Factors Affecting Interactions of Polyelectrolytes During Charge Analysis.

Abstract

The streaming current method is commonly used for charge analysis in water treatment and the paper industry. The working mechanism of this method is understood to involve polyelectrolyte complex (PEC) formation. Since the formation of PECs is important for many applications, the factors that could affect this complexation have been widely investigated. Studies have found increasing deviations from 1:1 stoichiometry of complexation with increasing salt concentration. The colloidal charge of water recirculated within a paper mill can affect process efficiency and product quality. With increased conductivities due to water recirculation and reuse, the need for accurate and reliable charge measurements at high conductivities has become more important. This research focused on experiments that define the range of sample types and electrical conductivity where it is possible to achieve accurate and reliable results from streaming current titrations.

Other studies related to the theories of polyelectrolyte complexation involved titrations carried out between solutions of a strong poly-acid and a strong poly-base over a range of salt concentrations. The results showed that deviations from 1:1 stoichiometry are consistent with earlier studies. In addition, it was found that the stoichiometry of PEC complexation depended on the direction of the titration. An excess over the stoichiometric amount was required to achieve a streaming current reading of zero. Theoretical models based on non-equilibrium complexation are proposed to explain the current results. Other tests, such as turbidity and electrophoretic mobility also have been used to evaluate the theoretical models. The results are consistent with those from the streaming current analysis.

Factors that can influence the polyelectrolyte adsorption and PEC formation were studied also. It is thought that the charge density of anionic polyelectrolytes may affect the titration stoichiometry to some extent at high conductivities; however, no clear trend was found in this study with crboxylmethylcelluloses. It was also found that the shape of the titration curves indicates the strength of complexation between titrant and sample molecules.

Aluminum ion interferes with the stoichiometry of charge titrations in a different way when compared with other inorganic ions such as Ca²⁺, Cl⁻ and Na⁺. It is possible to consider the aluminum ion itself as one kind of cationic sample to be titrated with standard anionic titrant under certain conditions. Turbidity and zeta potential tests indicated that there could be polynuclear species existing under the conditions in which the aluminum ions can be titrated as a cationic sample.