Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

provides a medium for the study of water interacting with ions. Turning the problem around, we are also
studying the influence of water on the dynamics and structure of RTILs.
Optical heterodyne-detected optical Kerr effect (OHD-OKE) measurements on a series of 1-alkyl-
3-methylimidazolium tetrafluoroborate room-temperature ionic liquids (RTILs) as a function of chain length
and water concentration were performed. The pure RTIL reorientational dynamics are identical in form to
other molecular liquids studied previously by OHD-OKE (two power laws followed by a single exponential
decay at long times), but are much slower at room temperature. In contrast, the addition of water to the
longer alkyl chain RTILs caused the emergence of a long time bi-exponential orientational anisotropy
decay. Such distinctly bi-exponential decays have not been seen previously in OHD-OKE experiments
on any type of liquid and were analyzed using a wobbling-in-a-cone model. The slow component for the
longer chain RTILs does not obey the Debye-Stokes-Einstein (DSE) equation across the range of
solutions, and thus we attributed it to slow cation reorientational diffusion caused by a stiffening of cation
alkyl tail-tail associations. The fast component of the decay was assigned to the motions (wobbling) of
the tethered imidazolium head groups. The wobbling-in-a-cone analysis provided estimates of the range
of angles sampled by the imidazolium head group prior to the long time scale complete orientational
randomization. The heterogeneous dynamics and non-DSE behavior observed should have a significant
effect on reaction rates in water/RTIL co-solvent mixtures.
We are continuing and extending our studies, a few of which are briefly outlined above. More
information on our recent work is contained in the papers listed below. 2D IR vibrational echo
experiments and IR polarization selective pump-probe experiments are being conducted on very dilute
D2O in the RTIL butylmethyl-ImidazoliumPF6 at such low water concentration that the water is interacting
with ions but not with other water molecules. These are the first experiments that can study the dynamics
of isolated water molecules in an ionic solution. The results will provide information on the dynamics and
structure of the RTIL and the nature of single water molecules interacting with ions. We are also
conducting detailed 2D IR studies and pump-probe studies of water in salt solutions such as MgSO4. We
are comparing the influence mono and divalent cations and anions on water dynamics. Several types of
experiments are being conducted on polyelectrolyte fuel cell membranes. These studies build on our
earlier work on Nafion by extending the methods we developed to our membranes. IR pump-probe
experiments on water in the membranes are being used to study the nature of water environments in the
nanoscopic channels and water hydrogen bond dynamics. Orientational relaxation measures the time
dependence of hydrogen bond reorganization. We will also use 2D IR vibrational echo experiments to
investigate water dynamics in the membrane channels. In addition we are using photoacids in the
membrane channels to study proton transport using time dependent fluorescence measurements
including population dynamics and orientational relaxation. In both the IR experiments and the
fluorescence experiments, the studies are being conducted as a function of the amount of water in the
fuel cell membrane channels. We are also using fluorescence studies to investigate the influence of the
addition of lithium ions to RTILs that can be useful in battery applications. The orientational relaxation of
a probe chromophore, perylene, that locates in the hydrocarbon regions of the RTILS is used as a probe
of structure and dynamics as a function of lithium ion concentration. We are also beginning studies of the
dynamics and interactions of water in membranes made of carbon nanotubes and graphene oxide.
Publication from DOE Sponsored Research 2009 – present
(1) “Ion-Water Hydrogen Bond Switching Observed with 2D IR Vibrational Echo Chemical Exchange
Spectroscopy,” David E. Moilanen, Daryl Wong, Daniel E. Rosenfeld, Emily E. Fenn, and M. D.
Fayer Proc. Nat. Acad. Sci. U.S.A. 106, 375-380 (2009).
(2) “Water Dynamics and Interactions in Water-Polyether Binary Mixtures,” Emily E. Fenn, David E.
Moilanen, Nancy E. Levinger, and Michael D. Fayer J. Am. Chem. Soc. 131, 5530-5539 (2009).
(3) “Water Dynamics at the Interface in AOT Reverse Micelles,” David E. Moilanen, Emily E. Fenn,
Daryl Wong and M.D. Fayer J. Phys. Chem. B 113, 8560–8568 (2009).
(4) “Geometry and Nanolength Scales vs. Interface Interactions: Water Dynamics in AOT Lamellar
Structures and Reverse Micelles,” David E. Moilanen, Emily E. Fenn, Daryl Wong, and M.D.
Fayer J. Am. Chem. Soc. 131, 8318-8328 (2009).
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