RESUMO
HYPOTHESIS: As more sodium dodecylsulfate (SDS) monomers adsorb at the water/titanium dioxide (TiO2) nanoparticles interface, the particles become more stable against agglomeration and sediment more slowly. SDS micelles are not expected to adsorb on the particles and affect the stability against agglomeration or sedimentation. Since micelles are smaller than the 300 nm TiO2 nanoparticles studied, they may introduce depletion forces which may affect the dispersion stability. EXPERIMENTS AND MODELS: Sedimentation times were measured in water and in 100 mM NaCl for SDS concentrations from 0.1 to 200 mM. Adsorption densities of SDS and zeta potentials of particles were measured. Dynamic light scattering was used to measure average diameters of particles or particle agglomerates. Modeling of sedimentation/diffusion was done to predict sedimentation times of particles. Modeling of agglomeration rates was done to help predict sedimentation rates of clusters. FINDINGS: At SDS concentrations close to or above the cmc, up to 60 mM in water or 115 mM in 100 mM NaCl, the nanoparticles sediment most slowly without any agglomeration. At higher micelle concentration, SDS micelle depletion forces are very strong, causing fast flocculation, without coagulation. Then sedimentation occurs much faster. The effective micelle depletant size includes about 4 Debye lengths of the charged micelles or particles.