We report the transformation of ionic liquid films from isotropic bulk to a fluid-ordered state over micrometer length scales. Data from infrared and nonlinear spectroscopy measurements show clear transitions that, for varying ionic liquids, occur over time frames of 10 min to 2 h. These maturation times depend linearly on the chosen ionic liquids' bulk viscosities. Interestingly, the ionic liquids do not form solids upon ordering but do exhibit strong preferential alignments of molecules that persist throughout the fluid films' thicknesses. Our measurements characterize this ordering process and show that it is largely insensitive to substrate surface chemistry or small amounts of absorbed water. Additional experiments show the transition is observed across several of the most common ionic liquid cations and that the process is completely reversible. The driving force for this organization is attributed to electrostatic and steric forces combined with a slow shearing of the viscous ionic liquid. These interactions work together to slowly bring the molecules within the film to a preferred, global orientation. The physical length and time scales of this transformation are unexpected and intriguing and invite additional studies to develop an understanding and control of ionic liquid materials' behavior, particularly near surfaces, to benefit their uses in lubrication, capacitive energy storage, and heterogeneous catalysis.

We report the reversible transitions from two distinct chemical environments (interfacial and bulk) to a single, globally ordered dominant environment that extends to 800 nm, within two triflate-based ionic liquid (IL) films. Vibrational spectra for supported IL films exhibit multiple peaks for the same vibrational mode, indicating the presence of multiple chemical environments (interfacial and bulk) in the film. After a quiescent maturation time, the vibrational spectra show much simpler absorption profiles indicating coalescence of the IL molecules into a global preferred phase, which resembles the interfacial environment that propagates throughout the film as a function of time. Data analysis suggests significant reorientation of the triflate anion with small changes of the cation, indicating a weakly interacting cation−anion pair. The distal extent of the self-organization is much thicker than that generally reported for solid−fluid interfaces. The ordering is reversible on replenishing the film with fresh fluid. This report, describing propagation of interfacial molecular orientation to form extended ordered structures, is a motivation for future studies to apply this phenomenon toward the thoughtful design of new IL systems for use in materials and devices.

We report surface-dependent, long-range ordering behavior of two ionic liquids, 1-butyl-1-methylpyrrolidinium tetracyanoborate (P14 B­(CN)4) and 1-butyl-1-methylpyrrolidinium dicyanamide (P14 N­(CN)2). The ionic liquids are supported as liquid films and examined using vibrational spectroscopy and spectroscopic ellipsometry. Both systems show changes in the infrared peak profile characteristic of major dipole reorientations for both anions, but the changes observed are in different spectral regions. Specifically, tetracyanoborate shows changes in the B–C stretching modes, and dicyanamide displays changes in the CN and N–C modes. Surprisingly, B­(CN)4 induces a final film environment, which is identical to the solid–liquid interfacial orientation. However, the N­(CN)2 indicates propagation of the gas–liquid interfacial environment through the film toward the solid substrate. The extent of ordering within the films extends to 0.4 and 1.1 μm for P14 N­(CN)2 and P14 B­(CN)4, respectively. This observation is intriguing as it presents a possibility for controlling molecular (re)­orientations at 100 s of nanometer scales by tuning fundamental intermolecular interactions at surfaces. These results for the cyano-functionalized anions should be generally applied for other systems, contributing directly to a better understanding and design for applications in molecular electronics, lubrication, and energy storage devices.

The sorption of water in ionic liquids (ILs) is nearly impossible to prevent, and its presence is known to have a significant effect on the resulting mixtures’ bulk and interfacial properties. The so-called “saturation” water concentrations have been reported, but water sorption rates and mixing behaviors in ILs are often overlooked as variables that can significantly change the resulting mixtures’ physical properties over experimental time frames of several minutes to hours. The purpose of this work is to establish a range of these effects over similar time frames for two model ILs, protic ethylammonium nitrate (EAN) and aprotic butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1114 TFSI), as they are exposed to controlled dry and humid environments. We report the water sorption rates for these liquids (270 ± 30 ppm/min for EAN and 30 ± 3 ppm/min for N1114 TFSI), examine the accuracy and precision associated with common methods for reporting water content, and discuss implications of changing water concentrations on experimental data and results.

Electrochemical effects manifest as nonlinear responses to an applied electric field in electrochemical devices, and are linked intimately to the molecular orientation of ions in the electric double layer (EDL). Herein, we probe the origin of the electrochemical effect using a double-gate graphene field effect transistor (GFET) of ionic liquid N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) top-gate, paired with a ferroelectric PbLaZrTiO (PLZT) back-gate of compatible gating efficiency. The orientation of the interfacial molecular ions can be extracted by measuring the GFET Dirac point shift, and their dynamic response to ultraviolet-visible light and a gate electric field was quantified. We have observed that the strong electrochemical effect is due to the TFSI anions self-organizing on a treated GFET surface. Moreover, a reversible order-disorder transition of TFSI anions self-organized on the GFET surface can be triggered by illuminating the interface with ultraviolet-visible light, revealing that it is a useful method to control the surface ion configuration and the overall performance of the device.

Room Temperature Ionic liquids present an exciting opportunity to examine surface confined thin fluid films and extended interfacial systems that may prove useful for electrochemical applications. This work explores the behavior of thin (1-5000 nm) films and bulk RTIL interfaces in contact with air and silver metal surfaces. Thin films are introduced by a highly controlled dynamic wetting process. Bulk interfaces are prepared by immersing the metal electrode into an IL solution. Data from ellipsometry, vibrational spectroscopy, contact angle, and electrochemical measurements are reported for a series of Cn-3-methylimidazolium based liquids with varying length alkane tails. Results correlate chemical nature of the liquid and substrates with resulting molecular structure of the interface and behavior of the wetting film. Descriptors include chemical interactions i.e. hydrophobic forces, hydrogen bonding, van der Waals, electrostatics, etc.

We report the interfacial structures and chemical environments of ionic liquid films as a function of dilution with molecular solvents and over a range of film thicknesses (a few micrometers). Data from spectroscopic ellipsometry and infrared spectroscopy measurements show differences between films comprised of neat ionic liquids, as well as films comprised of ionic liquids diluted with two molecular solvents (water and acetonitrile). While the water-diluted IL films follow thickness trends predicted by the Landau–Levich model, neat IL and IL/MeCN films deviate significantly from predicted behaviors. Specifically, these film thicknesses are far greater than the predicted values, suggesting enhanced intermolecular interactions or other non-Newtonian behaviors not captured by the theory. We correlate film thicknesses with trends in the infrared intensity profiles across film thicknesses and IL-solvent dilution conditions and interpret the changes from expected behaviors as varying amounts of the film volume existing in isotropic (bulk) vs anisotropic (interfacial) states. The hydrogen bonding network of water-diluted ionic liquids is implicated in the agreement of this system with the Landau–Levich model’s thickness predictions.

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