Triplet-state photoexcitation dipole-jump relaxation method to observe adsorption/desorption kinetics at a...

Electronic excitation of a probe chromophore can lead to a change in dipole moment that influences its activity or solubility in solution and changes its relative affinity for partitioning between two phases. Photoexcitation of a probe molecule can, therefore, perturb a sorption equilibrium, and the relaxation kinetics of the probe to the new equilibrium conditions can be monitored in a time-resolved luminescence experiment. The adsorption/desorption kinetics of rose bengal, distributed between a C-18 derivatized porous-silica surface and a liquid mobile-phase solution, were investigated. These kinetics were determined by observing their effect on the phosphorescence decay of the triplet state of rose bengal and its quenching by ferricyanide. The methanol/water solvent compositions were varied to alter the fraction of adsorbed rose bengal. The adsorption rate constant for the triplet state was determined from the dependence of the phosphorescence relaxation rate on dye concentration in solution. The results indicate that the adsorption kinetics are diffusion controlled and that the relaxation is influenced by efficient triplet-energy transfer between excited- and ground-state rose bengal at the C-18 silica/solution interface.