Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, PR China 230026.
Langmuir. 2011 Aug 16;27(16):9895-901. doi: 10.1021/la201057h. Epub 2011 Jul 20.
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 Na(2)SO(4) induces more collapse of the grafted layer than that of NaClO(3) 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 Na(2)SO(4), indicating the decrease of layer thickness or the chain collapse. However, ΔRU exhibits a minimum as the ionic strength increases in the case of NaClO(3). 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 Na(2)SO(4) or NaClO(3), 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.
聚[(2-二甲氨基)乙基甲基丙烯酸酯]（PDEM）在 pH 值为 4、7 和 10 时分别完全带正电、部分带正电和不带电。我们通过石英晶体微天平（QCM-D）和表面等离子体共振（SPR）研究了盐效应对不同 pH 值下接枝在表面上的 PDEM 链构象变化的影响。QCM-D 测量中的频率（Δf）和耗散（ΔD）变化表明，在 pH 值为 4 和 7 时，构象行为受抗衡离子凝聚控制，但在 pH 值为 10 时受非静电阴离子吸附控制。在 pH 值为 4 和 7 时，与 NaClO3 相比，Na2SO4 的加入导致接枝层的塌陷更多。然而，在 pH 值为 10 时，它们具有相似的效果。SPR 测量中的共振单元位移（ΔRU）反映了层厚度和层折射率的变化。在 pH 值为 4 时，在存在 Na2SO4 的情况下，ΔRU 随离子强度的增加而减小，表明层厚度或链塌陷减小。然而，在 NaClO3 的情况下，随着离子强度的增加，ΔRU 呈现出最小值。这是因为在低离子强度和高离子强度区域，分别是层厚度和折射率的影响占主导地位。在 pH 值为 7 时，在 Na2SO4 或 NaClO3 的情况下，ΔRU 随离子强度的变化很小，表明在层塌陷过程中，层厚度和折射率的影响相当。在 pH 值为 10 时，ΔRU 的移动表明非静电阴离子吸附控制 PDEM 链的构象行为。
