Ionic concentration- and pH-dependent electrophoretic mobility as studied by single colloidelectrophoresis

Abstract

Optical tweezers are employed to measure separately the complex electrophoretic mobilityof a single colloid and the complex electroosmotic response of the surrounding medium in aspecially designed microfluidic cell. Using the very same colloid both quantities aredetermined in dependence on the concentration of the aqueous salt solution (10 − 5–10 − 1 mol l − 1), the valenceof the ions (K + , Ca2 + ) and the pH (2.5–8.5). A pronounced effect is observed for all these examined parameters.The dependence on ion concentration agrees qualitatively—for the monovalent case—withthe predictions of the standard electrokinetic model.