DETERMINING THE FORCES BETWEEN POLYSTYRENE LATEX SPHERES USING DIFFERENTIAL ELECTROPHORESIS

The attractive force holding two polystyrene latex spheres in a double t was measured by the method of differential electrophoresis. The two spheres of each doublet had different surface chemistries (e.g., sulfa te and carboxylate) and different zeta potentials zeta(1) and zeta(2). The doublet acted as a dipole, and an applied electric field (E(infin ity)) caused the doublet to rotate such that the less negative sphere pointed in the direction of the field. Once the doublet was aligned, t he tendency of the spheres to translate at different velocities produc ed a tension, the ''electrophoretic displacement force''. This force, proportional to zeta(2) - zeta(1) and the applied electric field E(inf inity), is calculated from solutions to the electrostatic and hydrodyn amic equations. For our systems (5 mu m diameter spheres, zeta(2) - ze ta(1) approximate to 40 mV, E(infinity) approximate to 200 V/cm) the e lectrophoretic displacement force was 20-50 pN, which is more than a f actor of 10 greater than the maximum attractive force predicted by DLV O theory for doublets in a secondary minimum. In no case could we brea k the doublets with the electrophoretic displacement force. We conclud e that DLVO theory is inadequate for our colloidal system, either beca use the doublets were in a primary minimum (even though DLVO theory pr edicted an insurmountable energy barrier) or because the depth of the secondary minimum was more than a factor of 10 greater than predicted.