Donnan Electric Potential Dependence of Intraparticle Diffusion of Malachite Green in Single Cation Exchange Resin Particles: A Laser Trapping-Microspectroscopy Study

A laser trapping-microspectroscopy technique combined with excitation energy transfer from a fluorescent cationic dye (Rhodamine B, RB) to a non-fluorescent cationic dye (Malachite Green, MG) was employed to study pH effects on the diffusion coefficients of MG (D(MG)) in single cation-exchange resin microparticles with the diameters of 16 μm. When RB-pre-adsorbed resin particles were soaked in an aqueous MG solution, the RB fluorescence was quenched gradually with the soaking time. The time course of the quenching efficiency of RB by MG was then used to evaluate the D(MG) value in the particle. The D(MG) value increased from 1.1 × 10 to 4.3 × 10 cm·s on going the solution pH value from 9 to 4. The results were explained reasonably by a Donnan electric potential model.