skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: Direct observation of the double-layering quantized growth of mica-confined ionic liquids
Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL ( i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL–mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.  more » « less
Award ID(s):
1904486
PAR ID:
10332718
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Nanoscale
Volume:
13
Issue:
42
ISSN:
2040-3364
Page Range / eLocation ID:
17961 to 17971
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; (, RSC Advances)
    In this work, we perform atomic force microscopy (AFM) experiments to evaluate in situ the dependence of the structural morphology of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate ([P 6,6,6,14 ][DEHP]) ionic liquid (IL) on applied pressure. The experimental results obtained upon sliding a diamond-like-carbon-coated silicon AFM tip on mechanically polished steel at an applied pressure up to 5.5 ± 0.3 GPa indicate a structural transition of confined [P 6,6,6,14 ][DEHP] molecules. This pressure-induced morphological change of [P 6,6,6,14 ][DEHP] IL leads to the generation of a lubricious, solid-like interfacial layer, whose growth rate increases with applied pressure and temperature. The structural variation of [P 6,6,6,14 ][DEHP] IL is proposed to derive from the well-ordered layering of the polar groups of ions separated by the apolar tails. These results not only shed new light on the structural organization of phosphonium-based ILs under elevated pressure, but also provide novel insights into the normal pressure-dependent lubrication mechanisms of ILs in general. 
    more » « less
  2. ; ; ; ; (, Nanoscale)
    null (Ed.)
    Hydration layers are formed on hydrophilic crystalline surfaces immersed in water. Their existence has also been predicted for hydrophobic surfaces, yet the experimental evidence is controversial. Using 3D-AFM imaging, we probed the interfacial water structure of hydrophobic and hydrophilic surfaces with atomic-scale spatial resolution. We demonstrate that the atomic-scale structure of interfacial water on crystalline surfaces presents two antagonistic arrangements. On mica, a common hydrophilic crystalline surface, the interface is characterized by the formation of 2 to 3 hydration layers separated by approximately 0.3 nm. On hydrophobic surfaces such as graphite or hexagonal boron nitride (h-BN), the interface is characterized by the formation of 2 to 4 layers separated by about 0.5 nm. The latter interlayer distance indicates that water molecules are expelled from the vicinity of the surface and replaced by hydrocarbon molecules. This creates a new 1.5–2 nm thick interface between the hydrophobic surface and the bulk water. Molecular dynamics simulations reproduced the experimental data and confirmed the above interfacial water structures. 
    more » « less
  3. ; ; ; (, Advanced Functional Materials)
    Abstract Functional oxides have extensively been investigated as a promising class of materials in a broad range of innovative applications. Harnessing the novel properties of functional oxides in micro‐ to nano‐scale applications hinges on establishing advanced fabrication and manufacturing techniques able to synthesize these materials in an accurate and reliable manner. Oxidative scanning probe lithography (o‐SPL), an atomic force microscopy (AFM) technique based on anodic oxidation at the water meniscus formed at the tip/substrate contact, not only combines the advantages of both “top‐down” and “bottom‐up” fabrication approaches, but also offers the possibility of fabricating oxide nanomaterials with high patterning accuracy. While the use of self‐assembled monolayers (SAMs) broadened the application of o‐SPL, significant challenges have emerged owing to the relatively limited number of SAM/solid surface combinations that can be employed for o‐SPL, which constrains the ability to control the chemistry and structure of oxides formed by o‐SPL. In this work, a new o‐SPL technique that utilizes room‐temperature ionic liquids (RTILs) as the functionalizing material to mediate the electrochemistry at AFM tip/substrate contacts is reported. The results show that the new IL‐mediated o‐SPL (IL‐o‐SPL) approach allows sub‐100 nm oxide features to be patterned on a model solid surface, namely steel, with an initiation voltage as low as −2 V. Moreover, this approach enables high tunability of both the chemical state and morphology of the patterned iron oxide structures. Owing to the high chemical compatibility of ILs, which derives from the possibility of synthesizing ILs able to adsorb on a wide variety of solid surfaces, IL‐o‐SPL can be extended to other material surfaces and provide the opportunity to accurately tailor the chemistry, morphology, and electronic properties within nanoscale domains, thus opening new pathways to the development of novel micro‐ and nano‐architectures for advanced integrated devices. 
    more » « less
  4. ; ; ; ; ; (, Advanced Materials Interfaces)
    Abstract Ionic liquids (ILs) are proposed as potentially ideal electrolytes for use in electrical double layer capacitors. However, recent discoveries of long‐range electrostatic screening in ILs have revealed that this understanding of the electrical double layer in highly concentrated solutions is still incomplete. Through precise time‐dependent measurements of wide‐angle X‐ray scattering and surface forces, novel molecular insight into their electrical double layer is provided. An ultraslow evolution of the nanostructure of three imidazolium ILs is observed, which reflects the reorganization of the ions in confined and unconfined (bulk) states. The observed phase transformation in the bulk consists of the ILs ordering over at least 20 h, reflected in an expansion or contraction of the spacing between the ions organized in domains of ≈10 nm. This transformation justifies the evolution of the electrical double layer measured in force measurements. Subtle differences between the ILs arise from the intricate balance between electrostatic and non‐electrostatic interactions. This work reveals a new time scale of the evolution of the IL structure and offers a new perspective for understanding the electrical double layer in ILs, with implications on diverse areas of inquiry, such as energy storage, lubrication, as well as micro‐ and nanoelectronics devices. 
    more » « less
  5. ; ; ; ; ; ; ; ; ; ; et al (, Droplet)
    Abstract Ionic liquids (ILs) have attracted intensive research interest due to their outstanding physiochemical properties. However, comprehensive design is necessary for targeted applications and has rarely been conducted. As a result, the industry‐scale application of ILs is still very limited. In this academia–industry collaborative research among the University of Pittsburgh, Virginia Tech. University, and Seagate Technology LLC, we report the design, synthesis, molecular dynamics (MD) simulation, and characterization of a nanometer‐thick IL, which contains abundant fluorinated segments and a hydroxyl endgroup, as the next‐generation nano‐lubricant for hard disk drives (HDDs). The lab‐ and industry‐level testing results indicate that the IL lubricant performs significantly better than the state‐of‐the‐art lubricant, that is, perfluoropolyether (PFPE) that has been utilized for three decades in the HDD industry in two key functions: thermal stability and fly clearance. Meanwhile, the IL lubricant also shows excellent lubricity and durability. The outstanding performance of the IL has been attributed to its unique molecular structure on the solid substrate, which is supported by MD simulation results. Our work establishes the IL as a promising candidate among the next‐generation media lubricants in HDD industry. Meanwhile, the finding obtained here has important implications in many other applications involving nano‐lubricants. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email