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Polyelectrolyte complexes (PECs) formed by mixing oppositely charged polyelectrolytes are widely used as flocculation agents of aqueous solid/liquid dispersions. The complexes have been found superior to single polyelectrolytes in showing a substantially wider optimum concentration range for flocculation. In this work the flocculation performance and mechanisms induced by premixed complexes of cationic starch and anionic sodium polyacrylate (NaPA) on semidilute calcite dispersions were investigated by measuring the particle size and the dynamic mobility. The flocculation performance was studied as a function of the ratio of NaPA to starch, and the total amount of polyelectrolyte added. The influence of pretreatment of the calcite dispersions on the complex-induced flocculation was also investigated. The investigation clearly shows that, by using appropriate amounts of premixed NaPA and starch, the flocculation of the calcite dispersions was strongly enhanced compared to the case of using single NaPA or starch, respectively. Several mechanisms are involved in the enhanced flocculation induced by the two oppositely charged polyelectrolytes. These mechanisms are strongly dependent on the ratio of NaPA to starch, and the total amount of polyelectrolytes added. However, interparticle bridging by the polyelectrolyte complexes, and charge neutralization, induced by the deposition of the complexes, were found as the main reasons for the enhanced flocculation. Pretreating the bare calcite with anionic sodium polyacrylate changed the charge characteristics of the calcite from cationic to anionic. Despite the change in charge characteristics, the pretreatment did not drastically change the flocculation behavior of the semidilute calcite dispersions induced by the subsequent addition of premixed polyelectrolyte complexes. However, the pretreatment resulted in a slight shift in the ratio of NaPA to starch required for optimum flocculation.

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A series of wet‐cationized starch granules from waxy maize with different degrees of substitution (DS) were solubilized with either 2.2M HCl (lintnerization) or with the α‐amylase of Bacillus amyloliquefaciens. The maximum rate of the enzymatic hydrolysis occurred in starches with intermediate DS. It appeared that the cationic substituents interfered with the binding to the active site of the enzyme at high levels of substitution. The DS remained fairly constant in the granular residues after the enzymatic attack. The rate of the acidic hydrolysis increased with increasing DS but the final level of solubilization slightly decreased. The DS of the residual starch material decreased to 40% of the original level, showing that a large part of the cationic groups was found within the amorphous parts of the granules. A dry‐cationized sample with a high DS was also treated with the acid and lost a major part of its substituents at low levels of lintnerization. Probably most of the substituents were associated with the surface and channels of these granules. The cationized starches possessed branches that were resistant to isoamylase attack and the samples also contained β‐amylolysis resistant dextrins. The proportion of resistant dextrins in the granular residues decreased after lintnerization, but remained constant after the enzymatic hydrolysis.