COLLOIDAL CHARGE DETERMINATION IN CONCENTRATED LIQUID DISPERSIONS USING TORSIONAL RESONANCE OSCILLATION

The high-frequency theology of concentrated liquid charge-stabilized d ispersions has been measured using a surface-loaded torsional resonato r. The high-frequency shear moduli are extracted from the measurement as a function of ionic strength and surface acid content. A theory whi ch enables the determination of the effective surface charge of the co ncentrated dispersions is presented. At high ionic strengths the theor y simplifies to an analytic formula relating the effective surface cha rge to the high-frequency limiting shear modulus in terms of an effect ive hard-sphere diameter. The effective hard-sphere diameter is obtain ed experimentally from the high concentration asymptote of the zero-sh ear viscosity, The resulting effective surface charge increases with a dded electrolyte content in agreement with charge renormalization theo ry. The effective charge also increases with added weak and strong sur face acid content. The method, which operates on concentrated dispersi ons, is found to yield effective charges that agree with dilute measur ements of the electrophoretic mobility. Moreover, the effective hard-s phere scaling reduces the high-frequency viscosity to a master curve i ndependent of salt content. (C) 1998 Academic Press.