Picosecond Fluorescence Studies of the Surface Morphology of Charged Polystyrene Latex Particles

Two negatively charged polystyrene latex dispersions with particles of similar diameter (around 100 nm) but different surface charge density (1.7 μC/cm² for CG18 and 2.3 μC/cm² for CG5) were compared in terms of their surface morphology. Since they were synthesized in the presence of a redox initiator (potassium persulfate/sodium metabisulfite), particles bore sulfate and sulfonate charged groups at the surface. Fluorescence decay measurements were performed in situ on these latex samples, using a pair of fluorescent cationic dyes adsorbed onto the surface:  Rhodamine 6G (R6G) as the energy transfer donor and malachite green carbinol hydrochloride (MG) as the acceptor. Since the kinetics of the direct nonradiative energy transfer process between the dyes is sensitive to the donor and acceptor distributions, fluorescence decay measurements provide information about the binding distribution of the cationic dyes, which reflects the distribution of the negative charges at the latex particle surface. For the highly charged sample (CG5), we found a dye distribution reflecting an interface thickness with an average value of δ = 2.9 ± 0.2 nm. For the CG18 sample, having a lower surface charge density, a thinner dye distribution was evidenced, with an interface thickness δ = 2.0 ± 0.1 nm. The broadening of the distribution with the increase of surface charge density was interpreted in terms of the steric hindrance and the electrostatic repulsion between the charged polymer chain ends extending into the aqueous medium.