Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, Wageningen, The Netherlands.
J Phys Chem A. 2012 Jun 28;116(25):6574-81. doi: 10.1021/jp212640w. Epub 2012 Mar 30.
The use of thin layers of a surface bound (polyelectrolyte) hydrogels for measuring the concentration of metal ions from electrolyte solutions is our motivation for modeling such hydrogels. The gels are composed of polymeric species with conformational degrees of freedom on the nanometer scale. The polymer conformations are affected by the presence of cross-links in the gel on a five to ten times larger length scale, and the repulsive interactions generated by the charges along the chains. Here we present a hybrid computational Monte Carlo Self-consistent field (MC-SCF) approach to model such hydrogels. The SCF formalism is used to evaluate the conformational properties of the chains, implementing a freely jointed chain model, in between featureless cross-links. The Monte Carlo simulation method is used to sample the (restricted) translational degrees of freedom of the cross-links in the gel. We consider the case that the polymers in the gel have an affinity for surface positioned at the edge of the simulation volume. The polymer density decays as a power-law from the surface to the gel-density with an exponent close to -4/3. The gel features relatively large density fluctuations which is natural for a gel with a low density (φ ≈ 0.035), a low degree of cross-linking (average of three chainparts per cross-link), and relatively large chains (N = 50) in between the cross-links. Some parts of the gel can break loose from the gel and sample the adjoining volume. Representative snapshots exemplify large density fluctuations, which explain the large pore size distribution observed in experimental counterparts.
我们的动机是为了模拟这种水凝胶，因此使用表面结合（聚电解质）水凝胶的薄层来测量电解质溶液中金属离子的浓度。凝胶由具有纳米尺度构象自由度的聚合物种组成。聚合物构象受到凝胶中交联的影响，交联的长度尺度为五到十倍，并且受到链上电荷产生的排斥相互作用的影响。在这里，我们提出了一种混合计算蒙特卡罗自洽场（MC-SCF）方法来模拟这种水凝胶。SCF 形式用于评估链的构象特性，在无特征的交联之间实现自由连接链模型。蒙特卡罗模拟方法用于对凝胶中交联的（受限）平移自由度进行采样。我们考虑了凝胶中的聚合物对位于模拟体积边缘的表面具有亲和力的情况。聚合物密度从表面到凝胶密度呈幂律衰减，指数接近-4/3。凝胶具有相对较大的密度波动，这对于密度较低（φ≈0.035）、交联度较低（平均每个交联三个链段）且交联之间的链较长（N=50）的凝胶来说是自然的。凝胶的某些部分可以从凝胶中脱离出来并在相邻的体积中采样。代表性快照示例说明了较大的密度波动，这解释了在实验对应物中观察到的较大孔径分布。
