The dynamics of room temperature ionic liquids confined in the pores of polyether sulfone membranes (PES200) with an average pore size of ~350 nm and in the bulk liquids were studied. PES membranes are being considered for applications in CO2 capture as Supported Ionic Liquid Membranes (SILMs). A number of dynamical experimental methods were applied. Two dimensional infrared (2D IR) and polarization selective pump-probe (PSPP) experiments were used to study two vibrational probes, SeCN- and CO2, in EmimNTf2 (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in the bulk liquid and in the membrane pores. Both the spectral diffusion measured with 2D IR and the orientational relaxation measured with PSPP slowed substantially in membrane pores in spite of their large size. Time correlated single photon counting measurements of the fluorescence of the fluorophore Coumarin 153 (C153) were used to observe the time dependent Stokes shift (solvation dynamics) in CnmimNTf2, n = 2, 4, 6, 10: ethyl – Emim; butyl – Bmim; hexyl – Hmim; decyl – Dmim bulk liquids and in the PES200 pores. In addition, optical heterodyne detected optical Kerr effect (OHD-OKE) experiments investigated the time for complete structural relaxation of the four bulk liquids. In the bulk liquids, the slowest component of the C153 time dependent Stokes shift was identical to the OHD-OKE final exponential decay for complete structural randomization. Therefore, the slowest component of the Stokes shift is the time for complete randomization of each RTIL. In the membrane pores, the slowest component of Stokes shift for EmimNTf2 was longer by a factor of 4.4, a substantial slowing of the structural dynamics. However, for BminNTf2, HminNTf2, and DminNTf2, the dynamics slowed by factors of 2.0, 1.1, and 1.0, respectively. Thus, as the alkyl chain length increased, the effect of mesoscopic confinement decreased and vanished. These results are discussed in terms of a model in which long range ordering by the polymer interface is in competition with the driving force to form extended apolar regions. Finally, new reflection enhance 2D IR method was applied to very thin (~100 nm) planar films of RTILs using SeCN- as the vibrational probe. The new method enables excellent data to be acquired on thin films. The results showed dramatic slowing of the structural dynamics compared to the equivalent bulk liquids.

Influence of mesoscopic confinement on the dynamics of room temperature ionic liquids

255th National Meeting (2018)

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