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Jaydeb Chakrabarti: „Solute Rotation in Polar liquids: Microscopic Basis for the SED model”
SN Bose National Centre for Basic Sciences, Kolkata, India

We present a molecular level framework for the celebrated Stokes-Einstein-Debye (SED) formula which explores reasons behind the surprising success of the SED model in describing dipolar solute rotation in complex polar media. Relative importance of solvent viscosity and solute-solvent dipolar interaction is quantified via a self-consistent treatment for the total friction on a rotating solute where the hydrodynamic contribution is modified by the friction arising from the longer-ranged solute-solvent dipolar interaction. While the solute-solvent dipolar coupling is obtained via the Mori-Zwanzig formalism, the inclusion of solvent structure via the wave vector dependent viscosity in the hydrodynamic contribution incorporates solvent molecularity in the present theory.

This approach satisfactorily describes the experimental rotation times measured using a dipolar solute, coumarin 153 (C153), in protic and aprotic polar liquids, and more importantly, reveals conditions for dielectric friction to be operative in solvents of different polarity and viscosity. In addition, we derive a microscopic relation between the average rotation and solvation times. Note that, the present approach can be expanded to other kind of solute-solvent combinations, for instance, dipolar solute in electrolyte solutions, ionic liquids and even ionic solute in the above media.