Effect of a double-chain surfactant on the stabilization of suspensions of silica and titania particles against agglomeration and sedimentation

Dispersions of vesicles of didodecyldimethylammonium bromide (DDAB) were prepared with magnetic stirring of multilamellar liposomes followed by ultrasonication (SS) in order to generate dispersions that contained only unilamellar vesicles, unlike the stirring-only (S) method which also produces many large liposomes. With the SS method, the vesicles had average diameters ranging from 68 to 80 nm, which were much smaller than the diameters generated with the S method. The new vesicle dispersions had nearly infinite viscosities at very low shear stresses at DDAB weight fractions wD from 0.025 to 0.027, even though they were not close-packed, in contrast to the S dispersions. The new SS vesicle dispersions stabilized completely against sedimentation polydisperse nonspherical crystalline titania particles with sizes ranging from ca. 96–156 nm, as well as suspensions of monodisperse spherical amorphous silica particles with diameters of dsed = 454 nm, 691 nm, and 826 nm. These results establish that DDAB vesicle dispersions can stabilize against sedimentation a broad range of particle types and sizes. In addition, at the low values of wD = 0.009 and 0.018, the effective viscosities, ηeff, of the DDAB dispersions, determined from the sedimentation velocities, ranged from 1.35-1.87 cP and 4.34–5.57 cP, respectively. At higher shear stresses, however, these dispersions were highly shear-thinning and still flowable in a capillary under gravity. This behavior is critical for the practical applications of such dispersions for inkjet printing and possibly paints.