The effect of cationic surfactants on wetting, colloid stability and flotation of silica

Contact angle, colloid stability and flotation measurements have been carried out for silica as a function of the concentration of cationic surfactants at different salt concentrations. The results are compared with surfactant adsorption and surface charge isotherms. The surfactants used are dodecyl and cetyl pyridinium chloride, and dodecyl trimethyl-ammonium bromide. Colloid instability, contact angles and flotation recovery all pass through a maximum at about the charge compensation point that corresponds for the present system to the iso-electric point and the (near) common intersection point of surfactant isotherms measured at a given pH and different ionic strength values. Before the charge compensation point the surfactant progressively neutralises the negative surface charge by head-on adsorption and simultaneously the surfactant tail causes an increase in hydrophobicity. Beyond the charge compensation point admicelles are formed, leading to a net positive particle charge and an increase in hydrophilicity. In the charge compensation point the maximum of hydrophobicity occurs together with the minimal net particle charge. Therefore, the minimum in the stability and the maximum in the flotation recovery are related to both the low particle charge and the relatively large particle hydrophobicity around the charge compensation point. (C) 1999 Elsevier Science B.V. All rights reserved.