003, United States.
Langmuir : the ACS Journal of Surfaces and Colloids
|June 27, 2025
Summary
Hydroxyl-functionalized ionic liquids (ILs) and perfluoropolyethers (PFPEs) show stable submonolayer film formation on amorphous nitrogenated carbon (CNx) surfaces. Non-functionalized ILs lack stability, indicating the critical role of hydroxyl groups for robust IL film adhesion.
Area of Science:
- Materials Science
- Surface Chemistry
- Computational Chemistry
Background:
- Submonolayer films are crucial for surface modification and advanced material applications.
- Understanding the interfacial behavior of ionic liquids (ILs) on various surfaces is key to designing functional materials.
- Amorphous nitrogenated carbon (CNx) presents unique surface properties for potential applications in electronics and catalysis.
Purpose of the Study:
- To investigate the stability of submonolayer ionic liquid (IL) films on amorphous nitrogenated carbon (CNx) surfaces.
- To compare the performance of hydroxyl-functionalized ILs with non-functionalized ILs and perfluoropolyethers (PFPEs).
- To elucidate the bonding and evaporation kinetics, surface energy, and the role of hydrogen bonding in film stability.
Main Methods:
- Experimental investigation of bonding and evaporation kinetics.
- Surface energy measurements as a function of film thickness.
- Ab initio computational modeling of bimolecular complexes to assess hydrogen bond strength.
- Quantification of hydrogen bond stability against water displacement.
Main Results:
- Stable submonolayer film formation and thickness stability were observed only for hydroxyl-functionalized ILs and PFPEs.
- Non-functionalized ILs exhibited weak bonding, debonded under ambient conditions, and lost significant thickness.
- Polar surface energy decreased with increasing film thickness only for functionalized ILs.
- Computational modeling indicated that hydrogen bonds between ILs and CNx are moderately stable, with an equilibrium constant for IL debonding by water near 1.
Conclusions:
- Hydroxyl functionalization is critical for achieving stable submonolayer ionic liquid films on CNx surfaces.
- The stability is attributed to strong hydrogen bonding interactions between the hydroxyl groups and the CNx surface.
- Water molecules can compete with ILs for surface binding sites, influencing overall film stability.
- These findings provide insights for designing robust IL-based surface coatings and functional interfaces.
