Solute rotation in polar liquids: microscopic basis for the Stokes-Einstein-Debye model

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• The Stokes-Einstein-Debye (SED) formula successfully describes dipolar solute rotation in polar media.
• Understanding the molecular basis of SED formula's success is crucial for complex systems.

Purpose of the Study:

• To develop a molecular-level framework for the Stokes-Einstein-Debye (SED) formula.
• To quantify the relative importance of solvent viscosity and solute-solvent dipolar interactions on solute rotation.

Main Methods:

• A self-consistent treatment for total friction on a rotating solute.
• Incorporating wave vector-dependent viscosity to account for solvent structure and molecularity.
• Utilizing the Mori-Zwanzig formalism for solute-solvent dipolar coupling.

Main Results:

• The developed framework explains the success of the SED model in describing dipolar solute rotation.
• Experimental rotation times of coumarin 153 (C153) in various polar liquids were accurately reproduced.
• Demonstrated the insignificant contribution of electrical interactions to solute rotation, unlike ionic mobility.

Conclusions:

• The study provides a microscopic explanation for solute rotation dynamics in polar solvents.
• The framework successfully integrates hydrodynamic and dipolar interaction effects.
• The theory can be extended to study solute rotation in diverse non-dipolar solvent systems.