Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany.
Soft Matter. 2023 Feb 1;19(5):938-950. doi: 10.1039/d2sm01301k.
The network charge of polyampholyte microgels can be tuned by varying the pH of the surrounding solution, and a charge reversal from a positively charged microgel at low pH to a negatively charged microgel at high pH can be achieved. In a titration experiment, it is difficult to tell apart the ionisation of the acidic and basic monomers in the network and to determine the distribution of charges in the network, whereas using Metropolis Monte Carlo simulations, both the degree of ionisation and the distribution of ionised monomers can be determined separately for both species. Building on our earlier work on alternating polyampholyte microgels, we now investigated the pH-dependent ionisation and the swelling behaviour of polyampholyte core-shell microgels under good solvent conditions. For this purpose, we performed Metropolis Monte Carlo simulations for a bead-spring model using the constant-pH method. As in our previous study on alternating microgels, the width of the U-shaped curve of the microgels volume as a function of pH depends on the relative dissociation constants of acid and base, and the microgel volume can be approximated by a linear function of the total network charge. Due to the spatial separation of acid and base in core-shell systems, the ionisation is less enhanced compared to a microgel with an alternating distribution of the two species. Nevertheless, we still see an influence of the presence of one species on the ionisation behaviour of the other species under good solvent conditions. Furthermore, the isoelectric point is shifted towards higher pH, which is caused by a higher charge density in the core compared to that in the shell. Added salt changes the Donnan equilibrium, which determines the counterion distribution within and outside of the microgel. At the same time, it contributes to the electrostatic screening of the network charges, leading to a narrowing of the U-shaped volume transition curve.
聚两性电解质微凝胶的网络电荷可以通过改变周围溶液的 pH 值来调节，并且可以实现从低 pH 值下带正电荷的微凝胶到高 pH 值下带负电荷的微凝胶的电荷反转。在滴定实验中，很难区分网络中酸性和碱性单体的电离以及确定网络中的电荷分布，而使用 Metropolis 蒙特卡罗模拟，可以分别确定两种物质的电离度和离域单体的分布。在我们之前关于交替聚两性电解质微凝胶的工作基础上，我们现在研究了在良溶剂条件下聚两性电解质核壳微凝胶的 pH 依赖性电离和溶胀行为。为此，我们使用恒 pH 方法对珠-簧模型进行了 Metropolis 蒙特卡罗模拟。与我们之前关于交替微凝胶的研究一样，微凝胶体积随 pH 变化的 U 形曲线的宽度取决于酸和碱的相对离解常数，并且微凝胶体积可以用总网络电荷的线性函数来近似。由于核壳系统中酸和碱的空间分离，与两种物质交替分布的微凝胶相比，电离程度的增强较小。尽管如此，我们仍然看到在良溶剂条件下，一种物质的存在对另一种物质的电离行为的影响。此外，等电点向更高的 pH 值移动，这是由于与壳相比，核中的电荷密度更高。添加盐会改变决定微凝胶内外反离子分布的 Donnan 平衡。同时，它有助于网络电荷的静电屏蔽，导致 U 形体积转变曲线变窄。
