Biocolloid and Fluid Physics Research Group, Departamento de Física Aplicada, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva S/N, 18071 Granada, Spain.
J Chem Phys. 2013 Apr 7;138(13):134902. doi: 10.1063/1.4798708.
The mixture formed by charged (ionic) microgels in the presence of 1:1 added salt, with explicit consideration of a core-shell structure of the microgel particles, is studied. By solving numerically the three-component Ornstein-Zernike integral equations, the counter- and coion penetration inside the microgel network and the resulting effective microgel-microgel electrostatic interaction are calculated. This is done in the limit of very low microgel concentration, so that the resulting pair-wise effective potential is not affected by many-body particle-particle interactions. The ion-ion, microgel-ion, and microgel-microgel correlations are all treated within the Hypernetted-Chain approximation. The results obtained clearly show that the addition of salt to the microgel suspension has a deep impact on the screening of the bare charge of the particles, confirming an already well-known result: the strong reduction of the effective charge of the microgel occurring even for diluted electrolyte concentrations. We show that this effect becomes more important as we increase the shell size of the particle and derive a semi-empirical model for the effective charge as a function of the electrolyte concentration and the shell extension. The resulting microgel-microgel effective pair potential is analysed as a function of the shell extension and salt concentration. In all cases the interaction is a soft potential when particles overlap. For non-overlapping distances, our theoretical results indicate that microgel particles can be regarded as hard spherical colloids bearing an effective charge given by the net charge inside the particle and the microgel-microgel interaction shows a Yukawa-like behaviour as a function of the interparticle distance. It is also observed that increasing the bare-charge of the microgel induces a strong microgel-counterion coupling in the limit of very low electrolyte concentrations, which cannot be justified using linearized theories. This leads to an even more important adsorption of counterions inside the microgel network and to a reduction of the microgel-microgel effective repulsion.
在添加 1:1 盐的情况下，研究了带电荷（离子）的微凝胶在存在下形成的混合物，并明确考虑了微凝胶颗粒的核壳结构。通过数值求解三组分奥恩斯坦-泽尔尼克积分方程，计算了反离子和共离子在微凝胶网络中的渗透以及由此产生的有效微凝胶-微凝胶静电相互作用。这是在非常低的微凝胶浓度下完成的，因此所得的对有效势不受多体粒子-粒子相互作用的影响。离子-离子、微凝胶-离子和微凝胶-微凝胶相关性都在超网链近似中进行处理。得到的结果清楚地表明，向微凝胶悬浮液中添加盐会对颗粒的裸露电荷的屏蔽产生深远的影响，证实了一个已经众所周知的结果：即使在稀释的电解质浓度下，微凝胶的有效电荷也会强烈降低。我们表明，随着粒子壳层尺寸的增加，这种效应变得更加重要，并推导出有效电荷随电解质浓度和壳层延伸的半经验模型。所得的微凝胶-微凝胶有效对势作为壳层延伸和盐浓度的函数进行分析。在所有情况下，当粒子重叠时，相互作用都是软势。对于非重叠距离，我们的理论结果表明，微凝胶粒子可以被视为带有有效电荷的硬球形胶体，该有效电荷由粒子内的净电荷给出，并且微凝胶-微凝胶相互作用作为粒子间距离的函数表现出类 Yukawa 的行为。还观察到，增加微凝胶的裸露电荷会在非常低的电解质浓度下导致微凝胶-抗衡离子的强烈耦合，这不能用线性化理论来证明。这导致抗衡离子在微凝胶网络中的吸附更加强烈，并导致微凝胶-微凝胶有效排斥的减少。
