High temperature or γ-radiation pretreatment of pyrex glass and vitreous silica has been found to change the isoelectric point of the oxide, and decrease, substantially, the magnitude of the zeta potential at any pH. The changes observed can be adequately explained in terms of a decrease in the number of surface hydroxyl groups existing on the metal oxide. Model calculations of the zeta potential as a function of the number of surface hydroxyl groups, indicate that the surface density of hydroxyl groups after pretreatment is of the order of less than one hydroxyl group per nm2 This value is affirmed by independent contact angle measurements made on γ-irradiated quartz plates.

Surface charge–pH isotherms for TiO2 in K+, Li+, Mg2+ and tetramethylammonium electrolyte solutions are reported for a TiO2 colloid sample shown previously to be essentially holocrystalline and non-porous. The charge data are similar to other oxides with the exception of precipitated silica. The adsorption sequence obtained is Mg2+ Li+ > K+≃(CH3)4N+.

In order to facilitate analysis of potentiometric titration data on colloidal TiO2 dispersions, a detailed gas adsorption and tritium exchange study of the TiO2 colloidal material has been undertaken. Gas adsorption data revealed no significant porosity. The weight loss and tritium exchange yield the number of surface protons as 12.5 nm–2, which is consistent with the crystal being made up of exposed crystal planes. This concordance substantiates the essential non-porosity and absence of a gel layer at the TiO2–water interface.

A simplified general formalism of the physics of the charge-potential interdependence of the electrical double layer at ionisable group surfaces is presented. The equilibrium total double layer potential is obtained graphically and analytically for surfaces with acidic and basic sites, with amphoteric sites and with single acid sites. The deviations from Nernstian behaviour are given as a function of the acid ionization constants of the surface groups.

The tritium exchange technique has been used to detemine the maximum number of surface protons at the oxide-water interface for oxide colloids including silicas, TiO2 (rutile) and the iron oxides goethite, hematite and amorphous iron oxide. The effects of heat treatment, crystal structure and exchange conditions are considered and tritium exchange values for the number of surface protons are compared with values calculated from crystal structures.

The surface charge-pH, potentiometric titration data on precipitated and heat treated silicas has been analyzed with the aid of complimentary tritium exchange, gas adsorption and dissolution rate studies. While none of the silicas were found to have physical porosity, the interface for precipitated silica appears to be permeable to protons and counterions. This Silica when heated at less than 500°C is porous to protons but not to counterions whereas silica heated to 800°C is impermeable to protons and counterions. This new experimental evidence is used to assess the assumptions inherent of various models of the oxide-water interface.

Potentiometric titration studies of adsorption of nitrate, sulfate, and phosphate on goethite and α-chromia have been conducted to examine whether or not anion adsorption in these systems involves ligand exchange with surface groups. Both the extent and rate of adsorption are consistent with a ligand exchange mechanism for the adsorption of phosphate on goethite but this mechanism is kinetically inhibited on α-chromia, where a much lower adsorption of phosphate is observed. The adsorption of nitrate and sulfate is similar in both extent and rate on both oxides, so that ligand exchange cannot be of primary importance in their adsorption mechanisms.

A site-binding model of the oxide/aqueous electrolyte interface is introduced, in which it is proposed that the adsorbed counter ions form interfacial ion pairs with discrete charged surface groups. This model is used to calculate theoretical surface charge densities of the potential-determining (H+/OH–) ions and the potential at the Outer Helmholtz Plane, which are shown to be consistent with experimental data for oxides. An explanation is provided for the difference between silica and most other oxides in terms of the dissociation constants of the surface hydroxyl groups.

Since the 1st IMPC took place in London in 1952 there have been many developments in our understanding of flotation both in terms of chemical and physical factors influencing the process. Many of these developments have been captured in the Proceedings of the Congresses that have taken place since that first event as well as in numerous journal papers and special volumes dedicated to flotation. This paper attempts to highlight the individual contributions which many eminent researchers and engineers have made over the past 70 years towards extending our understanding of especially the effects of chemical factors on flotation. Although the focus is mainly on more fundamental aspects it seems that the 'pull' of practice has often preceded the 'push' from laboratory-based research. As the fundamental understanding of the role of chemical factors on flotation continues to be developed the major challenge remains to transfer effectively this knowledge into the domain of operations of this profoundly complex process.