Molecular Insight into the Ionic Conduction of Quaternary Ammonium and Phosphonium Cation-Based Ionic Liquids Using Dielectric and Spectroscopy Analyses.

We analyzed the primary properties of ionic liquids (ILs) comprising quaternary phosphonium cations and bis(trifluoromethylsulfonyl) amide anions and compared them with those of corresponding quaternary-ammonium-cation-based ILs. Broadband dielectric spectroscopy was used to confirm the coupling between the translational and orientational motions of ions, and our results demonstrated that the high ionic conductivity of the phosphonium-based ILs was attributed to their fast rotational dynamics. The differences between ILs with different cations were further evaluated using vibrational (Raman and terahertz) spectroscopy. The Raman spectroscopy data revealed that the cation structure affected the conformation and flexibility (conformational change) of the anion. Furthermore, terahertz spectroscopy allowed us to evaluate the relationship between ion transport and intermolecular interactions between the cation and anion of ILs.

Use of ionic liquid for X-ray micro-CT specimen preparation of imbibed seeds.

X-ray micro-CT is one of the most useful techniques to examine 3D cellular architecture inside dry seeds. However, the examination of imbibed seeds is difficult because immersion in water causes a decline in the image quality. Here, we examined the use of ionic liquids for specimen preparation of chemically fixed imbibed seeds of Arabidopsis. We found that treatment with high concentrations of ionic liquids after osmium tetroxide fixation helped not only to prevent the structural damage caused by seed shrinkage, but also to preserve the image quality. Under these conditions, the cellular architecture of seeds was also well maintained.

Charge transport and dipolar relaxations in phosphonium-based ionic liquids.

The role of anions in charge transport and localized dipolar relaxations in tributyloctylphosphonium ionic liquids is investigated by broadband dielectric spectroscopy and rheology. The dielectric spectra are quantitatively described by a combination of the random barrier model which accounts for ion transport and empirical Havriliak-Negami functions to characterize dipolar relaxations. Two secondary relaxations are observed at temperatures below the calorimetric glass transition temperature, where the primary structural relaxation is essentially frozen at the relevant experimental time scales. The faster process has an anion independent activation energy of 30 kJ/mol and is attributed to libration motion of the phosphonium cation. The slower relaxation is similar to a process previously assigned to a Johari-Goldstein relaxation in imidazolium-based ionic liquids; however, the activation energy is significantly higher in the phosphonium systems. For the charge transport dominated regime, it is observed that variation of the anion results in differences in the dc ionic conductivity and characteristic charge transport rates by ∼2.5 decades. Upon scaling by the calorimetric glass transition temperature, both transport quantities are observed to coincide. From these results, a picture of glass transition assisted hopping emerges as the underlying microscopic mechanism of ion conduction, in agreement with recent results obtained for other classes of aprotic ionic liquids.

Ion transport and structural dynamics in homologous ammonium and phosphonium-based room temperature ionic liquids.

Charge transport and structural dynamics in a homologous pair of ammonium and phosphonium based room temperature ionic liquids (ILs) have been characterized over a wide temperature range using broadband dielectric spectroscopy and quasi-elastic light scattering spectroscopy. We have found that the ionic conductivity of the phosphonium based IL is significantly enhanced relative to the ammonium homolog, and this increase is primarily a result of a lower glass transition temperature and higher ion mobility. Additionally, these ILs exhibit pronounced secondary relaxations which are strongly influenced by the atomic identity of the cation charge center. While the secondary relaxation in the phosphonium IL has the expected Arrhenius temperature dependence characteristic of local beta relaxations, the corresponding relaxation process in the ammonium IL was found to exhibit a mildly non-Arrhenius temperature dependence in the measured temperature range-indicative of molecular cooperativity. These differences in both local and long-range molecular dynamics are a direct reflection of the subtly different inter-ionic interactions and mesoscale structures found in these homologous ILs.

Simple observation of Streptococcus mutans biofilm by scanning electron microscopy using ionic liquids.

Scanning electron microscopy (SEM) has been successfully used to image biofilms because of its high resolution and magnification. However, conventional SEM requires dehydration and metal coating of biological samples before observation, and because biofilms consist mainly of water, sample dehydration may influence the biofilm structure. When coated with an ionic liquid, which is a kind of salt that exists in the liquid state at room temperature, biological samples for SEM observation do not require dehydration or metal coating because ionic liquids do not evaporate under vacuum conditions and are electrically conductive. This study investigates the ability of ionic liquids to allow SEM observation of Streptococcus mutans biofilms compared with conventional coating methods. Two hydrophilic and two hydrophobic ionic liquids, all of which are electronic conductors, are used. Compared with samples prepared by the conventional method, the ionic-liquid-treated samples do not exhibit a fibrous extracellular matrix structure and cracking on the biofilm surface. The hydrophilic ionic liquids give clearer images of the biofilm structure than those of the hydrophobic ionic liquids. This study finds that ionic liquids are useful for allowing the observation of biofilms by SEM without preparation by dehydration and metal coating.

The structural classification of the highly disordered crystal phases of [Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6] salts (Nn(+) = tetraalkylammonium and Pn(+) = tetraalkylphosphonium).

The structures of 16 symmetric tetraalkylammonium (Nn(+)) and tetraalkylphosphonium (Pn(+)) salts ([Nn][BF4], [Nn][PF6], [Pn][BF4], and [Pn][PF6], where n = 1 to 4, and denotes the number of carbon atoms in each alkyl chain) have been investigated by X-ray diffraction in order to elucidate the effect of ion size on the disordered structure of organic salts. All the salts exhibit one or more solid-solid phase transitions in differential scanning calorimetric curves. Powder X-ray diffraction revealed that the highest temperature solid phase of these salts belongs to a crystal system with a high cubic or hexagonal symmetry. The structures are classified into 5 different types: CsCl', NaCl, NaCl', inverse NiAs, and TBPPF6. The CsCl'-type whose octant corresponds to the original CsCl unit cell is observed for [N1][PF6] owing to the orientational difference for the cation or the anion. The NaCl-type structure is observed for the N2(+) and P2(+) salts while the NaCl'-type structure is observed for [N3][PF6], where the configuration of ions is based on the NaCl-type but the four equivalent positions in the original NaCl lattice split into two sets of equivalent positions (three and one). The inverse NiAs structure is observed for [P3][PF6]. Single-crystal X-ray diffraction reveals that the disordering of ions in [P4][PF6] becomes more significant with increasing temperature. The new structure of a cubic phase, the TBPPF6-type structure, is found for the salts with long alkyl chains. The structure is roughly determined at 333 K and the ions therein are highly disordered but not rotating. The validity of the radius ratio rule is confirmed through appropriate assessment of the ion size.

SEM observation of hydrous superabsorbent polymer pretreated with room-temperature ionic liquids.

Room-temperature ionic liquid (RTIL), which is a liquid salt at or below room temperature, shows peculiar physicochemical properties such as negligible vapor pressure and relatively-high ionic conductivity. In this investigation, we used six types of RTILs as a liquid material in the pretreatment process for scanning electron microscope (SEM) observation of hydrous superabsorbent polymer (SAP) particles. Very clear SEM images of the hydrous SAP particles were obtained if the neat RTILs were used for the pretreatment process. Of them, tri-n-butylmethylphosphonium dimethylphosphate ([P(4, 4, 4, 1)][DMP]) provided the best result. On the other hand, the surface morphology of the hydrous SAP particles pretreated with 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) was damaged. The results of SEM observation and thermogravimetry analysis of the hydrous SAP pretreated with the RTILs strongly suggested that most water in the SAP particles are replaced with RTIL during the pretreatment process.

Physicochemical properties of tri-n-butylalkylphosphonium cation-based room-temperature ionic liquids.

The physicochemical properties of novel four tri-n-butylalkylphosphonium-based room-temperature ionic liquids (RTILs), tri-n-butylmethylphosphonium dimethylphosphate ([P(4,4,4,1)][DMP]), tri-n-butyl(2-hydroxymethyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P(4,4,4,2OH)][Tf2N]), tetra-n-butylphosphonium O,O'-diethylphosphorodithioate ([P(4,4,4,4)][DEPDT]), and tri-n-butyldodecylphosphonium 3,5-bis(methoxycarbonyl)benzenesulfonate ([P(4,4,4,12)][MCBS]), were examined in this study. All RTILs showed a favorable thermal decomposition temperature exceeding 560 K. Of these, [P(4,4,4,12)][MCBS] exhibited a fairly high thermal stability compared with common phosphonium cation-based RTILs reported to date. Interestingly [P(4,4,4,1)][DMP] formed an ionic plastic crystal phase within a range of 279-290 K, but that was not the case with [P(4,4,4,4)][DEPDT], which is similar in the cation and anion structures to the [P(4,4,4,1)](+) and [DMP](-). [P(4,4,4,2OH)][Tf2N] showed a relatively high conductivity of 0.48 mS cm(-1) at 303 K among the RTILs consisting of tri-n-butylalkylphosphonium cation and usual fluoroanion.

Physicochemical properties and plastic crystal structures of phosphonium fluorohydrogenate salts.

Fluorohydrogenate salts of quaternary phosphonium cations with alkyl and methoxy groups (tetraethylphosphonium (P(2222)(+)), triethyl-n-pentylphosphonium (P(2225)(+)), triethyl-n-octylphosphonium (P(2228)(+)), and triethylmethoxymethylphosphonium (P(222(101))(+))) have been synthesized by the metatheses of anhydrous hydrogen fluoride and the corresponding phosphonium bromide or chloride precursors. The three salts with asymmetric cations, P(222m)(FH)(2.1)F (m = 5, 8, and 101), are room temperature ionic liquids (ILs) and are characterized by differential scanning calorimetry, density, viscosity, and conductivity measurements. Linear sweep voltammetry using a glassy carbon working electrode shows these phosphonium fluorohydrogenate ILs have wide electrochemical windows (>4.9 V) with the lowest viscosity and highest conductivity in the known phosphonium-based ILs. Thermogravimetry shows their thermal stabilities are also improved compared to previously reported alkylammonium cation-based fluorohydrogenate salts. Differential scanning calorimetry and X-ray diffraction revealed that tetraethylphosphonium fluorohydrogenate salt, P(2222)(FH)(2)F, exhibits two plastic crystal phases. The high temperature phase has a hexagonal lattice, which is the first example of a plastic crystal phase with an inverse nickel arsenide-type structure, and the low-temperature phase has an orthorhombic lattice. The high-temperature plastic crystal phase exhibits a conductivity of 5 mS cm(-1) at 50 °C, which is the highest value for the neat plastic crystals.

Thermal and transport properties of ionic liquids based on benzyl-substituted phosphonium cations.

The physicochemical properties of two novel ionic liquids based on benzyltriethylphosphonium and benzyltributylphosphonium cations are described in this report. It was found that both benzyl-substituted phosphonium cations gave low-melting salts in combination with a bis(trifluoromethylsulfonyl)amide anion. The thermogravimetric analysis suggested that the benzyl-substituted phosphonium ionic liquids showed higher thermal stability than those of not only the alkyl-substituted phosphonium ILs but also the corresponding benzyl-substituted ammonium compounds. The benzyl-substituted phosphonium ionic liquids also exhibited relatively high conductivities when compared to those of the corresponding ammonium compounds. These results indicate an improving effect of introducing a benzyl group into the phosphonium cations on both the thermal stability and the conductivity.