Adsorption and electrokinetic properties of polyethylenimine on silica surfaces

The adsorption on silica wafers of a hyperbranched, high molecular weight polyethylenimine (PEI) was investigated using reflectometry. The pH and ionic strength dependence of PEI adsorption kinetics and that of the adsorbed amount were interpreted according to the complex balance of segment/segment and segment/surface site interactions. The observed adsorption properties show significant deviations from the recently studied features of polyvinylamine adsorption on cellulose coated silicas. The different behavior can be attributed to the pronounced difference in the nonelectrostatic affinity of polyamines toward the two different types of surfaces. The role of electrostatic interactions was also characterized by electrokinetic measurements. Due to the adsorption of PEI, significant charge reversal and shift in the isoelectric point of silica wafers occur. The potential-pH curves show a maximum, which can be interpreted qualitatively by the adsorption characteristics of PEI. An attempt was also made to interrelate the adsorption and electrokinetic data via comparison of different estimates of the diffuse double layer charge of the PEI/silica system.