Effects of Light Dispersed Particles on the Stability of Dense Suspended Particles Against Sedimentation

Abstract

A novel method in which vesicular dispersions of the double-chain cationic surfactant DDAB (didodecyldimethylammonium bromide) stabilize suspensions of high density titania particles was recently presented (Yang, Y.-J; Corti, D.S.; Franses, E. I. Langmuir 2015, 31, 8802–8808). At high enough DDAB concentration, the vesicles form a close-packed structure, providing strong resistance to the sedimentation of the titania particles, while the dispersions remain highly shear-thinning with moderate limiting viscosities. Here, to elucidate the key factors of the mechanism by which vesicles or other nonsettling particles stabilize high density particles against sedimentation, we use Brownian dynamics simulations (BDS) to examine the sedimentation behavior of mixtures of “dense particles” that settle rapidly on their own and “light particles” that represent nonsettling “rigid vesicles”. BDS confirm that for large enough values of the volume fraction ϕ2 of the light particles, the dense particles should remain suspended. The rheological behavior of the mixtures is also computed with BDS. The observed shear-thinning behavior of the light particle dispersion suggests that the suspensions of the dense particles are still flowable at high shear stresses. Furthermore, the local viscosity of light particles around the dense particles significantly increases with increasing ϕ2, particularly when the same gravitational force applied in the BDS is exerted on a dense particle. The arrangement of light particles around the moving dense particles is an important factor in determining the stability of the dense particles against sedimentation. The BDS results indicate that dispersions of nonsettling particles provide a general method for the stabilization against sedimentation of high density particles.

Effects of the Method of Preparation and Dispersion Media on the Optical Properties and Particle Sizes of Aqueous Dispersions of a Double-Chain Cationic Surfactant

“…Interparticle interactions, which lead to dependent scattering, can also cause a significant increase in the viscosity of DDAB dispersions, as has been shown before. 15,16 Such rheological changes are the reason for the use of DDAB S or SS dispersions to stabilize suspensions against sedimentation. For the SE and SS dispersions in water, dependent scattering probably arises as a result of the repulsive electrostatic interactions among the vesicles, which should be long-ranged since the ionic strength in water is very low (about 10 −4 mM).…”

“…Single and independent scattering was, however, only observed for the salt solutions at w ≤ 0.00100 and 0.000500 for SE and SS dispersions, respectively. Therefore, the scattering in these dispersions at w ≥ w * is primarily multiple, at least up to w = 0.01, for which the viscosity of the vesicular dispersions increases greatly. , …”

“…Aqueous dispersions of vesicles (unilamellar particles) and liposomes (multilamellar onion-like particles) are used in many applications, such as drug or vaccine encapsulation, , controlled release, , biocides, , and material synthesis. , Vesicles have also served as useful models of biomembranes. , Among the various available double-chain surfactants that usually form liposomes and vesicles, the cationic surfactant didodecyldimethylammonium bromide (DDAB) has been studied for medical and biocidal applications. − Moreover, certain vesicular dispersions of DDAB at high vesicle volume fractions were recently found to have unique abilities to stabilize indefinitely dense particle dispersions against sedimentation. − …”