Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva S/N, 18071 Granada, Spain.
J Chem Phys. 2013 Aug 14;139(6):064906. doi: 10.1063/1.4817852.
In this work the system formed by charged (ionic) microgels in the presence of monovalent salt is investigated by solving numerically the Ornstein-Zernike integral equations within the Hypernetted-Chain approximation. The ionic density profiles, effective interaction between microgel particles, and the effective charge of the particles are calculated. In addition to the electrostatic interaction, the excluded-volume repulsion between the microgel particle and the ions is also explicitly taken into account. Although this steric interaction is irrelevant in the swollen state (when the packing fraction of the polymer network is low), it becomes a very important contribution close to the de-swollen state, hindering the counterion penetration inside the microgel mesh. The theoretical predictions indicate that the ionic density profiles are strongly affected by the degree of swelling, going from a volumetric distribution of counterions in the swollen state to a surface accumulation outside the particle that becomes more important as the particle shrinks. The electrostatic effective interaction between pairs of microgel particles is shown to be the result of a complex interplay between electrostatic and depletion effects that strongly depend on the bare charge density of the particle. For sufficiently charged microgel particles, the steric exclusion leads to a less efficient screening of the microgel charge near the de-swollen configuration, and so to a significant increase of the effective charge of the microgel particle.
在这项工作中，通过求解超网链近似下的奥恩斯坦-泽尼克积分方程，研究了单价盐存在下带电荷（离子）微凝胶形成的体系。计算了离子密度分布、微凝胶颗粒之间的有效相互作用和颗粒的有效电荷。除了静电相互作用外，还明确考虑了微凝胶颗粒与离子之间的排斥体积。尽管这种空间相互作用在溶胀状态（聚合物网络的堆积分数较低时）无关紧要，但在接近去溶胀状态时，它成为一个非常重要的贡献，阻碍了反离子渗透到微凝胶网格内部。理论预测表明，离子密度分布受到溶胀程度的强烈影响，从溶胀状态下的反离子体积分布到颗粒外部的表面堆积，随着颗粒的收缩，这种堆积变得更加重要。对微凝胶颗粒对之间的静电有效相互作用表明，这是静电和耗尽效应之间复杂相互作用的结果，而这强烈依赖于颗粒的裸电荷密度。对于带足够电荷的微凝胶颗粒，空间排斥作用导致微凝胶电荷在去溶胀状态附近的屏蔽效率降低，因此微凝胶颗粒的有效电荷显著增加。
