Dielectric friction, violation of the Stokes-Einstein-Debye relation, and non-Gaussian transport dynamics of dipolar solutes in water

Abstract

The phenomenon of dielectric friction predicts slowing down of rotational and translational diffusion of a dipolar tracer in a polar medium due to retarded response of the medium polarization. This problem is studied here by numerical simulations in which the dipole moment of the tracer (solute) is continuously increased. The rotational time of the solute increases linearly with its squared dipole moment. A more pronounced effect of the solute dipole is found on the relaxation time of the electric field of the medium acting on the dipole: We report two orders of magnitude retardation for the electric field dynamics. With this strong retardation, classical theories of dielectric friction fail to describe slowing down of rotational diffusion. This failure is traced back to the breakdown of additivity between friction produced by van der Waals and electrostatic forces and torques. In contrast to the neglect of their correlations by traditional models, electrostatic and van der Waals forces and torques are strongly correlated. Electrostatic interactions bring to linear transport coefficients a number of features typically associated with the dynamics of low-temperature and supercooled liquids. The translational diffusion coefficient becomes strongly anisotropic in the solute's body frame, which translates to non-Gaussian translational dynamics. The Stokes-Einstein-Debye relation connecting the translational and rotational single-particle dynamics deviates from the hydrodynamic limit in linear proportion to the squared solute dipole. It is also found to increase with lowering temperature in qualitative agreement with phenomenology of supercooled liquids.