We theoretically study the hydrodynamic dispersion by fully developed electroosmotic flow in soft slit microchannels of dense polyelectrolyte layer (PEL). When the PEL is dense, the smallness of its permittivity with respect to the fluid leads to the rearrangement of ions, a phenomenon known as ion partitioning. Moreover, the fluid viscosity gets much higher within the PEL while the opposite happens to molecular diffusivity. Adopting the Debye-Hückel linearization, we obtain analytical solutions for electric potential and velocity distributions by considering different values of permittivity and viscosity inside and outside the PEL. The velocity profile is then used to derive expressions for long-term analyte concentration and dispersion coefficient. Full numerical simulations are also performed to track the broadening of an analyte band from the time of injection. The results indicate that ion partitioning gives rise to the amplification of fluid velocity and, hence, the hydrodynamic dispersion, whereas the opposite is true for the increase in the dynamic viscosity within the PEL. Moreover, even though the reduction of the molecular diffusivity inside the PEL leads to the increase of the dispersion coefficient, its influence is much weaker than those of the ion partitioning and the viscosity variations.
