Pasi Mikkola

The influence of pH and NaCl on the zeta potential and rheology of anatase dispersions

Abstract The rheological behaviour of anatase dispersions was compared with the ζ potential, when the pH (from 3 to 10) and sodium chloride concentration (from 0 to 1 mol dm−3) was varied over a wide range. The isoelectric point, pHiep, is shifted to higher values with increasing salt concentration until a charge reversal was no longer observed at a concentration of 1 mol dm−3 NaCl. The rheological properties were analyzed in the form of the critical yield stress and the shear stress and viscosity at arbitrarily selected high and low shear rates, respectively. The maximum of the rheological properties was found to correlate with to the pHiep. The electrokinetic and rheological features of the system were related to a recent theory of Hunter et al. [J. Colloid Int. Sci. 28 (1968) 250] by plotting the yield stress against the square of the ζ potential for each salt concentration. The acidic branch, i.e. when pH pHiep), however, the curves did not show the expected linear behaviour. Also when comparing the influence of pH and sodium chloride concentration on the rheology, it was found that on the acidic yield stress branch the dependency of pH and salt concentration was very different from the one on the alkaline branch. On the acidic branch, low ionic strengths shared a common straight line. On the contrary, the alkaline branch had separate linear dependencies for each salt concentration. Moreover, the lines on the alkaline side seemed to be quite parallel for medium salt concentrations.

Effect of impurities on dispersion properties of alpha-alumina powder

Abstract Ceramic powders frequently have traces of different ions remaining from powder manufacturing. To understand how much the impurities will change the dispersion behaviour the powders were characterised very carefully. This study was focused on the surface composition and the dispersion behaviour of two α-Al 2 O 3 powders from different manufacturing processes in aqueous suspensions at various pH. Several techniques have been used to determine the degree of dispersion (dispersion stability). More attention has been paid to moderate particle concentrations where the dissolved impurities play a decisive role. The variation in surface composition between the powders led to differences in initial dispersion pH and in the magnitude and sign of the initial mobility (zeta potential) of the dispersion. Both observations show the importance of examining ceramic powder dispersions near the concentrations that are used to process the material, e.g., slip casting or dip-coating.

Deposition characteristics of ureido silane ethanol solutions onto E-glass fibres

The determination of the adsorption and the deposition characteristics of a silane monomer, which does not hydrolyse until it contacts the glass surface can be done by examining the effect of ultra dry solvent carriers. Most of the publications to date concentrate on amine and vinyl silanes. Many industrial processes use silanes with alternative organic functional groups, this study examines an industrial silane monomer with ureido functionality. The determination of how these monomeric, unhydrolysed silane solutions deposit, will improve our knowledge of the deposition behaviour of the much more complicated heterogeneous aqueous solutions. We have shown using diffuse reflectance Fourier transform infra red (DRIFT) how ureido silane deposition from ethanol results in the silane bonding in the OCH3 down orientation, with the ureido amine functions free for reaction with the polymer matrix. Furthermore, treatment with a series of solvents having solubility parameters which match (a) that of the unhydrolysed silane monomer and (b) that of the partially hydrolysed silane monomer can remove different fractions of the deposited silane and result in a certain amount of reorientation of the loosely bound physisorbed layers. X-ray photoelectron spectroscopy (XPS) results show that the silane was deposited in patches. Ultimately, precise control of the deposition conditions will determine the orientation of the silane monomer.

Colloidal processing of aluminum oxide powder for membrane applications

Abstract The properties of dip-coating slips were varied by adjusting pH, by using nonionic or ionic additive polymers and by using different electrolytes and concentrations. Electrostatic stabilization of the slip with poly(vinyl alcohol) (PVA) addition led to the smallest particle size resulting in the highest density and the lowest permeability of the dip coated layer. Only small amounts of nonionic PVA adsorbed on the particles, which decreased the zeta potential slightly by charge shielding. The measured particle size decreased when the PVA concentration increased. The PVA addition did not affect the isoelectric point (i.e.p.) of the alumina particles. Strongly adsorbed sodium poly(metacrylic acid) (Na-PMAA) changed the pH iep from ∼9 to ∼4. The smallest particle size was measured at pH values 7–9. The zeta potential of particles with Na-PMAA addition decreased when the ionic strength was increased. Low zeta potential values resulted in flocculation of the slip and larger flock size. The flocculated slips led to fast growth, high porosity and high permeability of the layer.