For many applications of colloidal dispersions, the particles must be suspended for a long time. This is often accomplished by preventing agglomeration, which generates aggregates with increasing sizes. Nevertheless, many colloidal dispersions of dense particles may settle even without agglomeration. To prevent sedimentation without significantly increasing the bulk dispersion viscosity is difficult, and has received little attention in the literature. To address this problem, for an example of TiO2 particles with a density of 4.2 g/cm3 and diameters of 300±200 nm in water, we first studied the effect of adding an anionic surfactant, sodium dodecylsulfate. We showed that the TiO2 particles settled by 0.5 cm in 45 h even without agglomeration .
To further improve the stability of the particles against sedimentation for weeks and months, we next used a double chain cationic surfactant as a stabilizer against both agglomeration and sedimentation. At surfactant concentrations of 0.5-2 wt% this surfactant forms vesicles with diameters of 400 nm or larger. For this diameter and a bilayer thickness of 3 nm, a vesicular dispersion with 2 wt% or 2 vol% surfactant will have a significantly large vesicles volume fraction, ϕv, of 0.46. Such dispersions are highly concentrated with respect to the vesicles. At 2 wt%, cryo-TEM images reveal that the vesicles may not pack as spheres, but are deformed to nearly polyhedral shapes, indicating that ϕv is larger than 0.7. At these conditions, the vesicles have very low mobilities, as evidenced by DLS measurements. The autocorrelation function shows an unusual and distinct behavior. It never decays to zero, implying some “memory” of the relative vesicles positions. Thus, the vesicles appear to be in a tightly-packed formation. They are “trapped,” or captive to each other. Furthermore, the vesicles in such dispersions have very low electrophoretic mobilities.
At 0.05-0.5 wt% double-chain surfactant, the TiO2 particles settle within a day. By contrast, at 2 wt% surfactant, they remain suspended for at least 6 months with no sedimentation observed. Cryo-TEM images show that the TiO2 particles are completely surrounded by the vesicles, which prevent them from moving. The gravitational force, being counteracted by the apparently large resistance of the close-packed vesicles microstructure, can no longer move the particles. Although the vesicles slow down drastically the sedimentation rate of the particles, the bulk dispersion viscosity at the high shear-stress attained in a capillary tube flow (ca. 5 Pa) is only 10 cP, ten times larger than that of water. The local shear stress exerted by, and applicable to, the individual TiO2 particles is much lower (10-3-10-2 Pa) than those at the walls of the capillary tube. Under such low shear stresses, the viscosities are quite high, ranging from 104 to over 106cP, effectively restricting or stopping the motion of the particles. This is the first report of the use of close-packed vesicular microstructures to stabilize the colloidal particles against sedimentation.
1. Yang, Y.-J.; Kelkar, A.V.; Zhu, X.; Bai, G.; Ng, H.T.; Corti, D.S.; Franses, E.I. "Effect of Sodium Dodecylsulfate Monomers and Micelles on the Stability of Aqueous Dispersions of Titanium Dioxide Pigment Nanoparticles against Agglomeration and Sedimentation." Journal of Colloid and Interface Science 450 (2015) 434-445.