Depletion interactions produced by nonadsorbing charged and uncharged spheroids

The effect of macromolecule shape on the depletion attraction between two h ard spherical particles in a solution with nonadsorbing hard spheroidal mac romolecules of arbitrary size and aspect ratio was investigated using a mod ified 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 g eneral spheroids, which could be either charged or uncharged. For the uncha rged case, a set of analytical expressions describing the depletion attract ion, 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 densi ty, rho (b), Or constant volume fraction, phi. It was found that increasing the spheroidal macromolecule aspect ratio (major axis length/minor axis le ngth) decreases the depletion attraction at constant rho (b), but increases the interaction at constant phi. In the latter case, the interaction produ ced by prolate macromolecules is greater than that produced by oblate macro molecules of equal axis lengths, while the opposite is true at constant rho (b) A simple scaling analysis is used to explain these trends. Surface cha rge 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 dispersio n of charged particles was examined. It was found that charged spheroids at concentrations of order 1% volume can produce secondary energy wells of su fficient magnitude to induce flocculation in a dispersion of charged spheri cal particles. (C) 2000 Academic Press.