Forces between glass surfaces in aqueous polyethylenimine solutions

Interaction forces between flame-polished glass surfaces were measured in a range of aqueous solutions of a branched cationic polyelectrolyte, polyethylenimine (PEI) MW approximate to 70 000 g/mol. Short incubation in 1 ppm PEI solution leads to neutralization of the glass negative surface charge. At this point the interaction forces are dominated by a bridging attraction detectable at separations below 10 nm. Prolonged incubation in the same solution results in charge reversal. Upon increasing the bulk polymer concentration, an additional adsorption takes place, and the magnitude of the observed charge reversal increases. In 50 ppm PEI an additional electrosteric force is present at short distances, and the force measured on approach remains repulsive at all separations. The pull-off forces measured on separation were shown to be dependent on the time the two surfaces spent in contact and increased with increasing contact time. In addition, the concentration dependence of the pull-off force in PEI solutions was compared to two linear, highly charged polyelectrolytes polyvinylamine (PVAm) and poly([2-(propionyloxy)ethyl]trimethylamonium chloride) (PCMA). It was found that the two linear polyelectrolytes induce lower adhesion between glass surfaces than PEI. Further, for the linear polyelectrolytes the adhesion force was not strongly influenced by the polyelectrolyte concentration for concentrations higher than 2 ppm. In contrast, PEI generates higher adhesive forces at low bulk concentrations, which then sharply decrease upon increasing the PEI concentration. These differences are discussed in terms of both the PEI highly branched structure and the fact that the PEI charge is concentration dependent due to changes in solution pH.