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 mole cules with chain lengths of eight or more monomers there is a single p olymer mass (0.1 mu g ml(-1)) which neutralizes the particle surface c harge and destabilizes the particle suspension in the presence of 0.00 5 hi NaCl at a particle concentration of 1.47 x 10(10) particles per m i. The coagulation kinetics at this optimum polymer dose are diffusion -limited, even for the largest polymer tested. Hence, the electrostati c ''patch'' model for the flocculation of negatively charged particles by cationic polymers proposed by Gregory (Gregory, J. Colloid Interfa ce Sci., 42 (1973) 448; 55 (1976) 35) does not apply to our system. Th e mechanism responsible for polymer flocculation in a given system app ears to depend upon the relative spacing of charged groups on the poly mer backbone and particle surface, the size and inherent flexibility o f the polymer in question, and the nature of the initial polymer-parti cle interaction.