Use of Close-Packed Vesicular Dispersions to Stabilize Colloidal Particle Dispersions against Sedimentation

Abstract

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 of increasing size. Nevertheless, many colloidal dispersions of dense particles may settle even without agglomeration. Preventing sedimentation without significantly increasing the bulk dispersion viscosity is difficult and has received little attention in the literature. However, settling can be drastically reduced through the novel use of close-packed vesicular dispersions at high enough concentrations, which are non-Newtonian shear-thinning fluids. Such dispersions have much higher viscosities at the low shear stresses "felt" by sedimenting colloidal particles than at the high shear stresses relevant to bulk dispersion flow. In a practical example, dense TiO 2 nanoparticles which normally would settle rapidly can remain suspended for at least 6 months without any observable sedimentation when they are introduced into a close-packed vesicular dispersion, while the dispersion retains its flowability. Cryo-TEM images reveal that the vesicles in these dispersions are tightly close-packed. Dynamic light scattering and electrophoretic mobility data also confirm that the vesicles in such dispersions have very low mobilities.

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

“…For each method of preparation and weight fraction, the absorbance decreases with increasing wavelength from 350 to 700 nm, as expected for RDG scattering. In water, the absorbances are higher for the S dispersions, which contain a lot of liposomes (see Figure S2 in the SI), along with many large vesicles that formed during the stirring process, as shown previously with cryo-TEM . The absorbances are higher for the SE dispersions than for the SS dispersions.…”

“…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. , …”

“…However, no liposomes were detected for SE and SS dispersions in water and in salt solution (see Figures S2 and S3 in the SI). Furthermore, for SS dispersions in water, only vesicles were identified in cryo-TEM micrographs …”

“…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. − …”

“…Nonetheless, for most double-chain surfactants, additional treatments are needed for the liposomes to break up into vesicles. These methods include vigorous stirring, 15 extrusion through an ultrafiltration membrane, 21 sonication, 22 or reverse evaporation. 23 Since the properties of these vesicles depend on the method of preparation, their formation is apparently not spontaneous.…”