Molecules 2023 , 28 , 7984
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4.2.3. Retention Mechanism (a) Single-component systems According to the theory of colloidal aggregation‚ the retention mechanism can be divided into three modes: coagulation, flocculation, and agglomeration. Coagulation implies that for negatively charged colloidal particles such as fine fibers, talc powder, calcium carbonate, titanium dioxide et al., the double layer becomes thinner and zeta potential falls under the action of electrolytes. Therefore, the electrostatic repulsion forces which commonly exist between colloid particles decrease‚ resulting in the instability of colloid particles. The retention is produced by the formation of small aggregates (also known as “soft aggregates”). Colloidal particles are bonded together by polymers with a high molecular weight (greater than 100,000) due to the instability phenomenon [109]. The coagulant adsorbs onto the surface of colloidal particles in a random coil conformation through electrostatic or non-electrostatic forces. These adsorbed polymers form so-called “hard aggregates” by reacting with another colloidal particle. We call this process flocculation. Agglomeration refers to the instability phenomenon of colloidal suspensions caused by the action of polymers with low molecular weight (less than 100,000) and high charge density. Due to the high positive charge density of retention aids‚ they adsorbed on the surface of negatively charged particles through ion bonds‚ forming so-called ‘patch’ [110]. These positive charge patches will agglomerate with the negative charge of another particle by “bridging” (see Figure 5).
Figure5. Mechanism diagrams of “Patch” and “Bridging”.
(b) Dual-polymeric retention system There are some defects in single-component systems. The formed flocs are sensitive to hydrodynamic forces, leading to flocs fragment exposed to turbulent shear forces. While the shear force slows down or disappears, these dispersed flocs will not be restored, making its retention ability greatly reduced. A novel retention system has emerged, which is so that that the synergistic effect is generally achieved through dual components [111]. Research has found that hard and tough flocs can be generated by combining two types of polyelectrolytes with different charges and controlling adding order. If a cationic polyelectrolyte is added before adding an anionic polyelectrolyte‚ the retention effect is far better than that of single-component systems [112]. The acting mechanism may be explained as follows: After adding cationic polyelectrolytes with low molecular weight and high charge den- sity‚ polyelectrolytes are adsorbed by negatively charged particles in a flat configuration‚ causing cationic patches. These adsorbed polyelectrolytes only have very few loop and tails extending into the aqueous solution‚ but providing anchoring points for anionic polyelec- trolytes. Anionic polyelectrolytes with high molecular weight and low charge density are then bonded onto cationic patches. However, many segments of anionic polyelectrolytes will extend into aqueous solutions due to the repulsive force of negative charges on the
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