Long-Range Ordering of Ionic Liquid Fluid Films

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

Structural Changes in Acetophenone Fluid Films as a Function of Nanoscale Thickness

We report experimental observations of a developing fluid/solid interface by examining acetophenone films of varying thicknesses, supported on solid silver substrates. A dynamic wetting technique provides experimental control of fluid film thickness, as a function of rotational velocity. Ellipsometry and infrared reflection absorption spectroscopy data are analyzed to provide absolute film thickness and details of the changing chemical environment for varying film thickness. These data are compared to theoretical models that predict fluid film thicknesses, based on physical-chemical properties of the acetophenone/silver pair. As the velocity of the substrate is varied from 0.003 cm s^–1 to 1.872 cm s^–1, the fluid film’s thickness changes from a ca. 200 nm to 2 μm. This increase in film thickness with increasing velocity follows a Landau trend, which is linear with respect to velocity^2/3. Our data also show clear evidence of molecular orientation changes, as a function of film thickness, which occur as the thinner films are increasingly comprised of acetophenone molecules within a confined, interfacial environment. The spectral changes for the thinnest fluid films (<100 nm) are shown to exhibit features similar to transmission Fourier transform infrared (FTIR) data of frozen acetophenone, suggesting that these films are highly ordered, as a result of their nanometer-scale confinement

Pyridine and Pyridinium Electrochemistry on Polycrystalline Gold Electrodes and Implications for CO₂ Reduction

We examine the electrochemical behavior of pyridine (py) and pyridinium ion (pyrH⁺) on gold electrodes in inert nitrogen (N₂) and carbon dioxide (CO₂) environments to evaluate potential catalytic roles of nitrogen heterocycles in electrochemical CO₂ reduction. Analysis of the pyridine and pyridinium systems shows that gold electrodes exhibit unique pyrH⁺ and CO₂ electrochemistry compared with previous work on platinum electrodes or photoelectrode systems. Specifically, analysis of the data shows specific adsorption of pyridine/pyridinium, an irreversible reduction wave at −1.0 V vs Ag/AgCl associated with the one-electron reduction of pyridinium, and an enhanced reductive current when CO₂ and pyrH⁺ are included together in the aqueous solution. Our results show no evidence to support formation of carbon-containing reduction products and implicate CO₂ as a possible weak acid catalyst for production of dihydrogen

Effects of Fluid Confinement and Temperature in Supported Acetophenone Films

Solid–liquid phase transitions are thought to be well understood in bulk phases of matter, but in thin films or interfacial volumes, melting and freezing transitions can exhibit significant departures from expected behaviors. Here, we show multiple solid–liquid phase transitions in thin films (50–500 nm) of the molecular fluid acetophenone. Transitions are driven by both geometric confinement and temperature, as characterized by spectroscopy. Fluid film confinement is controlled by systematic variation of the supported film thicknesses, and the same films are passed through cooling–heating cycles to generate amorphous or crystalline films with distinctly different molecular environments. Specifically, multiple temperature cycles reveal a distinct conditioning dependence, wherein phase transitions may or may not exhibit significant changes in the infrared absorption profile over the temperature cycle, indicating distinct crystalline and liquid-like phases. Significant effects of supercooling are also observed as a result of the highly confined nature of the thin-film sampling geometry. Interestingly, the spectral profiles recorded as a function of film temperature show clear evidence of molecular reordering phase transitions, which is similar to observations in variable thickness films held at constant temperature. The changes in spectral absorption profiles confirm the confinement-induced crystalline ordering and provide evidence that molecular confinement effects can extend beyond 100 nm from a surface, which is much larger than conventionally accepted “interfacial” volumes. Ultimately, the extended crystalline ordering within liquid films could offer important new avenues to tune the physical properties of designer interfaces

Structure of Aqueous Water Films on Textured −OH-Terminated Self-Assembled Monolayers

We report the thickness and interfacial molecular structure of thin (1–3 nm) aqueous films supported on hydroxyl-terminated self-assembled monolayers over a silver substrate. The water film structure is studied as a function of varying the monolayer’s methylene chain lengths. Analysis techniques include ellipsometry, contact angle, and polarization modulation reflection adsorption infrared spectroscopy. The aqueous film thicknesses follow 4-mercaptobutanol (4-MBU) > 11-mercaptoundecanol (11-MUD) > 6-mercaptohexanol (6-MHE) > 9-mercaptononanol (9-MNO). Water contact angle measurements across the same surfaces are very similar; however, vibrational spectroscopic analysis of the films shows that intermolecular bonding patterns of D₂O are significantly different from those of bulk D₂O. This evokes unique interfacial molecular architectures for each of these films. The structural differences depend on the nature of the SAM structure and resulting water–SAM interactions, which are evident from PM-IRRAS data. Spectroscopic peak intensity ratios of ν­(O–D) modes suggest more asymmetric hydrogen-bonded D₂O character near 9-MNO surfaces, whereas 4-MDU, 6-MHE, and 11-MUD surfaces exhibit increasingly symmetric hydrogen-bonded D₂O character. From this, we propose a model for film structure

Adventitious Water Sorption in a Hydrophilic and a Hydrophobic Ionic Liquid: Analysis and Implications

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

Large-Amplitude Fourier-Transformed AC Voltammetric Study of the Capacitive Electrochemical Behavior of the 1‑Butyl-3-methylimidazolium Tetrafluoroborate–Polycrystalline Gold Electrode Interface

In this paper, the capacitive electrochemical behavior of the 1-butyl-3-methylimidazolium tetrafluoroborate (Bmim BF₄)–polycrystalline gold electrode interface is reported over the potential range from −0.37 to 0.53 V vs Fc/Fc⁺ (Fc = ferrocene). Experimental results are generated by analysis of data (RC model) obtained from large-amplitude Fourier-transformed alternating current voltammetry (FT-ACV) over the frequency range of 10 Hz to 1 kHz. Results suggest a parabolic, U-shaped capacitance versus potential relationship, in stark contrast to present ionic liquid (IL) electrochemical double-layer (EDL) theory. The potential range analyzed was carefully selected to be free of Faradaic current and displays minimal hysteresis with respect to the potential scan direction. Over the selected potential window spanning 0.9 V, the capacitance versus potential curve at 9 Hz exhibits a U-shape, with a capacitance minimum of 19.9 ± 1.3 μF cm^–2 at 0.13 ± 0.04 V, flanked by maximum values of 21.2 ± 1.3 and 20.8 ± 1.4 μF cm^–2 at −0.37 and 0.53 V vs Fc/Fc⁺, respectively. This capacitance versus potential profile is consistent with traditional Gouy–Chapman–Stern theory for dilute aqueous electrolyte solutions and high-temperature molten salts but distinctly misaligned with bell- or camel-shaped relationships that have recently been proposed in IL model systems. The minimum capacitance exhibits a small level of frequency dispersion, which increases linearly versus the logarithm of the applied frequency. The potential at which the minimum capacitance is located is also slightly dependent on frequency. This work demonstrates that large-amplitude FT-ACV provides a sensitive probe of the EDL from a single experiment and advances the convergence between theoretical predictions and experimental observations of IL–electrode EDL systems

Double-Layer Capacitance at Ionic Liquid–Boron-Doped Diamond Electrode Interfaces Studied by Fourier Transformed Alternating Current Voltammetry

This article reports the electrochemical double layer behavior at the interfaces of ionic liquids (ILs) and a boron-doped diamond (BDD) electrode as measured by large-amplitude Fourier transformed alternating current (AC) voltammetry (FT-ACV). Data are collected over a ≥2 V potential range and fitted to a simple resistor–capacitor circuit model. The absence of significant higher-order AC harmonic components implies nearly ideal capacitive behavior in the potential ranges examined. Capacitance values for two protic ILs and three aprotic ILs range from 3 to 8 μF cm^–2 and generally increase (1–2 μF cm^–2 V^–1) as the potential is swept from negative to positive values. Capacitance–potential data display little dependence on the composition of the IL. The generally featureless, linear dependence of capacitance on potential over a wide potential range is similar to that reported for BDD electrodes in aqueous electrolyte media, suggesting that the BDD electrode is largely insensitive to the nature of the electrolyte media. The present study concludes that FT-ACV affords an efficient approach to probe the IL–electrode interface, with minimal capacitive hysteresis based on the potential scanning direction

Ru(II) Complexes with a Chemical and Redox-Active S₂N₂ Ligand: Structures, Electrochemistry, and Metal–Ligand Cooperativity

Here we describe the synthesis, structures, and reactivity of Ru complexes containing a triaryl, redox-active S₂N₂ ligand derived from <i>o</i>-phenylenediamine and thioanisole subunits. The coordination chemistry of <i>N</i>,<i>N′</i>-bis­[2-(methylthio)­phenyl]-1,2-diaminobenzene [H₂(^MeSNNS^Me)] was established by treating RuCl₂(PPh₃)₃ with H₂(^MeSNNS^Me) to yield {Ru­[H₂(^MeSNNS^Me)]­Cl­(PPh₃)}Cl (<b>1</b>). Coordinated H₂(^MeSNNS^Me) was sequentially deprotonated to form Ru­[H­(^MeSNNS^Me)]­Cl­(PPh₃) (<b>2</b>) followed by the five-coordinate, square pyramidal complex Ru­(^MeSNNS^Me)­(PPh₃) (<b>3</b>). Single-crystal X-ray diffraction (XRD) studies revealed that the ligand structurally rearranged around the metal at each deprotonation step to conjugate the adjacent aryl groups with the <i>o</i>-phenylenediamine backbone. Deprotonation of <b>2</b> with NaBH₄ or treatment of <b>3</b> with BH₃·tetrahydrofuran (THF) yielded Ru­[(μ-H)­BH₂]­(^MeSNNS^Me)­(PPh₃) (<b>5</b>) with BH₃ bound across a Ru–N bond in a metal–ligand cooperative fashion. The cyclic voltammogram of <b>3</b> in THF revealed three redox events consistent with one-electron oxidations and reductions of the <i>o</i>-phenylenediamine backbone and the metal (Ru³⁺/Ru²⁺). Reactions of <b>3</b> with CO, HBF₄, and benzoic acid yielded the new complexes Ru­(^MeSNNS^Me)­(CO)­(PPh₃), {Ru­[H­(^MeSNNS^Me)]­(PPh₃)­(THF)}­BF₄, and Ru­[H­(^MeSNNS^Me)]­(PPh₃)­(PhCO₂), indicating broader suitability for small molecule binding and reactivity studies. Subsequent nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry data are reported in addition to molecular structures obtained from single-crystal XRD studies

Rapid Macrocycle Threading by a Fluorescent Dye–Polymer Conjugate in Water with Nanomolar Affinity

A macrocyclic tetralactam host is threaded by a highly fluorescent squaraine dye that is flanked by two polyethylene glycol (PEG) chains with nanomolar dissociation constants in water. Furthermore, the rates of bimolecular association are very fast with <i>k</i>_on ≈ 10⁶–10⁷ M^–1 s^–1. The association is effective under cell culture conditions and produces large changes in dye optical properties including turn-on near-infrared fluorescence that can be imaged using cell microscopy. Association constants in water are ∼1000 times higher than those in organic solvents and strongly enthalpically favored at 27 °C. The threading rate is hardly affected by the length of the PEG chains that flank the squaraine dye. For example, macrocycle threading by a dye conjugate with two appended PEG2000 chains is only three times slower than threading by a conjugate with triethylene glycol chains that are 20 times shorter. The results are a promising advance toward synthetic mimics of streptavidin/biotin