Adsorption of poly-L-lysine and poly-L-glutamic acid on silica surfaces

The adsorption of poly-L-lysine (PL) and poly-L-glutamic acid (PG) was determined by FTIR-ATR-spectrometry on negatively charged hydrophilic and neutral hydrophobic silica surfaces of flat silicon crystals. Position and shape of the amide bands correspond to a coil like adsorption of PL at all pH values and of PG at pH > 5. High affinity adsorption isotherms are obtained for both polyelectrolytes. On the hydrophobic surface the plateau amounts of PL and PG decrease with their ionization degree demonstrating flat adsorbed macromolecules in the highly charged state because of the repellent electrostatic segmental interactions. The plateau amounts of PL increase at increasing pH and of PG at decreasing pH and nearly the same maximal amounts are reached at the neutral state corresponding to a dominant role of hydrophobic interactions. On hydrophilic surfaces the additional attraction by the negatively charged SiOH groups increases the adsorption of PL, whereas the adsorbed amount of PG decreases by repulsion. Decreasing surface charge with decreasing pH reduces these effects. Screening by the electrolyte KBr has no effect at the neutral surface, whereas at the charged surface rite adsorbed amounts of PL and PG increase distinctly at C-KBr > 0.4 mol/l.