Submonolayer Ionic Liquids Boundary Lubricants for Hard Disk Drives

The stability of submonolayer ionic liquid (IL) films on the amorphous nitrogenated carbon (CNx) surface is investigated by bonding and evaporation kinetics and by surface energy measurements. A protic, an aprotic, and a hydroxyl-functionalized aprotic IL, each appended with the same perfluorinated -OCH₂CF₂-(OCF₂CF₂)₂-OC₄F₉ group on the cation fragment, are compared to submonolayer perfluoropolyether (PFPE) films of comparable molecular weight and functionality. Stable bonding to the CNx surface and film thickness stability are observed only for the hydroxyl-functionalized ILs and PFPEs at these very low molecular weights (∼1000-1300 amu). Nonfunctionalized ILs exhibit low bonding on the CNx surface, debond under ambient conditions, and lose about 30% of their initially applied thickness. The polar surface energy as a function of increasing IL film thickness on CNx decreases only for the functionalized IL. Model bimolecular complexes via ab initio computational modeling assess the strength of possible hydrogen bonds between the ILs and CNx. The stability of the hydrogen bonds to the presence of water molecules is quantified by displacement energetics. The enthalpy for the displacement of bonded IL by water is computed to be between -0.7 and +1.9 kcal/mol, depending upon the specific bimolecular complex. The computed equilibrium constant for IL debonding by water is close to 1, indicating the coexistence of both bonded and debonded IL molecules on CNx at room temperature.