The interpretation of electrokinetic potentials and the inaccuracy of the DLVO theory for anatase sols

• Published in: Journal of colloid and interface science 1974-01, Vol.49 (3), p.346-361
• Main Authors: Webb, Joseph T., Bhatnagar, P.D., Williams, Dale G.
• Format: Article
• Language: English
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/0021-9797(74)90379-8

The surface charge density, electrophoretic mobility, and colloidal stability were measured for hydroxide-dispersed anatase (TiO 2) sols at 6.2% solids. The results of ζ calculations from electrophoretic mobility indicated that the Wiersema-Loeb-Overbeek method of correcting for retardation and relaxation effects is more accurate than Henry's method. At a given electrolyte concentration, the hydrodynamic shear plane was at a constant distance from the surface over a wide range of pH and ψ δ . The distance decreased with increasing salt content and ranged between 12Åat 0.01 M Na + to 25Åat 0.0004 M Na +. The resulting relationships between ζ and ψ δ and the location of the shear plane were best explained by the Lyklema-Overbeek case III, immobilized water at the surface, with small corrections for viscosity variation with the electric field outside the ordered region. Dispersion stability was not satisfactorily indicated by any of the double-layer potentials [ζ, ψ δ, or ψ 0] when they were substituted into the DLVO theory of colloidal stability. The DLVO theory gave inadequate quantitative results based on the large and widely varying values of the Hamaker constant required. A more exact mathematical description of double-layer interaction for spherical particles is apparently needed, and/or the effect of water structure on double-layer interaction and the Poisson-Boltzmann equation needs clarification.