The properties of four completely saturated and one partially saturated salts consisting of methyl-tri-n-decylphosphonium cations and either chloride, chloride monohydrate, bromide, bromide monohydrate, or nitrate anions are reported. Their neat phase behavior has been examined by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The monohydrates form smectic phases that persist in >90 °C ranges and extend from below room temperature; they are ordered, room-temperature ionic liquid crystals. The nitrate salt becomes liquid-crystalline above room temperature and the other two salts remain as soft solids to their isotropization temperatures. The locations of two solvatochromic dyes, Nile Red and 1,1-dicyano-2-[6-(dimethylamino)naphthylen-2-yl]propene, within the ionic assemblies have been approximated from UV/vis absorption and fluorescence spectra, and the temperature (phase) dependence of the conductivities and dielectric constants of some of the salts have been measured. Some comparisons with properties of isotropic, room-temperature ionic liquids are made.

The use of small divalent anions is suggested as a strategy for engineering regular patterns of “holes” in ionic planes of crystalline ammonium and phosphonium salts with four equivalent long (i.e., ≳ 8 carbon atoms) n-alkyl chains. Crystals of the first example of such a salt, bis[tetra-dodecylammonium] hexafluorosilicate, are shown to include the desired regular pattern of holes (ca. 40 Å2 cross sections) within the ionic layers.

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