Light reflectivity study on the adsorption kinetics of poly(propylene imine) dendrimers on glass

The rate of adsorption of positively charged poly(propylene imine) dendrime rs on glass, an oppositely charged surface, has been studied as a function of generation and charge (by systematic adjustment of pH and ionic strength ) using scanning angle reflectometry and an impinging-jet cell. A compariso n of the mass transport conditions for this geometry and the bulk long-time self-diffusion coefficient obtained from pulsed field gradient NMR experim ents shows that a sticking probability of the order of 3% is needed to rela te the diffusion toward the surface with the long-time self-diffusion in th e bulk. The adsorption kinetics were mostly diffusion/convection controlled with a linear dependence on the bulk concentrations up to 10 mg/L at pH 7 and in 0.1 M NaCl. At higher bulk concentrations there is a drop in concent ration dependence into a 1/3 power law dependence. This crossover concentra tion shifts to higher concentrations with decreasing pH (increasing dendrim er charge).