Moncho-Jordá A, Adroher-Benítez I
Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva S/N, 18071 Granada, Spain.
Soft Matter. 2014 Aug 21;10(31):5810-23. doi: 10.1039/c4sm00243a. Epub 2014 Jun 30.
In this work we have performed a theoretical study of a system formed by ionic microgels in the presence of monovalent salt with the help of Ornstein-Zernike integral equations within the hypernetted-chain (HNC) approximation. We focus in particular on analysing the role that the short-range specific interactions between the polymer fibres of the microgel and the incoming ions have on the equilibrium ion distribution inside and outside the microgel. For this purpose, a theoretical model based on the equilibrium partitioning effect is developed to determine the interaction between the microgel particle and a single ion. The results indicate that when counterions are specifically attracted to the polymer fibres of the microgel, an enhanced counterion accumulation occurs that induces the charge inversion of the microgel and a strong increase of the microgel net charge (or overcharging). In the case of coions, the specific attraction is also able to provoke the coion adsorption even though they are electrostatically repelled, and so increasing the microgel charge (true overcharging). Moreover, we show that ion adsorption onto the microgel particle is very different in swollen and shrunken states due to the competition between specific attraction and steric repulsion. In particular, ion adsorption occurs preferentially in the internal core of the particle for swollen states, whereas it is mainly concentrated in the external shell for de-swollen configurations. Finally, we observe the existence of a critical salt concentration, where the net charge of the microgels vanishes; above this inversion point the net charge of the microgels increases again, thus leading to reentrant stability of microgel suspensions.
在这项工作中，我们借助超网链（HNC）近似下的奥恩斯坦 - 泽尔尼克积分方程，对在单价盐存在下由离子微凝胶形成的系统进行了理论研究。我们特别关注分析微凝胶的聚合物纤维与进入的离子之间的短程特异性相互作用对微凝胶内外平衡离子分布的作用。为此，开发了一个基于平衡分配效应的理论模型来确定微凝胶颗粒与单个离子之间的相互作用。结果表明，当反离子被特异性吸引到微凝胶的聚合物纤维上时，会发生增强的反离子积累，从而导致微凝胶的电荷反转和微凝胶净电荷的强烈增加（或过充电）。对于同离子，特异性吸引即使在它们受到静电排斥的情况下也能够引发同离子吸附，从而增加微凝胶电荷（真正的过充电）。此外，我们表明由于特异性吸引和空间排斥之间的竞争，离子在微凝胶颗粒上的吸附在溶胀和收缩状态下有很大不同。特别是，对于溶胀状态，离子吸附优先发生在颗粒的内部核心，而对于去溶胀构型，它主要集中在外壳。最后，我们观察到存在一个临界盐浓度，此时微凝胶的净电荷消失；超过这个反转点，微凝胶的净电荷再次增加，从而导致微凝胶悬浮液的再入稳定性。
