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The key issue of solution chemistry is to understand the delicate balance between
solvent-solvent, solute-solvent and solute-solute interactions as it governs solvation
phenomena and solute aggregation processes and thus determines the macroscopic properties
of solutions. This requires molecular level information on the structure and dynamics of
solutions as well as accurate thermodynamic and transport data to benchmark computer
simulations and theoretical calculations. Accordingly, a plethora of powerful techniques are
used now in solution chemistry [1].
Spectroscopic techniques, such as NMR, Raman or time-resolved infrared
spectroscopy, have provided valuable information on solvation and ion-binding phenomena
[2]. However, these powerful methods are generally only sensitive to next-neighbour
interactions and/or the reorientation dynamics of individual molecules so that mesoscopic
structural correlations, including already solvent-shared ion pairs, or cooperative dynamical
processes elude detailed investigation. Here dielectric relaxation spectroscopy (DRS) can step
in. DRS probes fluctuations of the macroscopic dipole moment and thus yields information on
the cooperative and molecular dynamics of the sample on a timescale ranging in principle
from ~10-13 to 103 s [3]. Processes relevant in solution chemistry typically are in the pico- to
nanosecond range and encompass the reorientation and libration of permanent dipole
moments (e.g. solvent molecules, ion pairs), intermolecular vibrations (e.g. of anions and
cations relative to each other), and motions of counterions bound to micelles or polyions.
In this contribution a short introduction into the principles of dielectric
relaxation spectroscopy will be given before proceeding to examples from our laboratory on
applications of this technique in solution chemistry. One focus will be on the solvation and
ion binding of electrolytes and non-electrolytes in aqueous solutions, highlighting the
different behavior of hydrophilic and hydrophobic solutes. The second focus will be on the
use of in the investigation of ionic liquids and their mixtures with polar solvents.
Additionally, the merits of DRS for the investigation of counter-ion condensation on charged
micelles will be briefly discussed. |