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Permittivities of 1-alkyl-3-methylimidazolium tetrafluoroborates, hexafluorophosphates, and tetrafluoroborate-chloride mixtures (alkyl = butyl, hexyl, octyl or decyl) were determined from the capacitance of a specially designed capacitor, which was measured as the function of frequency over the range from 0.001 Hz to 5 MHz. The measurement setup was tested against molecular liquids. Water and methanol gave relatively stable results, which comply with the permittivities known from the literature over a frequency range from 30 kHz to 3 MHz. The apparent permittivity measured at lower frequencies was highly overestimated. With low-temperature ionic liquids, the plateau in the apparent permittivity as the function of frequency was narrower, and it was observed between I and 3 MHz. Yet, the apparent permittivities in the plateau region are still overestimated, at least for 1-butyl-3-methylimidazolium salts.
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