Unusual trend of viscosities and densities for four ionic liquids containing a tetraalkyl phosphonium cation and the anion bis(2,4,4-trimethylpentyl) phosphinate

•Densities and viscosities are reported for 3 phosphonium phosphinate ionic liquids.•The VFT equation represents well the viscosities of ionic liquids studied here.•Cation alkyl-chain length affects density and viscosity unpredictably.

Densities and viscosities are reported for three similar ionic liquids, all with anion bis(2,4,4-trimethylpentyl) phosphinate [TMPP]. The hydrocarbon chains attached to the phosphonium cation vary in length; the three cations are tetrabutylphosphonium [P4444], trimethyloctylphosphonium [P8111] and tributylmethylphosphonium [P1444]. Contrary to expectation, neither the densities nor the viscosities show a monotonic trend with the length of the hydrocarbon chains on the cation.

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