(114f) Electrostatic and steric stabilization of precipitated nanoscaled barium sulphate

The production of nanoscaled particles of various particle size distributions, morphology and other product properties plays an important role in industry. However the stability of colloidal suspensions in different media is a challenge to be met when the product properties in synthesis processes should be tailored. At this work the precipitation of barium sulphate in a static T-(micro)-mixer and especially the following secondary processes namely agglomeration, aggregation and disaggregation are investigated.

Brownian motion and shear forces in the turbulent flow field in the mixer lead to particle-particle collisions of nanoscaled particles and consequently aggregation occurs. There are two different ways to prevent the contact between two particles which is the key for stabilizing colloidal systems. Stabilization can be reached by free ions which adsorb on the particle surface and impart an electrical charge to the surface which leads to an electrostatic repulsion between the nano-particles. Another possibility is adding polymers to the colloidal system which adsorb on the surface and enable the particles to repel one another (steric stabilization). In this context the possibilities to stabilize precipitated nanoscaled barium sulphate is investigated experimentally.

The barite particles are precipitated using barium chloride and sulphuric acid as educts in a T-mixer at various volume rates and feed concentrations. More details about the experimental setup and the operating procedure can be seen in Schwarzer and Peukert 2004.

Electrostatic stabilization is reached by increasing the barium excess in the feed concentrations. Therefore experimental investigations with different initial molar ratio R of barium to sulphate ions are conducted. One minute after precipitation the particle size distribution is measured by dynamic light scattering. It can be shown that with increasing R the particle size distribution is shifted to smaller particles. For R > 1.5 no significant change in particle size can be noticed. Barium ions adsorb on the barite surface and thus the repulsive forces of the positive charged particles stabilize the suspension itself. The ionic strength was increased by adding potassium chloride to the suspension without changing the surface charge density due to the insignificant adsorption of potassium ions on barium sulphate. Increasing ion strength leads to a compression of the electric-double layer, reduced stability and thus increased aggregation kinetics. Further details on aggregation kinetics at nanoparticles and the electrostatic stabilization of barium sulphate can be seen in Schwarzer and Peukert 2005.

Although electrostatic stabilization of barite in aqueous media can be demonstrated successfully this method is not applicable for all materials. Most colloid systems have to be stabilized by adding polymers which adsorb on the particle surface to prevent aggregation sterically. The adsorption time of the polymers on the barium sulphate surface plays an important role and has to be faster than the aggregation kinetics to stabilize sufficiently. Poly vinyl alcohol (PVA) is used as polymer stabilizer with different functional acid groups (80% and 99%-hydrolyzed). In a polar media the 99 %-hydrolyzed PVA shows a compact molecule structure in contrast to the 80 %-hydrolyzed PVA (open molecule structure). The influence of different polymer structures on the adsorption behaviour and consequently on the stabilization of the colloid suspension is being investigated. Additionally to the three-dimensional molecule structure in aqueous solutions the added amount of PVA is varied to determine the concentration which is necessary to cover the surface of the barium sulphate particle sufficiently. The stabilization efficiency should increase with increasing PVA concentration. Besides the structure and the concentration of the polymer the molecule weight is varied. Small molecules (M ≈ 20000) show a higher diffusive mobility than larger polymers (M ≈ 200000) and thus the adsorptions time of PVA on the barite surface is changed. How far the molecule weight of the polymer is an influencing factor for the steric stabilization of the suspension is investigated.

Besides the aggregation kinetics the polymers in the mixer influence the flow field. The turbulence is changed and as a consequence the supersaturation built-up and thus indirectly the solid formation process is influenced. The turbulence can be reduced as well as increased depending on the polymer molecule structure or the molecule weight. Polymers can be added in the educt solution (BaCl₂ + PVA) before precipitation process is started as well as subsequently in the barium sulphate suspension. Solid formation (nucleation and growth) should be finished before the adsorption of the polymers on the barite surface begins and thus there should be no sufficient deviation in the results as long as the flow field is not influenced by added polymers.

Schwarzer H.-C.; Peukert, W.; (2004) Tailoring particle size through nanoparticles precipitation, Chem. Eng. Comm., 191, 580-606

Schwarzer H.-C.; Peukert, W.; (2005) Prediction of aggregation kinetics based on surface properties of nanoparticles; Chem. Eng. Sci., 60, 11-25