Electrophoretic motion of a spherical particle with a thick double layer in bounded flows

The electrophoretic mobility of a spherical colloidal particle with low zet a potential near a solid charged boundary is calculated numerically for arb itrary values of the double layer thickness by a generalization of Teubner' s method to the case of bounded flow. Three examples are considered: a sphe re near a nonconducting planar wall with electric field parallel to the wal l, near a perfectly conducting planar wall with electric field perpendicula r to the wall, and on the axis of a cylindrical pore with electric field pa rallel to the axis. The results are compared with recent analytical calcula tions using the method of reflections. For the case of a charged sphere nea r a neutral surface, the reflection results are quite good, provided there is no double layer overlap, in which case there can be extra effects for co nstant potential particles that are entirely missed by the analytical expre ssions. For a neutral sphere near a charged surface, the reflection results are less successful. The main reason is that the particle feels the profil e of the electroosmotic flow, an effect ignored by construction in the meth od of reflections. The general case is a combination of these, so that the reflections are more reliable when the electrophoretic motion dominates the electroosmotic flow. The effect on particle mobility of particle-wall inte ractions follows the trend expected on geometric grounds in that sphere-pla ne interactions are stronger than sphere-sphere interactions and the effect on a sphere in a cylindrical pore is stronger still. In the latter case, p article mobility can fall by more than 50% for thick double layers and a sp here half the diameter of the pore. The agreement between numerical results and analytical results follows the same trend, being worst for the sphere in a pore. Nevertheless, the reflections can be reliable for some geometrie s if there is no double layer overlap. This is demonstrated for a specific example where reflection results have previously been compared with experim ents on protein mobility through a membrane (J. Ennis et at, 1996, J. Membr ane Sci. 119, 47). (C) 1999 Academic Press.