Hydrophobic coagulation and aggregation of hematite particles with sodium dodecylsulfate

Synthesized hematite sols with particle diameter of ∼50 nm (Hs50) and ∼100 nm (Hs100) revealed points of zero charge (pzc) at pH ∼ 9.0. Therefore, their hydrophobic coagulation behaviour with sodium dodecylsulfate (SDS), was monitored using turbidimetric titrations in the pH range between 2.5 and 7.5, i.e. when the particles are positively charged. As expected, the critical hydrophobic coagulation concentration (chc) of the SDS required for the particle coagulation, determined by turbidimetric titrations, was dependent upon dispersion pH. Similar trends in chc were found for both the Hs50 and the Hs100 hematite particles. At dispersion pHs ≤ 3.5 the chc was constant, indicating the limitation due to acid/base character of Fe2O3 surface. The chc values decreased 4-fold, with a constant solid content (i.e. volume fraction), when using the smaller particles, due to a 4-fold increase in surface area. Similarly, a 4-fold increase in Hs50 particle concentration caused approximately a 4-fold decrease in chc, over the investigated pH range. Varying the dosing rate of SDS, the hydrophobic coagulation kinetics at pH ∼ 6 was affected and different chc values were detected. However, the differences disappeared when the concentration of Hs50 particles was greater than 300 mg L−1. The dependence of chc upon particle concentration was fitted with a semiempirical exponential decay function. The fitting parameters are discussed and indicate the significant influence of effective interparticle collisions and the adsorption state of the SDS molecules on the nanometer sized hematite particles. A defined type of hematite aggregates coagulated with SDS were prepared and viewed with an Environmental Scanning Electron Microscope. We found four types of aggregates, depending on the concentration of SDS used: ≤1.4 × 10−4 M gave typical framboidal aggregates, ∼1.0 × 10−3 M compact aggregates, ∼1.0 × 10−2 M “chain-like” aggregates and at ∼2.5 × 10−1 M small porous aggregates were found.