Zeta potential and flocculation

Fig. 9 Zeta potential and flocculation rate of a parenteral emulsion in the presence of a non-specifically adsorbing electrolyte (A) and a specifically adsorbing electrolyte (B).

Cationic Polymers., The relation between zeta potential and flocculation by a polymer has been studied by Rjes (3IS), who pointed out that as soon as a colloidal particle is coated with polymer it bears the same charge as the polymer and is redispersed. Similar studies by Ries and Meyers (316) involved the use of microphoresis and electron microscope observations of model colloids and polymeric flocculants. Polyamine type flocculants appeared to extend out from the particle surface for a distance of 20-300 A. Flocculation occurs simultaneously through charge neutralization and bridging of polymer chains from particle to particle then excess polymer reverses the potential and redispersion occurs. Adsorption of poly [(1,2-dimethylvinylpyridinium) methylsulfate] on silica was similarly studied by Shyluk (317), who concluded that the polymer chains lay flat along the surface when no excess polymer was present. 

Size enlargement of fine particles in liquid suspension can be accomplished in a number of ways. Electrolytes can be added to a suspension to cause a reduction in zeta potential and allow colliding particles to cohere. Examples include the use of trivalent aluminum and iron ions to flocculate the particles responsible for the turbidity of many water supplies and the flocculation of metallurgical slimes by pH adjustment to the isoelectric point. Alternatively, polymeric flocculants can be added to suspensions to bridge between the particles. A wide range of such polymeric agents [1] is available today to aid the removal of fine particles from water. 

The absolute value of the zeta potential decreases until a plateau is reached at a certain polyanion concentration. A contrary effect is obtained in the case of adding a polycation. A partial stabilization of the kaolin particles can be realized due to an adsorption of anionic charged macromolecules at the edges of the kaolin platelets. By adding a polycation to the kaolin dispersion, an adsorption at the negative basal surface becomes possible, and the iep of the particles is reached very quickly at low PEI concentrations. A further addition of PEI leads to an increase of the zeta potential while flocculation was observed. This is because the adsorption of cationic polymer can cause a face-to-face association that can generate polymer-kaolin complexes. 

Changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated by sodium dodecyl sulfate (SDS) are shown in Fig. 5.9. When SDS was... 

Fig. 5.9 The zeta potential and turbidity of a-Fe20s sols flocculated by SDS as a function of surfactant concentration closed symbols, zeta potential open symbols, turbidity arrow, turbidity of sols in the absence of surfactant. (From Ref. 65. Re-produced by permission of Elsevier Science Publishers.)...

Figure 5.10 shows changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated with NFIOO. The optimum flocculation concentration was about 3 X 10 mM NFIOO. The sols were redispersed by NF7 or NP7.5, a hydrocarbon-type nonionic surfactant (polyoxyethylene nonylphenyl ether with a polyoxyethylene chain of average 7.5 EO). The turbidity increased sharply. The zeta potential changed only a little, as expected for a nonionic surfactant. Sols flocculated by NFIOO were not redispersed by SDS. The inability of SDS, an anionic hydrocarbon surfactant, to redisperse the sols was attributed... 

Changes in zeta potential and turbidity of iron(III) oxide hydrate sols flocculated with lithium perfluorooctanesulfonate (LiFOS) are shown in Fig. 5.11. The nonionic surfactants NF7 and NP7.5 redispersed the sols. However, the anionic hydrocarbon surfactant LiDS (lithium dodecyl sulfate) had no significant effect. Accordingly, sols flocculated by LiDS were redispersed by a nonionic surfactant, NF7, but not by the anionic surfactant LiFOS (Fig. 5.12). 

Wu, W., Giese, R. F., and van Oss, C. J. (1994a) Linkage between zeta-potential and electron donicity of charged polar surfaces 1. Implications for the mechanism of flocculation of particle suspensions with plurivalent counterions Coll. Surf. A, 89, 241-252. 

The well-known DLVO theory of coUoid stabiUty (10) attributes the state of flocculation to the balance between the van der Waals attractive forces and the repulsive electric double-layer forces at the Hquid—soHd interface. The potential at the double layer, called the zeta potential, is measured indirectly by electrophoretic mobiUty or streaming potential. The bridging flocculation by which polymer molecules are adsorbed on more than one particle results from charge effects, van der Waals forces, or hydrogen bonding (see Colloids). 

Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

A reduction in the electrical charge is known to increase the flocculation and coalescence rates. Sufficient high zeta potential (> — 30 mV) ensures a stable emulsion by causing repulsion of adjacent droplets. The selection of suitable surfactants can help to optimize droplet surface charges and thus enhance emulsion stability. Lipid particles with either positive or negative surface charges are more stable and are cleared from the bloodstream more rapidly than those with neutral charge [192, 193]. 

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