(325e) Using Close-Packed Vesicular Dispersions (CPVDs) for Stabilizing Suspensions of Dense Particles Against Sedimentation

Many colloidal suspensions of dense particles, such as titania (with density of 4.2 g/cm³) and silica (with density of 2.0 g/cm³), in water are unstable, settling in hours and days, even without agglomeration, which generally would make them settle even faster. In many applications, such as for inks and paints, the particles, however, must remain suspended for weeks and possibly months. Stabilizing such dispersions against both sedimentation and agglomeration, while still maintaining a low bulk viscosity to allow for fast flow in these applications, is quite challenging and has received little attention in the literature. We are continuing to investigate a new method for preventing dense particles from settling, without a significant increase in the bulk viscosity, by using a double chain cationic surfactant, DDAB (didodecyldimethylammonium bromide). DDAB can form unilamellar vesicles, or “vesicles,” which under certain conditions can also form close-packed vesicular dispersions (CPVDs) (Yang et al., Langmuir 31, 8802, 2015). When dense particles are introduced into CPVDs, the particles remain suspended almost indefinitely. To be most effective, the CPVDs should be comprised of vesicles of high volume fractions, 0.50-0.70, but at low surfactant weight fractions, 0.01-0.05.

The ability of DDAB vesicles to form both in water and in aqueous sodium bromide, NaBr, at 25°C first requires, however, the formation of multilamellar liposomes, or fluid microcrystallites, of a lamellar liquid crystalline phase, at low surfactant concentrations. The dispersed liposomes are then broken up with stirring, sonication, extrusion, or a combination of such methods in order to prepare deformable vesicles of controlled sizes. The weight fraction of the liposomes that are ultimately broken to form vesicles and the final average vesicle size depend strongly on the method of preparation, as has been known previously.

Upon magnetic stirring, DDAB in water forms easily deformable vesicles with diameters of 400-600 nm, as inferred by cryo-TEM and dynamic light scattering measurements. Such vesicular dispersions, at 2.0 wt% DDAB, were found to be quite effective for stabilizing against settling, for over 18 months, otherwise fast-settling suspensions of 1-10 wt% titania particles, with diameters of 300 ± 200 nm, and (nearly monodisperse) silica particles of 1-5 wt% with diameters of 500 or 750 nm. Such DDAB dispersions, which could form CPVDs, are quite viscous at low shear rates, but highly shear-thinning and free-flowing at high shear rates. The following methods were used to produce vesicles or liposomes: magnetic stirring (S), stirring followed by sonication (SS) and stirring followed by extrusion through a membrane (SE). Sonication of the stirred 2.0 wt % DDAB dispersions broke almost all remaining liposomes and produced mostly vesicles of sizes smaller than 100 nm. Since the estimated volume fraction of the vesicles in the dispersion was then smaller than 0.5 at 2 wt%, the dispersion failed to produce a CPVD at this concentration, but formed a CPVD at higher concentrations.

The phase and dispersion behavior of DDAB at 0.1-1.0 wt % NaBr was also examined. Densitometry revealed substantial chain melting of DDAB in the liposomes and vesicles for 0-0.5 wt% NaBr, but not with 1.0 wt % NaBr. At 0.1-0.5 wt% NaBr, the liposome formation was similar as to that with water only, but the volume fractions of the formed vesicles and their average sizes were smaller. At 0.75-1.0 wt% NaBr, a much more viscous lamellar liquid crystal was observed, and no fluid liposomes or vesicles could be prepared with the preparation methods used. Hence, the ability of DDAB to form CPVDs and stabilize suspensions against sedimentation depends strongly on the salt concentration, which affects strongly the phase behavior of DDAB.