Specific Anion Effects on the Growth and Stability of Poly (2-ethyl-2-oxazoline)/Tannic Acid Layer by Layer Films

The control of pH responsiveness of hydrogen bonded (H-bonded) layer-by-layer films is especially important in biomedical applications. Specific anion effects are known to vary the cloud point temperature of poly(2-ethyl-2-oxazoline) (PEOX) solutions. By growing H-bonded multilayers of PEOX and Tannic Acid (TA) in the presence of nine different sodium salts at different salt concentrations, we show that the growth profile and pH stability of the multilayers can be controlled in a wide range. The observed changes in the average bilayer (BL) thickness and critical disintegration pH are in accordance with the Hofmeister series of anions. Going from the most kosmotropic (Na2CO3) to the most chaotropic (NaSCN), BL thickness decreases by ~11.5 nm while the critical disintegration pH increases up to 10. Conformational variations in PEOX chains from compact coils due to dehydration by kosmotropic anions to extended chains due to direct ion binding of chaotropic anions contribute to these changes. ITC investigation of PEOX/TA interaction in the presence of sodium salts in solution confirm the enhanced stability in the direction of the most chaotropic anion. The results clearly show that properties of H-bonded multilayers can be easily tuned by specific anion effects as desired by applications.