Spherical α-Al2O3 suspensions layered sequentially with anionic and cationic polyelectrolytes: Chemistry, rheology and TEM images

Abstract

The zeta potential-pH and yield stress-pH behaviour of washed spherical α-Al2O3 suspensions with and without adsorbed multilayer polyelectrolytes up to 4 layers of poly(styrene sulphonic acid) sodium salt or PSSNa (100 kDa) and polyethyleneimine or PEI (70 kDa) adsorbed sequentially were characterised. The maximum yield stress (τymax) of washed spherical α-Al2O3 is 81.8 Pa located at pHξ=0 at ~9.2. Particles with the first layer (layer 1) of PSSNa displayed negative zeta potential over the whole pH range of 3 to 8. Particles containing 3 layers with layer 3 being PSSNa also displayed negative zeta potential over the same pH range. The magnitude of the Layer 3 PSSNa is smaller. Similarly, the zeta potential remained positive for layer 2 and layer 4 PEI over the whole pH range of 3 to 8. At the dosage of 0.5 dwb% PSSNa and 0.1 dwb% PEI, monolayer coverage of PSSNa for layer 1 and 3 and of PEI for layer 2 and 4 should be achieved if the adsorption is uniform. The maximum yield stress of layer 1 PSSNa and layer 2 PEI was much larger than that without adsorbed additives despite the zeta potential being not zero. Additional attractive forces such as bridging, hydrophobic and hydrogen bonding must be present. TEM images showed that the adsorbed PSSNa-PEI layer surface is undulating and not uniform in thickness. This suggests that adsorption of these polyelectrolyes is not uniform. Extensive particle bridging showing the merging of layers at the interacting surface of the particles was observed from these TEM images. In contrast, α-Al2O3 suspensions with bilayer DIBMA-PEI, trilayer DIBMA-PEI-DIBMA and 4 layers DIBMA-PEI-DIBMA-PEI formed by 14 kDa DIBMA and 1.8 kDa PEI performed effectively as steric layer reducing the maximum yield stress significantly. TEM image of α-Al2O3 particles with bilayer DIBMA-PEI did not show the presence of polyelectrolyte layer suggesting that the polyelectrolyte layer is too thin to be observable.