(4eu) Molecular Structures of Solid-Confined Ionic Liquids and Their Applications As Media Lubricants in Hard Disk Drives

Media lubricants are critical to the reliability of hard disc drives (HDDs). Ionic liquids (ILs) are promising candidates as the next-generation media lubricants because of their excellent physiochemical properties. Both the fundamental understandings of the interfacial molecular structure at the IL/solid interfaces and the novel designs of the IL chemical structures are critical to the application of ILs as media lubricants. The IL/solid interfaces always play a critical role in many other important applications such as energy storage and catalysis.

First, we have investigated the molecular structure of mica-confined ILs. Experimental evidence from ATR-FTIR, contact angle measurement, and AFM shows that tuning relative humidity (RH) is an effective approach manipulating the molecular arrangement and macroscopic wettability of ILs on mica. More water adsorbs on mica at higher RH, which mobilizes the surface K⁺ and consequently initiates the layering of IL cations/anions. Additionally, unique double-layering quantized growths of mica-confined ILs with various cation alkyl chain lengths have been directly observed under AFM. The IL nanofilms initially grow only by covering more solid surface areas at the constant film thickness of 2 monolayers (ML) until a quantized thickness increase by another 2 ML. A double-layer model describing the molecular structure of IL cations and anions on the mica surface is proposed, which can be explained as the result of several competing interactions at the IL-mica interface. Time-dependent AFM results reveal the slower mobility of ILs with longer alkyl chains due to more ordered packing between longer alkyl chains.

Second, we have assessed the potential of nanometer-thick ILs as media lubricants. The TGA results indicate the commercially-available [Bmim][FAP] has higher thermal stability than the state-of-the-art perfluoropolyether (PFPE) lubricants. More importantly, AFM surface roughness results demonstrate that ILs have much lower ML thickness than PFPEs on carbon overcoat (COC) due to the smaller molecular sizes, which provides the opportunity to scale down the lubricant thickness and increase the areal density. Then a fluorinated IL (FIL) with a highly fluorinated cation alkyl chain has been synthesized to realize a reduced surface tension that is comparable to PFPEs. To enhance the bonding of ILs on COC and further reduce the surface tension, we have synthesized a novel IL, HFIL-OH, containing a hydroxyl endgroup and a fluorinated alkyl chain in the cation and a highly fluorinated anion. The successful development of the IL lubricants is a major step forward for ILs to be utilized as the next-generation media lubricants.