Layer-by-Layer Assembly of Thick, Cu²⁺-Chelating Films

Layer-by-layer adsorption of protonated poly­(allylamine) (PAH) and deprotonated poly­(N,N-dicarboxymethylallylamine) (PDCMAA) yields thick films with a high density of iminodiacetic acid (IDA) ligands that bind metal ions. When film deposition occurs at pH 3.0, PAH/PDCMAA bilayer thicknesses reach 200 nm, and Cu²⁺ binding capacities are ∼2.5 mmol per cm³ of film. (PAH/PDCMAA)₁₀ films deposited at pH 3.0 are 4–8-fold thicker than films formed at pH 5.0, 7.0, or 9.0, presumably because of the low charge density on PDCMAA chains at pH 3.0. However, with normalization to film thickness, all films bind similar amounts of Cu²⁺ from pH 4.1 solutions of CuSO₄. In micrometer-thick films, equilibration of binding sites with Cu²⁺ requires ∼4 h due to a low Cu²⁺ diffusion coefficient (∼2.6 × 10^–12 cm²/s). Sorption isotherms determined at several temperatures show that Cu²⁺ binding is endothermic with a positive entropy (binding constants increase with increasing temperature), presumably because metal-ion complexation involves displacement of both a proton from IDA and water molecules from Cu²⁺. (PAH/PDCMAA)₁₀ films retain their binding capacity over four absorption/elution cycles and may prove useful in metal-ion scavenging, catalysis, and protein binding.