In situ studies of molecular transfer between microparticles by second-harmonic generation

We have demonstrated that second-harmonic generation (SHG) can be used to i nvestigate the in situ transfer of molecules between colloidal microparticl es. The transfer kinetics of organic cations, malachite green (MG) initiall y adsorbed on to spherical polystyrene sulfate (PSS) microparticles, to dis k-shaped clay particles in aqueous solution was obtained. The surface popul ation of MG on the PSS particles during the transfer process can be measure d in real time. In the present studies, the adsorption free energy and the number of adsorption sites were found from the adsorption isotherm measured by the SHG method (Wang, H.; Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. P hys. Chem. B 1998, 102, 4446). The kinetics of transfer was observed to dep end linearly on the particle density of clay. The observed transfer was bie xponential with decay times of 17 +/- 1 s and 217 +/- 4 s for the case of a clay particle density of 3 x 10(9)/cm(3). This method was further applied to study the transfer of MG between PSS microparticles. At a PSS density of 4 x 10(8)/cm(3), the decay time was 40 s. Because of the coherent nature o f the SHG signal, the transfer kinetics can be obtained in situ by the SHG method. A kinetic model is used to describe the transfer process from which the desorption and adsorption rate constants of MG from and onto PSS parti cles were obtained.