Critical zeta potential and the Hamaker constant of oxides in water

Abstract

The critical zeta potential characterises the flocculated–dispersed state transition of a colloidal dispersion. For many colloidal dispersions, yield stress displayed a linear relationship with the square of zeta potential, indicating that they obeyed the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. From this relationship, the critical zeta potential is obtained from the intercept at the zeta potential axis at a yield stress of zero. The critical zeta potential is a measure of the repulsive potential required to exactly counter the maximum attractive potential between particles in dispersion in the flocculated state. When the forces of interaction between particles in the dispersion are only the van der Waals and electrostatic forces, then the critical zeta potential is indirectly a measure of the van der Waals attractive potential and, hence, it may be used to determine the Hamaker constant of solids in water. This potential is proportional to the square root of the solids Hamaker constant in water. At present, only the ratio of Hamaker constant between two oxides was obtained and compared with that obtained by other techniques. These oxides were ultrapure anatase TiO2 and γ-Al2O3, and they displayed a linear relationship between yield stress and the square of zeta potential. At the conductivity (or ionic strength) of about 3000 μS/cm, the critical zeta potential for both TiO2 and Al2O3 is ∼47 and ∼32 mV, respectively. These critical zeta potential data give a value of 2.2 for the ratio of Hamaker constant of anatase TiO2/H2O/TiO2 to γ-Al2O3/H2O/γ-Al2O3. This ratio compares well with a value ranging from 1.0 to 2.18 for rutile TiO2/H2O/TiO2 to α-Al2O3/H2O/α-Al2O3 where their Hamaker constants were calculated from the Lifshitz theory using full optical spectral data.