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Ionic liquids (ILs) with long alkyl substituents are amphiphilic, which leads to a bicontinuous liquid structure. The strongly interacting anionic and cationic head groups form a long range charge network, with the hydrocarbon tails forming a nonpolar domain. Such nonpolar domains have been shown to dissolve a variety of neutral organic solvents. In mixtures of ILs with solvents the neutral organic molecules residing in the nonpolar domains experience different environments and friction from the charged cations and anions. Thus, the neutral molecules diffuse much faster than predicted by hydrodynamic scaling using the average viscosity of the mixture. Here, we reportmore » studies on the structure and transport properties of mixtures of 1-octanol with the IL trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (P_6,6,6,14⁺/NTf₂^–). The majority of the atom fraction in the P_6,6,6,14⁺ cation comprises four hydrocarbon substituents. The unique amphiphilic nature of ILs with the P_6,6,6,14⁺ cation makes 1-octanol fully miscible with the IL at ambient temperatures. X-ray scattering experiments show that the IL structure persists in the mixtures for 1-octanol mole fractions as large as x_oct = 0.90. The self-diffusion coefficients of the three molecular species in the mixtures were measured by NMR experiments. The self-diffusion of the P_6,6,6,14⁺ cation is well described by the Stokes–Einstein equation, while the diffusivity of the NTf₂^– anion is slightly lower than the hydrodynamic prediction. The measured diffusivities of octanol in these mixtures are 1.3–4 times higher than the hydrodynamic predictions.« less

The role that interfaces play in the dynamics of liquids is a fundamental scientific problem with vast importance in technological applications. From material science to biology, e. g., batteries to cell membranes, liquid properties at interfaces are frequently determinant in the nature of chemical processes. For most liquids, like water, the influence of an interfaces falls off on a ~1 nm distance scale. Room temperature ionic liquids (RTILs) are a vast class of unusual liquids composed of complex cations and anions that are liquid salts at room temperature. They are unusual liquids with properties that can be finely tuned bymore » selecting the structure of the cation and anion. RTILs are being used or developed in applications such as batteries, CO2 capture, and recently, liquids for biological processes. Here, it is demonstrated quantitatively that the influence of an interface on RTIL properties is profoundly different from that observed in other classes of liquids. The dynamics of planar thin films of the room temperature ionic liquid, 1- butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2), were investigated using two-dimensional infrared spectroscopy (2D IR) with the CN stretch of SeCN- as the vibrational probe. The structural dynamics (spectral diffusion) of the thin films with controlled nanometer thicknesses were measured and compared to the dynamics of the bulk liquid. The samples were prepared by spin coating the RTIL, together with the vibrational probe, onto a surface functionalized with an ionic monolayer that mimics the structure of the BmimNTf2. Near-Brewster’s angle reflection pump-probe geometry 2D IR, facilitated the detection of the exceedingly small signals from the films, some of which were only 14 nm thick. Even in quarter micron (250 nm) thick films, the observed dynamics were much slower than those of the bulk liquid. Using a new theoretical description, the correlation length (exponential fall-off of the influence of the interfaces) was found to be 28 ± 5 nm. This very long correlation length, ~30 times greater than that of water, has major implications for the use of RTILs in devices and other applications.« less

Zwitterionic liquids (Zw-ILs) have been developed that are homologous to monovalent ionic liquids (ILs) and show great promise for controlled dissolution of cellulosic biomass. Using both high energy X-ray scattering and atomistic molecular simulations, this article compares the bulk liquid structural properties for novel Zw-ILs with their homologous ILs. It is shown that the significant localization of the charges on Zw-ILs leads to charge ordering similar to that observed for conventional ionic liquids with monovalent anions and cations. A low-intensity first sharp diffraction peak in the liquid structure factor S(q) is observed for both the Zw-IL and the IL. Thismore » is unexpected since both the Zw-IL and IL have a 2-(2-methoxyethoxy)ethyl (diether) functional group on the cationic imidazolium ring and ether functional groups are known to suppress this peak. Detailed analyses show that this intermediate range order in the liquid structure arises for slightly different reasons in the Zw-IL vs. the IL. For the Zw-IL, the ether tails in the liquid are shown to aggregate into nanoscale domains.« less

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Recently, we have reported a systematic study of photoinduced electron-transfer reactions in ionic liquid solvents using neutral and anionic electron donors and a series of cyano-substituted anthracene acceptors [Wu, B.; Maroncelli, M.; Castner, E. W., Jr.Photoinduced Bimolecular Electron Transfer in Ionic Liquids. J. Am. Chem. Soc.139, 2017, 14568]. In this paper, we report complementary results for a cationic class of 1-alkyl-4-dimethylaminopyridinium electron donors. Reductive quenching of cyano-substituted anthracene fluorophores by these cationic quenchers is studied in solutions of acetonitrile and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Varying the length of the alkyl chain permits tuning of the quencher diffusivities in solution.more » The observed quenching kinetics are interpreted using a diffusion-reaction analysis. Finally, together with results from the prior study, these results show that the intrinsic electron-transfer rate constant does not depend on the quencher charge in this family of reactions.« less

Ionic liquids with cationic organosilicon groups have been shownto have a number of useful properties, including reduced viscosities relative to the homologous cations with hydrocarbon substituents on the cations. Herein, we report structural and dynamical properties of four ionic liquids having a trimethylsilylpropyl functional group, including 1-methyl-3-trimethylsilylpropylimidazolium (Si–C₃-mim⁺) cation paired with three anions: bis(fluorosulfonyl)imide (FSI ), bis(trifluoromethanesulfonyl)imide (NTf$${^–_2}$$), and bis(pentafluoroethanesulfonyl)imide (BETI ), as well as the analogous N-methyl-Ntrimethylsilylpropylpyrrolidinium (Si–C₃-pyrr⁺) cation paired with NTf$${^–_2}$$. This choice of ionic liquids permits us to systematically study how increasing the size and hydrophobicity of the anions affects the structural and transport properties of themore » liquid. Structure factors for the ionic liquids were measured using high energy X-ray diffraction and calculated from molecular dynamics simulations. The liquid structure factors reveal first sharp diffraction peaks (FSDPs) for each of the four ionic liquids studied. Interestingly, the domain size for Si–C₃-mim⁺/NTf$${^–_2}$$ indicated by the maxima for these peaks is larger than for the more polar ionic liquid with a similar chain length, 1-pentamethyldisiloxymethyl- 3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (SiOSi-mim⁺/NTf$${^–_2}$$. For the series of Si–C₃-mim⁺ ionic liquids, as the size of the anion increases, the position of FSDP indicates that the intermediate range order domains decrease in size, contrary to expectation. Diffusivities for the anions and cations are compared for a series of both hydrocarbon-substituted and siliconsubstituted cations. All of the anions show the same scaling with temperature, size, and viscosity, while the cations show two distinct trends—one for hydrocarbon-substituted cations and another for organosilicon-substituted cations, with the latter displaying increased friction.« less

Significantly lower viscosities result when a single alkyl carbon is replaced by a silicon atom on the side chain of an ionic liquid cation. To further explore this effect, we compare liquid structure factors measured using high-energy X-ray scattering and calculated using molecular dynamics simulations. In this work, four ionic liquids are studied that each has a common anion, bis(trifluoromethylsulfonyl)amide (NTf$${^–_2}$$). The four cations for this series of NTf$${^–_2}$$-anion ionic liquids are 1-methyl-3-trimethylsilylmethylimidazolium (Si-mim⁺), 1-methyl-3-neopentylimidazolium (C-mim⁺), 1-methyl-3-pentamethyldisiloxymethylimidazolium (SiOSi-mim⁺), and 1-methyl-1- trimethylsilylmethylpyrrolidinium (Si-pyrr⁺). To achieve quantitative agreement between the structure factors measured using high-energy X-ray scattering and molecular dynamics simulations, newmore » transferable parameters for silicon were calibrated and added to the existing force fields.« less

Ionic liquids with cyano anions have long been used because of their unique combination of low-melting temperatures, reduced viscosities, and increased conductivities. Recently we showed that cyano anions in ionic liquids are particularly interesting for their potential use as electron donors to excited state photo-acceptors [B. Wu et al., J. Phys. Chem. B 119, 14790–14799 (2015)]. Here we report on bulk structural and quantum mechanical results for a series of ionic liquids based on the 1-ethyl-3-methylimidazolium cation, paired with the following five cyano anions: SeCN^-, SCN^-, N(CN)_{$$-\atop{2}$$}, C(CN)_{$$-\atop{3}$$}, and B(CN)_{$$-\atop{4}$$}. By combining molecular dynamics simulations, high-energy X-ray scattering measurements, andmore » periodic boundary condition DFT calculations, we are able to obtain a comprehensive description of the liquid landscape as well as the nature of the HOMO-LUMO states for these ionic liquids in the condensed phase. Features in the structure functions for these ionic liquids are somewhat different than the commonly observed adjacency, charge-charge, and polarity peaks, especially for the bulkiest B(CN)_{$$-\atop{4}$$} anion. While the other four cyano-anion ionic liquids present an anionic HOMO, the one for Im_{$$+\atop{2,1}$$}/B(CN)_{$$-\atop{4}$$} is cationic.« less

Ionic liquids with cyano anions have long been used because of their unique combination of low-melting temperatures, reduced viscosities, and increased conductivities. Recently we have shown that cyano anions in ionic liquids are particularly interesting for their potential use as electron donors to excited state photo-acceptors [B. Wu et al., J. Phys. Chem. B 119, 14790–14799 (2015)]. Here we report on bulk structural and quantum mechanical results for a series of ionic liquids based on the 1-ethyl-3-methylimidazolium cation, paired with the following five cyano anions: SeCN-, SCN-, N(CN)-2N(CN)2-, C(CN)-3C(CN)3-, and B(CN)-4B(CN)4-. By combining molecular dynamics simulations, high-energy X-ray scattering measurements,more » and periodic boundary condition DFT calculations, we are able to obtain a comprehensive description of the liquid landscape as well as the nature of the HOMO-LUMO states for these ionic liquids in the condensed phase. Features in the structure functions for these ionic liquids are somewhat different than the commonly observed adjacency, charge-charge, and polarity peaks, especially for the bulkiest B(CN)-4B(CN)4- anion. While the other four cyano-anion ionic liquids present an anionic HOMO, the one for Im+2,1Im2,1+/B(CN)-4B(CN)4- is cationic.« less