(51c) Effect of a Double-Chain Surfactant on the Stabilization of Suspensions of Silica and Titania Particles Against Both Agglomeration and Sedimentation

Aqueous vesicle dispersions formed from a cationic double-chain surfactant didodecyldimethylammonium bromide (DDAB) are shown to stabilize suspensions of silica and titania particles against both agglomeration and sedimentation. The densities of the silica and titania particles, r_p, were accurately determined via densitometry in which the densities of the particle suspensions were measured for particle weight fractions w_p from 0.001 to 0.020. The heights of the settling fronts of the particle suspensions in water at these weight fractions were visually monitored in order to measure the settling velocities, v_sed, of each suspension. The diameters of the particles d_sed were determined from these settling velocities and the use of Stokes’ law. The diameters of the silica particles with nominal diameters d_n = 500, 750, and 1000 nm were found to be d_sed = 450, 695, and 827 nm, respectively, which also indicates that no particle agglomeration took place in water. DDAB vesicle dispersions for DDAB weight fractions w_D = 0.010, 0.020, and 0.027 were prepared via stirring followed by sonication, and were allowed to age for 10 days. The particle suspensions were then mixed with the aged DDAB dispersions, with final values of w_D = 0.009, 0.018, and 0.025. The particle settling fronts were monitored by time-lapse photography to obtain the settling velocities of the silica or titania particles within the DDAB vesicle dispersions. At w_D = 0.009, v_sed was independent of time for all these sizes, implying that the particle sizes remained constant during sedimentation. Hence, the presence of the DDAB vesicles at this weight fraction caused no agglomeration of the particles. The settling velocities and Stokes’ law were also used to estimate the viscosities of the DDAB vesicle dispersions at the shear stresses induced by the settling particles. From the sedimentation of the silica suspensions with d_sed = 695 nm, the viscosities of the DDAB dispersions were estimated to be 1.35 cP at w_D = 0.009 and 5.02 cP at w_D = 0.018. For this particle size, the shear stress exerted by a particle is roughly estimated to be ca. 0.005 Pa. For w_D = 0.025 and d_sed = 695 nm, the settling velocities of the silica suspensions were essentially zero for at least 10 days, indicating that these particles were stabilized fully against both agglomeration and sedimentation. Moreover, the determined value of v_sed = 0 suggests that the viscosity at low shear stresses (ca. 0.005 Pa) was quite large, practically infinite (Yang, Franses, and Corti, J. Phys. Chem. B, 2019, 123, 922-935). Nevertheless, all of these DDAB vesicle dispersions remain flowable at the higher shear stresses (on average ca. 1 Pa) that develop during the flow of the bulk dispersions through small capillaries with diameters of 1 to 5 mm. Hence, their bulk viscosities remain finite even when the DDAB dispersions stabilize the particle suspensions against sedimentation. The DDAB dispersions at high enough values of w_D therefore exhibit properties similar to a soft gel with a yield stress lower than ca. 0.1 Pa. For d_sed = 827 nm and w_D = 0.018, some particles nevertheless sedimented much faster than expected for monodisperse sizes. These faster settling velocities suggest that agglomeration has occurred, probably due to depletion forces induced by the vesicles, an effect that was analogously generated by micelles (Yang et. al, J. Colloid Interface Sci., 2015, 450, 434-445). Nonetheless, for d_sed = 827 nm and w_D = 0.025, neither particle agglomeration nor sedimentation was detected, evidently because of the very high viscosity of the DDAB dispersion at low shear stresses. Overall, DDAB vesicle dispersions can stabilize many types and sizes of particles against both agglomeration and sedimentation, while still retaining some desirable properties for various practical applications.