Low-Shear Viscosities of (Semi-)Dilute, Aqueous Dispersions of Charged Boehmite Rods: Dynamic Scaling of Double Layer Effects

Abstract

Low-shear viscosities were measured of dilute aqueous dispersions of charged boehmite rods with an average aspect ratio of 22.5 as a function of the Debye length κ-1 and the particle volume fraction Φ. It is found that the low-shear viscosity of the dispersions scales with the effective overlap concentration ν/ν1*, where ν1* is the minimum overlap concentration of rods with an effective length L* = L + 5κ-1. This dynamic scaling is also valid for low-shear viscosity data of FD-virus solutions (Graf et al. J. Chem. Phys. 1993, 98, 4920). The rescaled relative viscosity curves can be described by the (Maron-Pierce) equation ηr = (1 − (ν/ν1*)/(ν/ν1*)max)-2. The effective overlap concentration at which the low-shear viscosity diverges, (ν/ν1*)max, is much lower for the boehmite rods than for the semiflexible FD-virus and hard rods without electric double layers. Extrapolation of the reduced viscosity to ν/ν1* = 0 yields an unrealistic high intrinsic viscosity [η]. Possibly the relative viscosity of dilute dispersions at very low ionic strength shows a nonanalytical concentration dependence, which renders the definition of [η] for colloids with thick double layers questionable.