Thickness of electrical double layer. Effect of ion size

The thickness of a single flat electrical double layer is considered. The e lectrostatic mean field and the excluded volume effect are taken into accou nt. A simple statistical mechanical approach is used, where the particles i n the solution are distributed over a lattice with an adjustable lattice co nstant. Different sizes of ions are described by different values of the la ttice constant. Two measures are introduced that describe an effective thic kness of the electrical double layer: a distance where the density of the n umber of counterions drops to a chosen fraction of its maximal value, and a distance defining a region that contains a chosen fraction of the excess o f the counterions. It is shown that the effective thickness of the electric al double layer increases with increasing counterion size and with decreasi ng bulk concentration of ions, whereas increasing the surface charge may ca use either a decrease or an increase of the effective thickness. It is show n that the description of the effective thickness of the electrical double layer by the Debye length differs qualitatively from the description presen ted for low bulk concentrations of ions. (C) 2001 Elsevier Science Ltd. All rights reserved.