Characterization of polystyrene latex surfaces by the adsorption of rhodamine 6G

The adsorption of the fluorescent dye rhodamine 6G (R6G) onto polystyrene (PS) latex was investigated as a potential tool for characterizing the surface of PS latex, in particular the density of negative surface charges and their distribution on the surface. The method exploited previous observations that R6G forms nonfluorescent dimers in concentrated solutions and that when R6G is adsorbed to surfaces, R6G dimerization is enhanced. This work examined the influence of ionic strength and particle concentration for variations in R6G adsorption. R6G adsorption was found to be charge-driven in the most dilute regime, and hydrophobically driven in the concentrated regime, with the crossover occurring near the neutralization of negative surface charge by the positive R6G. For negatively charged particles, adsorbed dyes were nearly completely quenched on the surface near the point of charge neutralization, where the dispersions were also unstable. At higher R6G surface loadings, the surface fluorescence was recovered and the dispersions restabilized. No surface quenching could be resolved on cationic latex due to weaker dye adsorption.