(654a) Effect of Salt Concentration on the Ability to Form Stable Close-Packed Vesicular Dispersions for Stabilizing Suspensions of Dense Particles Against Sedimentation

For many applications of colloidal dispersions or suspensions, such as inks and paints, the particles must be suspended for long times. While this is often accomplished by preventing agglomeration, dense particles may still settle rapidly even without agglomeration. Stabilizing dense particles with low bulk viscosity of the dispersion is challenging and has received little attention. We have presented a general method for preventing dense particles from settling by using close-packed vesicular dispersions of the cationic double-chain surfactant DDAB (didodecyldimethyl-ammonium bromide) (Yang et al., 2015, Langmuir, 31, 8802). To understand the feasibility of the vesicular stabilization mechanism under various salinities, we examined the effect of sodium bromide (NaBr) concentration on the stability of dense particle suspensions against sedimentation.

DDAB forms lamellar liquid microcrystallites, or “liposomes,” with stirring in water at 25 C, and it also easily forms dispersions of unilamellar vesicles at low concentrations. Such vesicular dispersions were found to be quite effective for stabilizing settling suspensions of 1-10 wt% titania particles, with diameters of 300 ± 200 nm and a density of 4.2 g/cm³. From cryo-TEM and Dynamic Light Scattering measurements it was found that the vesicles are deformable with diameters of 400-600 nm. At 0.5 wt% DDAB and an estimated volume fraction of less than 0.2, the vesicles are mobile. At 2.0 wt% DDAB, the vesicles have an estimated volume fraction of over 0.5 and are quite immobile, forming a close-packed vesicular dispersion. Both dispersions are highly shear-thinning, and free-flowing at high shear rates. Only the close-packed vesicles provide strong resistance against the settling of the dense particles. At 0.5 wt% DDAB, the titania particles settled completely by 1 cm in two months, while at 2.0 wt%, the particles remained suspended for over 18 months. We extended the study to silica particles, which can also be easily stabilized against sedimentation for long times.

To better understand the effect of salt on the use of DDAB vesicular dispersions to minimize sedimentation, the stabilization of titania and silica particles against sedimentation was studied for NaBr concentrations ranging from 1 to 1000 mM. At 1.0 wt% DDAB with NaBr concentrations below 10 mM, the suspensions were again found to be quite stable. At higher NaBr concentrations, the suspensions were unstable, evidently because no liposomes or vesicles were formed. Various thermophysical properties and the phase behavior of DDAB solutions as a function of salinity were also studied to gain insight into the impact of NaBr concentration on vesicle formation.