Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany.
Soft Matter. 2018 May 23;14(20):4087-4100. doi: 10.1039/C7SM02528A.
In this study, we investigated the effect of pH on single weak acidic polyelectrolyte microgels under salt-free conditions with (i) varying microgel concentration, (ii) varying content of acidic groups and (iii) different crosslinking densities using Monte Carlo simulations under explicit consideration of the protonation/deprotonation reaction. We assessed both global properties, such as the degree of ionization, the degree of swelling and the counterion distribution, and local properties such as the radial network ionization profile and the ionization along the polymer chains as a function of pH. We found a pronounced suppression of the pH-dependent ionization of the microgels, as compared to the ideal titration behavior and a shift of the titration curve to a higher pH originating in the proximity of acidic groups in the microgel. In contrast to macroscopic gels, counterions can leave the microgel, resulting in an effective charge of the network, which hinders the ionization. A decreasing microgel concentration leads to an increased effective charge of the microgel and a more pronounced shift of the titration curve. The number of acidic groups showed only a weak effect on the ionization behavior of the microgels. For two different microgels with different crosslinking densities, similar scaling of the gel size was observed. A distinct transition from an uncharged and unswollen to a highly charged and expanded polymer network was observed for all investigated microgels. The degree of swelling mainly depends on the degree of ionization. An inhomogeneous distribution of the degree of ionization along the radial profile of the microgel was found.
在这项研究中，我们使用蒙特卡罗模拟，在明确考虑质子化/去质子化反应的情况下，研究了在无盐条件下 pH 值对不同（i）微凝胶浓度、（ii）酸性基团含量和（iii）不同交联密度的单一弱酸性聚电解质微凝胶的影响。我们评估了全局性质，如离解度、溶胀度和反离子分布，以及局部性质，如径向网络离解轮廓和沿聚合物链的离解度随 pH 值的变化。与理想滴定行为相比，我们发现微凝胶的 pH 依赖性离解受到明显抑制，并且滴定曲线向更高 pH 值移动，这源于微凝胶中酸性基团的接近。与宏观凝胶不同，反离子可以离开微凝胶，从而产生网络的有效电荷，这阻碍了离解。微凝胶浓度的降低会导致微凝胶的有效电荷增加，并且滴定曲线的移动更加明显。酸性基团的数量对微凝胶的离解行为仅表现出微弱的影响。对于具有不同交联密度的两种不同微凝胶，观察到凝胶尺寸的相似缩放。对于所有研究的微凝胶，都观察到从无电荷和未溶胀到高电荷和膨胀聚合物网络的明显转变。溶胀度主要取决于离解度。沿着微凝胶的径向轮廓发现离解度的不均匀分布。
