Depletion Interactions Produced by Nonadsorbing Charged and Uncharged Spheroids.

The effect of macromolecule shape on the depletion attraction between two hard spherical particles in a solution with nonadsorbing hard spheroidal macromolecules of arbitrary size and aspect ratio was investigated using a modified form of the force-balance model of J. Y. Walz and A. Sharma (1994, J. Colloid Interface Sci. 168, 495). The macromolecules were represented as general spheroids, which could be either charged or uncharged. For the uncharged case, a set of analytical expressions describing the depletion attraction, valid for particles much larger than the characteristic macromolecule size, was developed. Comparisons with the case of spherical macromolecules were made under the condition of either constant macromolecule number density, rho(b), or constant volume fraction, phi. It was found that increasing the spheroidal macromolecule aspect ratio (major axis length/minor axis length) decreases the depletion attraction at constant rho(b), but increases the interaction at constant phi. In the latter case, the interaction produced by prolate macromolecules is greater than that produced by oblate macromolecules of equal axis lengths, while the opposite is true at constant rho(b). A simple scaling analysis is used to explain these trends. Surface charge is found to increase both the range and the magnitude of the depletion attraction; however, the general trends are the same as those found in the uncharged systems. Finally, the effect of the depletion attraction produced by spherical and spheroidal macromolecules on the stability of a dispersion of charged particles was examined. It was found that charged spheroids at concentrations of order 1% volume can produce secondary energy wells of sufficient magnitude to induce flocculation in a dispersion of charged spherical particles. Copyright 2000 Academic Press.