(60b) Effect of the Surfactant Molecular Structure on the Stabilization of Colloidal Suspensions Against Agglomeration and Sedimentation

Many aqueous colloidal suspensions, such as inks and paints, contain dense particles, such as titania (with a density of 4.2 g/cm³) and silica (with a density of 2.0 g/cm³), with sizes between about 50 and 1000 nm. These particles tend to settle in gravity within hours and days, even without agglomeration, which generally would make them settle even faster, when their Peclet number, which is defined as the ratio of their sedimentation rate to the diffusion rate, is higher than about 50 [1]. To maintain the useful properties of inks and paints suspensions in many applications, such particles must be suspended, without settling, for weeks and months. Stabilizing such suspensions against both agglomeration and sedimentation, while maintaining a low bulk suspension viscosity, to allow for fast flows in painting and inkjet printing applications, is quite challenging and has received little attention in the literature.

The key question is what surfactant molecular structures may produce the best dispersants. In our first empirical efforts towards that goal, we tested various single-chain ionic or nonionic surfactants, such as SDS (sodium dodecyl sulfate) and Triton X100, respectively [1, 2]. These surfactants adsorb at the titania particles/water surfaces below the cmc (critical micelle concentration), and can produce good stability against agglomeration via electrostatic or steric interactions. Nonetheless, the SDS micelles formed above the cmc provided little further improvement against agglomeration, and no improvement against sedimentation. In fact, at the higher micelle concentrations, the SDS micelles induced such strong depletion interaction forces that the particles flocculated and sedimented faster than without the micelles. The Triton X100 micelles increased the solution viscosity, and provided some improvement in the titania particles stability against sedimentation, but still showed a “masked” depletion interaction causing flocculation [2]. Hence, micelles formed by typical single-chain surfactants are of little use, or are detrimental, to the stability of particles against sedimentation. Different molecular structures that form non-micellar aggregates are needed.

We have discovered a new mechanism for stabilizing suspensions against sedimentation using a surfactant dispersant. Some of the physical and colloid chemistry and chemical engineering ideas which led to this discovery will be presented and explained. The chosen dispersant is a double chain cationic surfactant, DDAB (didodecyldimethylammonium bromide), which was known previously to form unilamellar vesicles, or simply “vesicles,” and close-packed vesicular dispersions (CPVDs) under certain conditions of temperature, salinity, and dispersion method [3]. DDAB does not form micelles. The key for such double-chain surfactants to form fluid vesicles is to have a “proper” phase behavior, namely the ability to form fluid multilamellar liquid crystals, or liposomes with a very high volume fraction of water by absorbing water spontaneously. Moreover, to be most effective and practically useful, the surfactant liposomes should be broken, upon stirring or sonication or extrusion through a porous membrane, to form fluid vesicles with high volume fractions but low surfactant weight fractions. This may be possible by controlling where possible the vesicles sizes. The reason for the favorable stabilization behavior is that the CPVDs are strongly shear-thinning, having low viscosities at high shear stresses relevant to flows in thin tubes but very high viscosities at the small shear stresses produced by the sedimenting particles in the dispersion media. When dense particles, titania or silica or other, are suspended in such CPVDs, they can remain suspended almost indefinitely.

1. Yang, Y.-J., Kelkar, A. Zhu, X., Bai, G., Ng, H.-T, Corti, D. S., and Franses, E. I., J. Colloid Interface Sci., 450, 434-445 (2015).
2. Yang, Y.-J., Corti, D. S., and Franses, E. I., Colloids Surfaces A, 516, 296-304 (2017).
3. Yang, Y.-J., Corti, D. S., and Franses, E. I., Langmuir, 31, 8802-8808 (2015).