Factors influencing the flocculation of colloidal particles by a model anionic polyelectrolyte

The flocculation of charged colloidal particles by oppositely charged polyelectrolytes is investigated, using as a model system, positively charged latex microspheres and negatively charged single-stranded DNA of varying chain length from 3-mers up through 1400-mers. For DNA molecules with chain lengths of eight or more monomers there is a single polymer mass (0.1 mu g ml(-1)) which neutralizes the particle surface charge and destabilizes the particle suspension in the presence of 0.005 hi NaCl at a particle concentration of 1.47 x 10(10) particles per mi. The coagulation kinetics at this optimum polymer dose are diffusion-limited, even for the largest polymer tested. Hence, the electrostatic ''patch'' model for the flocculation of negatively charged particles by cationic polymers proposed by Gregory (Gregory, J. Colloid Interface Sci., 42 (1973) 448; 55 (1976) 35) does not apply to our system. The mechanism responsible for polymer flocculation in a given system appears to depend upon the relative spacing of charged groups on the polymer backbone and particle surface, the size and inherent flexibility of the polymer in question, and the nature of the initial polymer-particle interaction.