Staffan Wall

Aggregation of nanosized colloidal silica in the presence of various alkali cations investigated by the electrospray technique.

The slow aggregation process of a concentrated silica dispersion (Bindzil 40/220) in the presence of alkali chlorides (LiCl, NaCl, KCl, RbCl, and CsCl) was investigated by means of mobility measurements. At intervals during the aggregation, particles and aggregates were transferred from the liquid phase to the gas phase via electrospray (ES) and subsequently size selected and counted using a scanning mobility particle sizer (SMPS). This method enables the acquisition of particle and aggregate size distributions with a time resolution of minutes. To our knowledge, this is the first time that the method has been applied to study the process of colloidal aggregation. The obtained results indicate that, independent of the type of counterion, a sufficient dilution of the formed gel will cause the particles to redisperse. Hence, the silica particles are, at least initially, reversibly aggregated. The reversibility of the aggregation indicates additional non-DLVO repulsive steric interactions that are likely due to the presence of a gel layer at the surface. The size of the disintegrating aggregates was monitored as a function of the time after dilution. It was found that the most stable aggregates were formed by the ions that adsorb most strongly on the particle surface. This attractive effect was ascribed to an ion-ion correlation interaction.

Electrostatic characterisation of Al-modified, nanosized silica particles

Abstract Al-modified, nanosized silica particles have been studied with an electroacoustic technique. Also slightly aggregated silica particles have been investigated with this method. ζ-potentials calculates from the measured dynamic mobilities were found to be in good agreement with previous reported ζ-potentials obtained from streaming potential measurements. This indicates that the magnitude of the dynamic mobility is very similar to the electrophoretic mobility for moderately charged nanosized particles. The degree of Al-substitution was varied so that between 0 and 16% of the surface silicon atoms were replaced by aluminium. It was found that the magnitude of the titratable surface charge density increased with increasing degree of Al-substitution. This was especially pronounced in the neutral pH region. The behaviour of the ζ-potential as a function of Al-substitution was more complex than the behaviour of the surface charge density: increasing the degree of Al-substitution stepwise up to 7%, raised the magnitude of the ζ-potential both in 2 and 10 mM NaCl. A further increase of the degree of Al-substitution lead to a reduction of the magnitude of the ζ-potential. However, in 50 mM NaCl, it was favourable to include as much as 16% aluminium in the surface structure of the particle. The behaviour of the ζ-potential as a function of pH for the Al-modified samples can be explained in terms of adsorbed and non-adsorbed aluminium.

The kinetics of heteroflocculation in the system cationic starch and colloidal anionic silicic acid

Abstract The flocculation kinetics of cationic amylopectin (CAP) with hydrodynamic radii of 200–400 nm or cationic amylose (CA) with an anionic colloidal silicic acid (CSA) in the size range 5.5–21 nm in NaCl solutions have been studied with the stopped-flow technique. Turbidity has been used for detection. Electrostatic forces seem to be of greatest importance. For maximum flocculation it is found that the two polyions involved should carry equal or nearly equal amounts of charge. This condition must be closely fulfilled for small particles of colloidal silicic acid and at low concentrations of excess salt. With increasing salt concentration and particle size this condition decreases in importance and flocculation occurs over a fairly broad interval around the charge equivalence of the polyions involved. The experimental results can be explained by assuming the formation of bridges between the CAP molecules. These bridges can contain either a single CSA particle or two (or more) CSA particles. The two-particle bridge type is important when there is a high excess salt concentration and a comparatively high degree of substitution of cationic groups on the CAP molecules simultaneously with an excess of CSA particles. In other cases an excess of CSA particles stabilizes the system against flocculation. After an initial increase in turbidity in the time range 10 ms-1.5 s the turbidity decreases again. This can be interpreted as a contraction of the flocs due to an electrostatic screening of the positive charges of the CAP molecules. This contraction phenomenon is observed only for the smallest CSA particles.

Characterisation of colloidal silica particles with respect to size and shape by means of viscosity and dynamic light scattering measurements

A simple method has been developed to determine the axial ratios and solvation volume of small colloidal silica particles characterised as equivalent ellipsoids of revolution with the same hydrodynamic properties as the actual particles. The method is based on a combination of viscometry and dynamic light scattering and uses the relations of Simha and Perrin for the specific viscosity and frictional factors respectively of ellipsoids of revolution. The axes of the unsolvated equivalent ellipsoid and the solvation volume can be determined by knowing or assuming the thickness of the solvation shell. Silica particles form elongated aggregates and the method is applied to particles having three different degrees of aggregation.

A study of the interactions between cationic polymers and colloidal silicic acid

Abstract The kinetics of the interaction between small colloidal silicic acid (CSA) particles and cationic polymers have been studied with a stopped-flow technique. The cationic polymers used were cationic polyacrylamide (CPAM) and cationic amylopectin (CAP). The CSA particles were slightly aggregated. The size of the primary particles was 5.5 nm and the particles used were, consequently, somewhat larger. The components studied are used in papermaking as retention and dewatering aids. The kinetic processes were followed by turbidity measurements, and the parameters obtained from the experiments were the changes in initial rate of turbidity and the total turbidity. The dependence of these parameters on electrolyte content and the structure of the CSA particles was studied. A comparison between the flocculation behaviour of CAP and CPAM was made. Also, the effect of the aggregation state of the CSA on the flocculation behaviour was studied. We found differences between CAP and CPAM which could be referred to their structural differences. CAP is a very branched and, therefore, a rather compact molecule, whereas CPAM is a linear and extended polymer.

The primary electroviscous effect of prolate silica sols

The intrinsic viscosity and the dynamic mobility of four silica sols have been measured as a function of the ionic strength. It was found that intrinsic viscosity decreased with increasing ionic strength, which we attribute to the primary electroviscous effect. The geometry and the charge of the particles were fitted using experimental viscosity, light scattering, and dynamic mobility data, where the intrinsic viscosity measured at the highest ionic strength for a given sol was used as input data in our analysis. Further, the boundary element (BE) method was used to calculate the primary electroviscous effect and electrophoretic mobility of charged prolate ellipsoids. These calculations were then compared with experimental data, and the primary electroviscous effect was subtracted from the intrinsic viscosity at a given ionic strength, which led to a slightly altered geometry of the particles. This revised geometry was used as input data using the BE method, and the procedure was repeated iteratively until agreement was obtained at high ionic strength. In general, good agreement between theory and experiment was found.

Equilibrium and kinetic properties of alkylpyridinium sulfate micelles

Abstract Equilibrium and kinetic measurements on tetradecyl- and pentadecylpyridinium sulfate micelles have been carried out. The value of the degree of counter ion dissociation α is seen to be highly dependent on the method used for its determination. The critical micelle concentrations and the mean aggregation numbers are close to those of the corresponding bromides. The fast and slow relaxation times for the two substances were also measured and the different values of α were used for the calculation of the parameters obtainable from the relaxation times. The rate constants and the width of the micelle size distributions determined from the fast relaxation time are seen to be rather insensitive to the value of α, while the parameters characterizing the submicellar aggregates are highly dependent on the degree of dissociation. Furthermore, an explanation is given for the very different concentration dependences of the slow relaxation time for the alkylpyridinium sulfate and sodium alkyl sulfate micelles.