| Abstract
| - The validity of the hypothesis of electroneutrality outside the double layer of a suspended particle with anapplied ac electric field is analyzed. It is shown that the electrolyte solution remains electroneutral for distancesgreater than a few Debye lengths from the particle surface only when the diffusion coefficients of the two ionspecies are identical. On the contrary, in the general case, a volume charge density around the particle buildsup, which extends to distances that are proportional to the square root of the effective diffusion coefficientvalue divided by the frequency. These distances can easily attain many particle radii. Numerical results forboth uncharged and charged suspended particles are presented, and a correction to existing analytical expressionsfor the field-induced ion distributions around uncharged particles (J. Phys. Chem.2004, 108, 8397) is given.While the charge densities far from the particle are usually very weak, it is shown that they strongly contributeto the dipole coefficient value and, therefore, to the calculated values of the permittivity and conductivityincrements. The errors that would be committed if these charge densities were ignored, assuming localelectroneutrality and determining the dipole coefficient at a few Debye lengths from the particle surface, areanalyzed and shown to be substantial.
|
