Expansion and shrinkage of the electrical double layer in charge-asymmetric electrolytes

The spatial extension of the electrical double layer determines many thermodynamic and electrokinetic properties of charged colloids in solution. In the classical Debye-Hückel formalism of point-ions, the thickness of the electrical double layer surrounding a charged colloid can be characterized by the bulk Debye length of the supporting ionic fluid. As a result, that approach neglects the influence of the colloidal charge on the spatial extension of the ionic cloud. In this work, we study the thickness of the electrical double layer as a function of the colloidal charge in the presence of several binary charge-asymmetric 1:z electrolytes with monovalent counterions and multivalent coions. The thickness of the ionic cloud is quantified here via the capacitive compactness, which represents essentially the separation distance between two electrodes associated to the corresponding effective electrical double layer capacitor. Our theoretical calculations show that the electrical double layer may expand or shrink as a function of the surface charge density, in the presence of multivalent coions, which is confirmed by primitive model Monte Carlo simulations.