Conformational Behavior of Grafted Weak Polyelectrolyte Chains: Effects of Counterion Condensation and Nonelectrostatic Anion Adsorption

Poly[(2-dimethylamino)ethyl methacrylate] (PDEM) is completely charged, partially charged, and uncharged at pH 4, 7, and 10, respectively. We have investigated the salt effects on the conformational change of PDEM chains grafted on a surface at different pH by using quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance (SPR). The changes in frequency (Δf) and dissipation (ΔD) in QCM-D measurements demonstrate that the conformational behavior is governed by counterion condensation at pH 4 and 7 but by nonelectrostatic anion adsorption at pH 10. The addition of Na₂SO₄ induces more collapse of the grafted layer than that of NaClO₃ at pH 4 and 7. However, they have a similar effect at pH 10. The shift of resonance unit (ΔRU) in SPR measurements reflects the changes of layer thickness and layer refractive index. At pH 4, ΔRU decreases with ionic strength in the presence of Na₂SO₄, indicating the decrease of layer thickness or the chain collapse. However, ΔRU exhibits a minimum as the ionic strength increases in the case of NaClO₃. This is because the effects of the layer thickness and refractive index are dominant in the low and high ionic strength regimes, respectively. At pH 7, ΔRU slightly varies with ionic strength in the case of either Na₂SO₄ or NaClO₃, indicating that the effects of the layer thickness and refractive index are comparable during the layer collapse. At pH 10, the shift in ΔRU suggests that the nonelectrostatic anion adsorption governs the conformational behavior of the PDEM chains.