Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6](-) anion, nu(s)(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [C(n)C(1)im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C(6)C(1)im][PF6] and [C(8)C(1)im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C(4)C(1)im][PF6], which crystallizes above similar to 0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of nu(s)(PF6) with density observed in [C(6)C(1)im][PF6] and in [C(8)C(1)im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, <Delta omega(2)>, and the relaxation time of frequency fluctuation, tau(c), as a function of density by Raman band shape analysis of the nu(s)(PF6) mode of [C(6)C(1)im][PF6] and [C(8)C(1)im][PF6]. (C) 2013 AIP Publishing LLC.