Le Huynh

In situ particle film ATR FTIR spectroscopy of carboxymethyl cellulose adsorption on talc: binding mechanism, pH effects, and adsorption kinetics.

Carboxymethyl cellulose (CMC), in solution and adsorbed on the surface of talc, has been studied with ATR FTIR spectroscopy as a function of the solution pH. The solution spectra enable the calculation of the extent of ionization of the polymer (due to protonation and deprotonation of the carboxyl group) at various pH values, yielding a value of 3.50 for the pK(app)(1/2) (pH at which half of all carboxyl groups are ionized) in a simple electrolyte solution and a value of 3.37 for the pK(app)(1/2) in solutions containing magnesium ions (3.33 x 10(-4) M). The spectra of the adsorbed layer reveal that CMC interacts with the talc surface through a chemical complexation mechanism, via the carboxyl groups substituted on the polymer backbone. The binding mechanism is active at all pH values down to pH 2 and up to pH 11. The adsorbed layer spectra reveal that protonation and deprotonation of the polymer are affected by adsorption, with an increase in the pK(app)(1/2) to a value of 4.80. Spectra of the adsorbed polymer were also acquired as a function of the adsorption time. Adsorption kinetic data reveal that the polymer most likely has two different interactions with the talc surface, with a stronger interaction with the talc edge through chemical complexation and a weaker interaction with the talc basal plane presumably through the hydrophobic interaction.

A rheological and electrokinetic investigation of the interactions between pigment particles dispersed in aqueous solutions of short-chain phosphates

Abstract The effects of short-chain phosphate adsorption on the colloid and interfacial chemistry of concentrated metal oxide pigment dispersions were investigated with rheological, electrokinetic and adsorption isotherm measurements. The phosphates were observed to specifically adsorb at the solid–aqueous solution interface. This caused the isoelectric point of the pigment particles to shift to lower pH values. The magnitude of the shift in the isoelectric point was found to be a function of both phosphate concentration and chain length. At a fixed phosphate concentration, but with changing pH, the yield stress of the dispersion (and, hence, the interaction force between particles) was seen to vary with the square of the zeta potential of the particles. This behaviour was attributed solely to the variation of electrostatic forces with pH, in line with the DLVO theory of colloid stability. However, a reduction in the magnitude of the yield stresses at the isoelectric point was observed with either increasing phosphate concentration or chain length. This was ascribed to the presence of repulsive steric forces. At low and high coverage, the range of the force was found to be equivalent to approximately twice the minimum and maximum dimension, respectively, of the phosphate molecules.

Control of slime coatings by the use of anionic phosphates : A fundamental study

Abstract A model system comprising of chemically-dissimilar and oppositely-charged metal oxides has been used to investigate the removal of small particles from surfaces and, particularly, the role of interaction forces in this process. Techniques including reflectometry, rheology, microelectrophoresis, acoustophoresis and atomic force microscopy have been utilised. Silica particles adsorbed onto titania surfaces were removed by the addition of phosphate. The phosphate was shown to adsorb selectively to the titania. Desorption of the silica particles has been attributed to the ability of the phosphate to modify the interaction forces between the metal oxides. Phosphate was found to alter the electrostatic force between silica and titania from attractive to repulsive. It also introduced an additional repulsive force, which has been ascribed to the presence of a steric barrier. In addition, significant reductions in the adhesive and frictional forces acting between silica and titania were observed. The implications of the findings for ‘slimes’ control during the separation of mineral ores are discussed.

Modification of the rheological properties of concentrated slurries by control of mineral-solution interfacial chemistry

Abstract The rheological properties of a chalcopyrite slurry have been measured as a function of different chemical conditioning treatments. The magnitude of the yield stress (τB) was reduced when either hydrochloric, nitric or sulfuric acid was used to decrease the pH. These results were consistent with an increase in repulsive electrostatic forces between particles. Surface analysis of the particles in the slurry revealed that reducing the pH removed an overlayer of insoluble calcium sulfate. This left the underlying heavily oxidised chalcopyrite surfaces covered with metal hydroxide species. For hydrochloric and nitric acids, the viscosity (ηB) of the slurry was also reduced with decreasing pH — behaviour which was attributed to changes in the surface chemistry of the particles with pH. However, for sulfuric acid, ηB increased with decreasing pH. It is proposed that precipitation of insoluble calcium sulfate increased the solid content of the slurry as the pH was reduced, resulting in a significant increase in viscosity. Addition of phosphates to the slurry also reduced τB and ηB. Adsorbed phosphate apparently produces an enhanced repulsive force between particles due to the presence of long-range electrostatic and, possibly, short-range steric interactions. Surface analysis showed that the phosphates dispersed insoluble calcium sulfate from the surface of the chalcopyrite particles. Additional phosphates adsorbed onto the chalcopyrite surfaces. The rheological parameters determined were used to calculate the energy required to pump the slurry through a pipeline. The calculations indicated that the use of either acids or phosphates permits the solid content of the slurry to be increased by over 10 wt.% for the same pumping energy input.

The effect of adsorbing naphthalene sulfonate formaldehyde condensates upon the interactions between metal oxides

Abstract The effects of naphthalene sulfonate formaldehyde condensate (NSF) adsorption on the colloid and interfacial chemistry of titania particles have been investigated using acoustophoresis, rheology and atomic force microscopy. The NSFs were observed to specifically adsorb at the solid-aqueous solution interface with the isoelectric point of the titania particles shifting to lower pH values. The magnitude of the shift in the isoelectric point was found to be a function of NSF concentration. For fixed NSF concentration and at pH values greater than the isoelectric point of the titania, the yield stress of the dispersion (and, hence, the interaction force between particles) was seen to vary with the square of the zeta potential of the particles. This behaviour was attributed to variation of solely electrostatic forces over the pH range in question, in line with the DLVO theory of colloid stability. However, an increase in the magnitude of the yield stress was observed at pH values below the isoelectric point of the titania. This was ascribed to the presence of attractive forces between the particles due to bridging of the positively-charged titania surfaces by the oppositely-charged NSF molecules. The rheological data gathered for the concentrated dispersion exhibited qualitative agreement with atomic force microscopy measurements performed in (very) dilute solution.