Studies of surfactant behaviour and model surfaces relevant to flotation deinking

The objective of this work was to investigate the behaviour of some model surfactants used in the deinking process, namely, sodium oleate and two ethoxylated nonionic surfactants (C12E6 and C14E6) as well as to investigate the suitability of various model substrates for mimicking interactions in technical systems. The influence of the adsorption to the air � water interface has been measured by means of equilibrium and dynamic surface tension measurements, as well as foaming experiments. It was found that the solution pH and temperature influenced the dynamic and equilibrium surface tensions and the value of the cmc. Equilibrium surface tension measurements were also performed with mixtures of sodium oleate and nonionic surfactants and a strong synergistic effect was obtained, which means that a greater surface tension reduction can be obtained at lower surfactant concentrations. The foaming experiments, carried out with a Foam Scan Apparatus, showed an approximately constant total foam volume for both the pure surfactant systems and for the mixtures. In addition, the foam was slightly more stable for the mixtures than for the pure components. Friction and surface forces between solid surfaces in liquid were studied using the atomic force microscope (AFM) from which inferences about the adsorption to these interfaces could be drawn. The AFM measurements were performed with the colloidal probe technique using cellulose as colloidal probe and an alkyd resin as a model ink surface. Mica and silica were both used as models for hydrophilic surfaces. Adsorption was observed on the alkyd resin, both with sodium oleate and with C12E6. The adsorption was registered both as a change in normal surface force interaction and as a strong reduction in friction force and friction coefficient at increasing surfactant concentration. The magnitude of the friction force was observed to be dependent on the adhesion and varied monotonically with the surface roughness. Measurements of adhesion and friction forces in air were performed, and the same conclusions about the effect of roughness were drawn. Finally the friction force behaviour appears to be similar if the adhesion is caused by a vapour bridge in liquid, or by a liquid bridge in air, where the formation of a capillary bridge in air is strongly dependent on the relative humidity.QC 2010092