Picosecond fluorescence studies of the surface morphology of charged polystyrene latex particles

Two negatively charged polystyrene latex dispersions with particles of simi lar diameter (around 100 nm) but different surface charge density (1.7 muC/ cm(2) for CG18 and 2.3 muC/cm(2) for CG5) were compared in terms of their s urface morphology. Since they were synthesized in the presence of a redox i nitiator (potassium persulfate/sodium metabisulfite), particles bore sulfat e and sulfonate charged groups at the surface. Fluorescence decay measureme nts were performed in situ on these latex samples, using a pair of fluoresc ent cationic dyes adsorbed onto the surface: Rhodamine 6G (R6G) as the ener gy transfer donor and malachite green carbinol hydrochloride (MG) as the ac ceptor. Since the kinetics of the direct nonradiative energy transfer proce ss between the dyes is sensitive to the donor and acceptor distributions, f luorescence decay measurements provide information about the binding distri bution of the cationic dyes, which reflects the distribution of the negativ e charges at the latex particle surface. For the highly charged sample (CG5 ), we found a dye distribution reflecting an interface thickness with an av erage value of delta = 2.9 +/- 0.2 nm. For the CG18 sample, having a lower surface charge density, a thinner dye distribution was evidenced, with an i nterface thickness delta = 2.0 +/- 0.1 nm. The broadening of the distributi on with the increase of surface charge density was interpreted in terms of the steric hindrance and the electrostatic repulsion between the charged po lymer chain ends extending into the aqueous medium.