Rheological Studies of Threadlike Micellar Solutions and Spherical Particle Suspensions

Abstract

This paper is composed of two parts. The first part is a brief review of rheological behaviors of viscoelastic cationic surfactant solutions, and the second is a review of viscoelasticity of hard core suspensions.
A cationic surfactant, cetyltrimethylammonium bromide (CTAB), forms long and stable thread-like micelles in aqueous solutions with sodium salicylate (NaSal). Then, a CTAB:NaSal/Water system showed pronounced viscoelasticity depending on the concentration of NaSal, C_S, when the concentration of CTAB, C_D, was kept constant. In the case of C_S ? C_D, the system exhibited a unique viscoelasticity essentially the same as that of a Maxwell element with only one relaxation time. In this condition, the plateau modulus was proportional to the square of C_D. Thus, the origin of the elasticity in the system is entanglement effects between the threadlike micelles as in the concentrated polymer system. On the other hand, the relaxation time in this condition was controlled by the concentration of free Sal-anions, C*_S, which could be easily estimated from C_S and C_D as C*_S = C_S-C_D. A simple model (called the Phantom Network Model) proposed for entangling polymer systems can successfully explain the unique viscoelastic behavior of this CTAB:NaSal/Water system.
Suspensions of silica particles dispersed in ethylene glycol possess inter-particle forces very close to the hard core one. Viscoelastic behavior of the monodisperse spherical particle suspension with the hard core inter-particle force was discussed from a series of expeniments of particles with various radii. The zero shear viscosities and the high frequency limiting viscosities were functions of only volume fraction of the particle independent of the particle radius. Relaxation times for the monodisperse suspension were well described with times necessary for dispersed particles to migrate randomly by distance equal to their radii due to Brownian motion. Thus, the origin of viscoelasticity for the monodisperse suspension should be the Brownian motion of the suspended particle. Viscoelastic behavior for bimodal suspensions with the hard core inter-particle force was also investigated to clarify contribution of the distribution of particle radius to their viscoelastic behaviors. When a particle radius ratio was less than 3, shape of relaxation spectra for the bimodal suspension was essentially the same as that of the monodisperse suspension. However, when the ratio was larger than 5, the shape of the relaxation spectrum was much broader than that of the monodisperse suspension. These mean that motions of two kinds of particles are averaged into those of a hypothetical particle with the average radius in the case of the radius ratio less than 3. The zero shear viscosities of the bimodal suspension exhibited minima at compositions dependent on the particle radius ratio, while the high frequency limiting viscosities were essentially independent of the composition in the entire range of the particle radius ratio examined.