2-(p-toluidino)-6-naphthalene sulfonate as a fluorescent probe for flocculation studies of cationic potato amylopectin and nanosized silica particles. 2. Flocculation by binding of nanosized silica particles

2-(p-toluidino)-6-naphthalene sulfonate (TNS) is a probe that fluoresces st rongly when bound to certain proteins and polymers, but weakly in aqueous s olutions. The reversible association of TNS is used to monitor the binding of anionic nanosized silica particles (NSP) to cationic potato amylopectin starch (CApS) through the decreasing fluorescence emission as TNS is compet itively released by the particle binding. Steady-state fluorescence measure ments at different mixing ratios of CApS and NSP provide data on the equili brium binding. The isotherm derived is used to establish the fact that the most efficient flocculation between CApS and NSP occurs when the polymer co ils are nearly saturated by NSP, but still have positively charged parts le ft. This supports a patch-flocculation mechanism. Stopped-flow experiments show that NSP binding to CApS occurs within a few milli seconds. This obser vation allows turbidity changes which occur on longer timescales to be ascr ibed to particle-decorated polymers undergoing changes in the conformation or aggregation.