Differential Microscopic Mobility of Components within a Deep Eutectic Solvent

From macroscopic measurements of deep eutectic solvents such as glyceline (1:2 molar ratio of choline chloride to glycerol), the long-range translational diffusion of the larger cation (choline) is known to be slower compared to that of the smaller hydrogen bond donor (glycerol). However, when the diffusion dynamics are analyzed on the subnanometer length scale, we find that the displacements associated with the localized diffusive motions are actually larger for choline. This counterintuitive diffusive behavior can be understood as follows. The localized diffusive motions confined in the transient cage of neighbor particles, which precede the cage-breaking long-range diffusion jumps, are more spatially constrained for glycerol than for choline because of the stronger hydrogen bonds the former makes with chloride anions. The implications of such differential localized mobility of the constituents should be especially important for applications where deep eutectic solvents are confined on the nanometer length scale and their long-range translational diffusion is strongly inhibited (e.g., within microporous media)

Quantum Chemical Evaluation of Deep Eutectic Solvents for the Extractive Desulfurization of Fuel

Sulfur compounds in fuels are converted to SO_<i>x</i> during combustion, poisoning automotive catalytic converters and creating serious environmental concerns (e.g., acid rain). The efficient desulfurization of liquid fuel is thus a critical step toward minimizing SO_<i>x</i> emissions and their associated environmental impact. To address this problem, governments worldwide have passed stringent legislation regulating the maximal sulfur levels allowable in fuels. In the petroleum refining industry, the conventional method for removing sulfur from fuel is catalytic hydrodesulfurization which, while highly efficient for removing mercaptans, thioethers, and disulfides, shows limited performance in removing aromatic organosulfur compounds exemplified by dibenzothiophene. To meet these strict environmental targets, innovative strategies beyond hydrodesulfurization for the deep desulfurization of fuel are sought. One key strategy entails the oxidation of refractory organosulfur compounds in liquid fuel, coupled with efficient liquid/liquid extraction of the oxidized sulfur compounds using an immiscible solvent phase (i.e., oxidative desulfurization). In this study, we employ computational chemistry to gain atomistic-level insight into the specific interactions responsible for the extraction of key organosulfur compounds and their oxidation products from fuel using deep eutectic solvents (DESs). Specifically, we perform quantum chemical calculations involving the well-studied DESs reline (1:2 choline chloride:urea) and ethaline (1:2 choline chloride:ethylene glycol) to characterize the intermolecular interactions, charge transfer behavior, and thermodynamics associated with their application for organosulfur extraction. We observe that the model aromatic sulfur compounds (ASCs) benzothiophene and dibenzothiophene interact with choline and the hydrogen bond donor (HBD; i.e., urea or ethylene glycol) of the DES via a plurality of weak noncovalent interactions. However, the chloride ion is essentially noninteractive with the ASC due to retention of the conventional hydrogen bond network existing within the initial DES. Oxidation of the model ASCs to their respective sulfoxide and sulfone products was shown to enhance interactions with the DES components, particularly the HBD species due to its propensity for forming multiple hydrogen bonds. We further demonstrate that, upon oxidation, the ASCs exhibit significant and favorable free energies of solvation, suggesting that oxidation will aid in the partition of these sulfur compounds from liquid fuel to a conventional DES phase

Ionic Liquid Anion Controlled Nanoscale Gold Morphology Grown at a Liquid Interface

Two different ionic liquids comprising the tetrabutylphosphonium cation ([P₄₄₄₄]) paired with the strongly coordinating anions 6-aminocaproate ([6-AC]) or taurinate ([tau]) were prepared and employed in an aqueous/organic liquid bilayer system to generate nanoscale gold by Au­(OH)₄^– photoreduction. Generally, as the concentration of ionic liquid in the organic phase was increased, the resulting quasi-spherical gold nanoparticles were smaller in size and presented less aggregation, leading to marked increases in the catalytic efficiency for 4-nitrophenol reduction using borohydride. The diffusion of the ionic liquids across the liquid/liquid interface was also investigated, revealing partition coefficients of 6.0 and 7.6 for [P₄₄₄₄]­[6-AC] and [P₄₄₄₄]­[tau], respectively. Control studies elucidated that biphasic interfacial reduction was necessary to achieve stable nanoparticles possessing high catalytic activity. When the ionic liquid anion was instead replaced by the weakly coordinating bis­(trifluoro­methyl­sulfonyl)­imide ([Tf₂N]), photoreduction of Au­(OH)₄^– led to holey, wavy gold nanowires instead of spherical nanoparticles, indicating the dramatic morphological control exerted by the coordination strength of the ionic liquid anion. This strategy is straightforward and simple and opens up a number of intriguing avenues for controllably preparing plasmonic colloids for a range of applications from catalysis to optical sensing

Polyol Synthesis of Magnetite Nanocrystals in a Thermostable Ionic Liquid

We report on the development of a facile, one-pot synthesis of single-crystalline magnetite (Fe₃O₄) nanoparticles (NPs) based on the thermal decomposition of the nontoxic iron precursor iron­(III)­acetylacetonate within the ionic liquid trihexyltetradecylphosphonium bis­(trifluoromethylsulfonyl)­imide ([P_6,6,6,14]­[Tf₂N]) using 1,2-hexadecanediol as a polyol reducing agent in an “iono-polyol” process. In this expedient approach, the [P_6,6,6,14]­[Tf₂N] acts both as a low-volatility, thermostable solvent and as the colloid-stabilizing agent, eliminating the requirement for additional surface-capping agents. Performing the synthesis at 300 or 350 °C yielded quasi-spherical, monodispersed Fe₃O₄ NPs with a mean size of 14 nm. Evidence from thermogravimetry, X-ray fluorescence, and infrared analysis is consistent with nanocrystal coverage by a partial bilayer of [P_6,6,6,14]­[Tf₂N], accounting for the excellent dispersibility of the Fe₃O₄ NPs in solvents such as hexane, toluene, and methylene chloride. Time-dependent thermogravimetric analysis reveals that [P_6,6,6,14]­[Tf₂N] is transiently stable at 300 °C for 30 min (sufficient for nanocrystal formation) but rapidly degrades at 350 °C or higher. By employing a reaction temperature of 300 °C, the [P_6,6,6,14]­[Tf₂N] can be recycled and reused multiple times for the subsequent preparation of Fe₃O₄ NPs with no ill effects in terms of particle size, uniformity, or agglomeration. Finally, we demonstrate that the Fe₃O₄ NPs can be dispersed into [P_6,6,6,14]­[Tf₂N] as a solventless carrier fluid to produce an “iono-ferrofluid” responsive to an external magnetic field

Structure and spectroscopy of uranyl and thorium complexes with substituted phosphine oxide ligands

Phosphine oxide ligands are important in the chemistry of the nuclear fuel cycle. We have synthesized and characterized a series of phosphine oxide ligands with polycyclic aromatic hydrocarbon (PAH) groups to enhance the spectroscopic features of uranyl, UO_2^(2+), and to make detection more efficient. Complexation of OPPh_2R, R = C_(10)H_7 (naphthyl); C_(14)H_9 (phenanthrenyl); C_(14)H_9 (anthracenyl); and C_(16)H_9 (pyrenyl), to UO_2(NO_3)_2 afforded the eight-coordinate complexes, UO_2(NO_3)_2(OPPh_2R)_2. An eleven-coordinate complex, Th(NO_3)_4[OPPh_2(C_(14)H_9)]_3, C_(14)H_9 = phenanthrenyl, was structurally characterized, and was found to be the first thorium compound isolated with three phosphine oxide ligands bound. The phosphine oxide ligands were not fluorescent but the anthracenyl-substituted ligand showed broad, red-shifted emission at approximately 50 nm relative to typical anthracene, making this ligand set a possibility for use in detection. The synthesis and spectroscopy of the uranyl and thorium complexes are presented

Characterization of a Novel Ionic Liquid Monopropellant for Multi-Mode Propulsion

A deep eutectic 1:2 molar ratio mixture of choline-nitrate and glycerol [Cho][NO3] - glycerol is investigated as a fuel component in a binary mixture propellant for multi-mode micropropulsion. Specifically, binary mixtures of the novel ionic liquid fuel with hydroxyl-ammonium nitrate (HAN) and ammonium nitrate (AN) are considered and compared against our previously investigated propellant [Emim][EtSO4]-HAN. Chemical rocket performance simulations predict this new propellant to have higher performance (280 vs. 250 sec specific impulse) at lower combustion temperature (1300 vs. 1900K), relaxing catalyst melting temperature requirements and making it a promising alternative. Qualitative experimental investigation of synthesized propellants on a hot plate in atmosphere indicate the AN mixtures are significantly less reactive, and are therefore not investigated further. Quantitative reactivity studies using a microreactor indicate that both 65:35% and 80:20% by mass [Cho][NO3] - glycerol to HAN propellants have a decomposition temperature 26-88% higher than [Emim][EtSO4]-HAN, depending on the catalyst material. Additionally, the decomposition rate (or self-heating rate) was 2 to 17 times slower for [Cho][NO3] - glycerol - HAN on titanium and platinum catalysts, but the 65:35% propellant decomposition rate was approximately 10 °C/s (37%) faster on rhenium. It was also observed that propellants with the novel ionic liquid fuel contain endothermic reaction steps, and therefore higher input heat flux was required to maintain a temperature rise. Overall the results indicate [Emim][EtSO4]-HAN with platinum catalyst is still most promising as a multi-mode micropropulsion propellant

Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of Liquid Fuel

Extractive desulfurization of liquid fuel is a simple process that requires minimum energy input and can be operated via existing liquid–liquid extraction apparatuses. In particular, to achieve deep desulfurization, the conventional hydrodesulfurization (HDS) process has shown limitations in the removal of aromatic sulfur compounds. Recently, extractive desulfurization using a new type of nonvolatile solvent, ionic liquids (ILs), has yielded promising results. However, there is a lack of systematic evaluation of the effect of IL structure on desulfurization efficiency, and a lack of mechanistic understanding regarding how ILs lead to the partition of aromatic sulfur compounds from fuel to the IL phase. The present study examines a total of 71 ILs and two deep eutectic solvents (DESs) with combinations representing various cations and anions. We identify a number of ILs that yield high partition coefficients [up to 1.85 mg­(S) kg (IL)⁻¹/mg­(S) kg (oil)⁻¹] for the partition of aromatic sulfur compounds between ILs and <i>n</i>-octane or <i>n</i>-dodecane as surrogates for gasoline or diesel, respectively. We find that the high sulfur partition coefficient correlates with a high dipolarity/polarizability (π*) or a low solvent polarizability (SP) of ILs carrying the same cation and different anions, but correlates with a low dipolarity/polarizability (π*) for ILs carrying the same anion paired to cations bearing different alkyl chain lengths. We further demonstrate that a four-step extraction using ILs can achieve 99% dibenzothiophene (DBT) removal (i.e., an initial sulfur content of 500 ppm is reduced to <5 ppm following extraction)

Bacterial Cellulose Ionogels as Chemosensory Supports

To fully leverage the advantages of ionic liquids for many applications, it is necessary to immobilize or encapsulate the fluids within an inert, robust, quasi-solid-state format that does not disrupt their many desirable, inherent features. The formation of ionogels represents a promising approach; however, many earlier approaches suffer from solvent/matrix incompatibility, optical opacity, embrittlement, matrix-limited thermal stability, and/or inadequate ionic liquid loading. We offer a solution to these limitations by demonstrating a straightforward and effective strategy toward flexible and durable ionogels comprising bacterial cellulose supports hosting in excess of 99% ionic liquid by total weight. Termed bacterial cellulose ionogels (BCIGs), these gels are prepared using a facile solvent-exchange process equally amenable to water-miscible and water-immiscible ionic liquids. A suite of characterization tools were used to study the preliminary (thermo)­physical and structural properties of BCIGs, including no-deuterium nuclear magnetic resonance, differential scanning calorimetry, thermogravimetric analysis, scanning electron microscopy, and X-ray diffraction. Our analyses reveal that the weblike structure and high crystallinity of the host bacterial cellulose microfibrils are retained within the BCIG. Notably, not only can BCIGs be tailored in terms of shape, thickness, and choice of ionic liquid, they can also be designed to host virtually any desired active, functional species, including fluorescent probes, nanoparticles (e.g., quantum dots, carbon nanotubes), and gas-capture reagents. In this paper, we also present results for fluorescent designer BCIG chemosensor films responsive to ammonia or hydrogen sulfide vapors on the basis of incorporating selective fluorogenic probes within the ionogels. Additionally, a thermometric BCIG hosting the excimer-forming fluorophore 1,3-bis­(1-pyrenyl)­propane was devised which exhibited a ratiometric (two-color) fluorescence output that responded precisely to changes in local temperature. The ionogel approach introduced here is simple and has broad generality, offering intriguing potential in (bio)­analytical sensing, catalysis, membrane separations, electrochemistry, energy storage devices, and flexible electronics and displays