One-Dimensional Ion Transport in Self-Organized Columnar Ionic Liquids

New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts:  an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotropy. These materials would be useful for anisotropic transportation of ions at the nanometer level

Hydrophobicity/Hydrophilicity of 1-Butyl-2,3-dimethyl and 1-Ethyl-3-methylimodazolium Ions: Toward Characterization of Room Temperature Ionic Liquids

We continue to experimentally characterize the constituent ions of room temperature ionic liquids in terms of their interactions with H2O. By using the so-called 1-propanol probing methodology, we experimentally index the relative hydrophobicity/hydrophilicity of a test ion. In this paper, we examine 1-butyl-2,3 dimethylimidazolium (abbreviated as [C4C1mim]+) and 1-ethyl-3-methylimidazolium ([C2mim]+). We found that [C4C1mim]+ dissociates completely in dilute aqueous solution less than 0.006 mol fraction, and hence, its hydrophobicity/hydrophilicity could be determined. The results indicate that [C4C1mim]+ is highly amphiphilic with much stronger hydrophobicity and hydrophilicity than normal ions. Our earlier similar studies indicated the same conclusion for such typical constituent ions as 1-butyl-3-methylimidazolium ([C4mim]+), PF6−, CF3SO3−, and N(SO2CF3)2−. Hence, we suggest that the constituent ions of room temperature ionic liquids that we have studied so far are all amphiphiles with much stronger hydrophobicity and hydrophilicity than normal ions. We found, furthermore, that the hydrophobicity and hydrophilicity of [C4C1mim]+ are stronger than those for [C4mim]+. A possible reason for higher hydrpohilicity is discussed in terms of strong acidic character of H on the C(2) of the imidazolium ring, which tends to attract the delocalized positive charge toward itself on forming a hydrogen bond to H2O. On replacing it with CH3 in [C4C1mim]+, the lack of acidic H enhances the positive charge in the vicinity of N−C−N in the ring that interacts with the surrounding H2O strongly to an induced dipole of O of the H2O. For [C2mim]+, we found it does not dissociate completely, even in dilute aqueous solution, and hence, we could not characterize it within the present methodology

One-Dimensional Ion-Conductive Polymer Films: Alignment and Fixation of Ionic Channels Formed by Self-Organization of Polymerizable Columnar Liquid Crystals

We have prepared two types of one-dimensional ion-conductive polymer films containing ion nanochannels that are both perpendicular and parallel to the film surface. These films have been obtained by photopolymerization of aligned columnar liquid crystals of a fan-shaped imidazolium salt having acrylate groups at the periphery. In the columnar structure, the ionic part self-assembles into the inner part of the column. The column is oriented macroscopically in two directions by different methods:  orientation perpendicular to the modified surfaces of glass and indium tin oxide with 3-(aminopropyl)triethoxysilane and orientation parallel to a glass surface by mechanical shearing. Ionic conductivities have been measured for the films with columnar orientation vertical and parallel to the surface. Anisotropic ionic conductivities are observed for the oriented films fixed by photopolymerization. The ionic conductivities parallel to the columnar axis are higher than those perpendicular to the columnar axis because the lipophilic part functions as an ion-insulating part. The film with the columns oriented vertically to the surface shows an anisotropy of ionic conductivities higher than that of the film with the columns aligned parallel to the surface

Nano-Segregated Polymeric Film Exhibiting High Ionic Conductivities

Nanostructures can be used for the fabrication of highly functional materials transporting ions and charges. We demonstrate a new design strategy for polymeric higher ion-conductors. Phase-segregated layers of alternating mobile tetra(ethylene oxide)s (TEOs) and rigid aromatic cores where the TEO moieties are grafted from aromatic layers have been shown to be efficient to transport lithium triflate. Such segregated structures at the nanometer scale (nano-segregated structures) were prepared by in-situ photopolymerization of an aligned methacrylate liquid crystalline monomer comprising a terphenyl rigid rod mesogen having a TEO terminal chain. The ion-conductive TEO moiety remains in the highly mobile state even after polymerization, which is indicated by its low glass transition temperature (−45 °C). This nanostructured film exhibits an ionic conductivity parallel to the layer of 10-3 S cm-1 at room temperature. The highest ionic conductivity is in the level of 10-2 S cm-1 observed at 150 °C. The anisotropic ionic conductivities have been observed for the nano-segregated film

Noncovalent Approach to One-Dimensional Ion Conductors: Enhancement of Ionic Conductivities in Nanostructured Columnar Liquid Crystals

Noncovalent design of new liquid-crystalline (LC) columnar assemblies based on an ionic liquid has shown to be useful to achieve anisotropic high ionic conductivities. An equimolar mixture of an ionic liquid, 1-butyl-3-methylimidazolium bromide, and 3-[3,4,5-tri(octyloxy)benzoyloxy]propane-1,2-diol, which is partially miscible with the ionic liquid, exhibits an LC hexagonal columnar phase from −4 to 63 °C. This columnar supramolecular assembly forming the nanostructures shows the one-dimensional (1D) ionic conductivity of 3.9 × 10-3 S cm-1 at 50 °C along the column, which is more than 700 times higher than that of the corresponding covalent-type columnar ionic liquid, 1-methyl-3-[3,4,5-tri(octyloxy)benzyl]imidazolium bromide, which is 5.3 × 10-6 S cm-1 at 50 °C. This significant enhancement of the ionic conductivity is attributed to the increase of the mobility of the ionic part

Nanostructured Anisotropic Ion-Conductive Films

A flexible self-standing film with layered nanostructures was obtained by in situ photopolymerization of a new smectic liquid-crystalline monomer containing a tetra(oxyethylene) moiety, which forms a macroscopically oriented complex with lithium salts. The resultant films show two-dimensional ionic conductivity

Self-Organization of Room-Temperature Ionic Liquids Exhibiting Liquid-Crystalline Bicontinuous Cubic Phases: Formation of Nano-Ion Channel Networks

Three-dimensionally interconnected nano-ion channel networks are formed by room-temperature ionic liquids exhibiting thermotropic liquid-crystalline (LC) bicontinuous cubic phases. These LC ionic liquids are a new family of ion-conductive materials with self-organized nanostructures. The ionic liquids have fan-shaped block molecular structures composed of two immiscible molecular parts:  the ammonium moiety at the focal point and the lipophilic tris(alkyloxy)phenyl part. We demonstrate that the ionic conductivities of the materials that are alignment free in the LC bicontinuous cubic phases are higher than those observed in the LC columnar phases

Uniaxially Parallel Alignment of a Smectic A Liquid-Crystalline Rod−Coil Molecule and Its Lithium Salt Complexes Using Rubbed Polyimides

The alignment behavior of thermotropic smectic A liquid-crystalline molecule 1 and its complexes with lithium triflate on the thin films of rubbed polyimides has been examined. Compound 1 has a rod−coil block structure consisting of a tetra(ethylene oxide) chain that is polar and a phenylcyclohexyl mesogen that is nonpolar. The use of a rubbed polyimide having a linear and rigid molecular structure prepared from pyromellitic dianhydride and 1,4-phenylenediamine induces the uniaxially parallel alignment of 1 and its lithium triflate complexes. The lithium salt complexes of compound 1 that exhibit smectic A phases function as two-dimensional ion conductors. The ionic conductivities of the aligned complexes of 1 on the rubbed polyimides were measured by an ac impedance method. Anisotropic ionic conductivities have been observed for the aligned liquid-crystalline lithium salt complexes