Electrophoretic mobility of a "soft particle" with a nonuniformly charged surface layer as a model for cells

The electrophoretic mobility of a colloidal particle with a nonuniformly charged ion-penetrable surface layer in an electrolyte solution (as a model for a biological cell) is calculated for the case in which the surface charge layer consists of two oppositely charged sublayers. It is shown that the mobility and the surface potential (the latter of which is defined as the potential at the front surface of the surface charge layer) generally exhibit different behaviors, in contrast to the case of rigid particles, which is described by Smoluchowski's mobility formula. The mobility in some cases shows a minimum (or a maximum) when plotted as a function of the electrolyte concentration and alters its sign at a certain concentration, which is considered to be an isoelectric point of the mobility with respect to the electrolyte concentration. This isoelectric point is in general different from that of the surface potential.