pH-controlled adsorption of polyelectrolyte diblock copolymers at the solid/liquid interface

We investigated the pH dependence of the adsorption of polyelectrolyte dibl ock copolymers from aqueous solution onto the native oxide surface of silic on using spectroscopic ellipsometry. The observed adsorption behavior is cl osely related to the chemical structure and the hydrophilic-hydrophobic bal ance of the copolymers. These amphiphilic copolymers contain hydrophobic re sidues comprising either 2-(diethylamino)ethyl methacrylate (DEA) or 2-(dii sopropylamino)ethyl methacrylate (DPA). The copolymers also contain hydroph ilic residues that are either (1) methyl-quaternized or benzyl-quaternized 2-(dimethylamino)ethyl methacrylate (designated Me-DMA or Bz-DMA, respectiv ely) or (2) sulfopropyl betainized 2-(dimethylamino)ethyl methacrylate (Bet -DMA). The DEA and DPA residues can be tuned to become hydrophilic by adjus ting the solution pH. Thus, these diblock copolymers can be molecularly dis solved in acidic media without using cosolvents as a result of the protonat ion of the tertiary amine groups. At low solution pH, adsorption of the cop olymers is only about 0.5-1.5 mg/m(2), which is expected for polyelectrolyt e adsorption. Above a pH of 7, there is a pronounced increase in the adsorb ed amount. This change in adsorption coincides with the formation of copoly mer micelles in the bulk solution. Hence, it is likely that the interfacial layer consists of adsorbed micelles. In the Bz-DMA-b-DEA copolymer, only a small fraction of DEA blocks (22%) is needed to achieve a relatively large increase in the adsorption at higher pH. On the other hand, control experi ments confirm that the corresponding homopolymers show no sharp change in t he extent of adsorption with pH. Changing the hydrophobic residues from DEA to DPA does not significantly affect the extent of adsorption. However, su bstitution of the hydrophilic Bz-DMA residues with Bet-DMA significantly in creases the extent of adsorption at higher pH. This is probably because the electrically neutral betainized block is less hydrophilic than the cationi c Bz-DMA block. This work provides insight into the major influences on the block copolymer adsorption and thus creates a framework for tuning adsorpt ion behavior.