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Air- and water-stable 1-alkyl-3-methylimidazolium tetrafluoroborate salts with the general formula Cn-mimBF4 (n = 0–18) have been prepared by metathesis from the corresponding chloride or bromide salts. The salts have been characterised by 1H NMR and IR spectroscopy, microanalysis, polarising optical microscopy and differential scanning calorimetry. Those with short alkyl chains (n = 2–10) are isotropic ionic liquids at room temperature and exhibit a wide liquid range, whereas the longer chain analogues are low melting mesomorphic crystalline solids which display an enantiotropic smectic A mesophase. The thermal range of the mesophase increases with increasing chain length and in the case of the longest chain salt prepared, C18-mimBF4, the mesophase range is ca. 150 °C.
Holbrey et al. (Fri,) studied this question.