Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan.
J Chem Phys. 2012 Mar 28;136(12):124903. doi: 10.1063/1.3697477.
All-atom molecular dynamics simulations are used to study the condensation behavior of monovalent (Na(+)) and multivalent (Ca(2+)) salt counterions associated with the co-ions (Cl(-)) surrounding the charged poly(methacrylic acid) (PMAA) chain in water. The study is extended to the influences on chain conformation, local arrangement, and dynamics of water in the highly diluted aqueous solutions. We find that even when the salt ions are monovalent, they attract more than one charged monomer and act as a bridging agent within the chain, as the multivalent salt ions. In principle, the salt ions bridge between not only the "non-adjacent" but also the "adjacent" charged monomers, leading to a more coil-like and a locally stretched conformation, respectively. With an increase in the salt concentration, the amount of coiled-type condensed ions increase and reach a maximum when the chain conformation becomes the most collapsed; whereas, the stretched-type shows an opposite trend. Our results show that the attractive interactions through the condensed salt ions between the non-adjacent monomers are responsible for the conformational collapse. When the salt concentration increases high enough, a significant increase for the stretched-type condensed ions makes an expansion effect on the chain. These stretched-type salt ions, followed by the adsorption of the co-ions and water molecules, tend to form a multilayer organization outside surrounding the PMAA chain. Thus, the expansion degree of the chain conformation is greatly limited. When only the monovalent Na(+) ions are present in the solutions, water molecules are primarily adsorbed into either the condensed Na(+) ions or the COO(-) groups. These adsorbed water molecules form hydrogen bonds with each other and enhance the local bridging behavior associated with the Na(+) condensation on the resultant chain conformation. With an increase in the amount of multivalent Ca(2+) salt ions, more water molecules are bonded directly with the condensed Ca(2+) ions. In this case, only the condensed Ca(2+) ions provide a strong bridging effect within the polymer chain. We observe a significant shift towards a higher frequency of the oxygen vibration spectrum and only a slight shift towards a higher frequency of the hydrogen spectrum for the water molecules associated with the ion condensation.
使用全原子分子动力学模拟研究了单价（Na(+)）和多价（Ca(2+））盐反离子与带电荷的聚（甲基丙烯酸）（PMAA）链周围的共离子（Cl(-））在水中的凝聚行为。研究扩展到对高度稀释水溶液中链构象、局部排列和水分子动力学的影响。我们发现，即使盐离子是单价的，它们也会吸引一个以上的带电单体，并在链内充当桥接剂，就像多价盐离子一样。原则上，盐离子不仅在“非相邻”，而且在“相邻”带电单体之间桥接，分别导致更卷曲和局部拉伸的构象。随着盐浓度的增加，卷曲型凝聚离子的数量增加，并在链构象最坍塌时达到最大值；而拉伸型则呈现相反的趋势。我们的结果表明，通过非相邻单体之间凝聚盐离子的吸引力是导致构象坍塌的原因。当盐浓度增加到足够高时，大量拉伸型凝聚离子的增加会对链产生扩展效应。这些拉伸型盐离子，以及随后共离子和水分子的吸附，往往会在 PMAA 链周围形成多层组织。因此，链构象的扩展程度受到很大限制。当溶液中仅存在单价 Na(+)离子时，水分子主要被吸附到凝聚的 Na(+)离子或 COO(-)基团中。这些吸附的水分子相互形成氢键，并增强与 Na(+)凝聚相关的局部桥接行为，从而影响到所得链构象。随着多价 Ca(2+)盐离子数量的增加，更多的水分子与凝聚的 Ca(2+)离子直接键合。在这种情况下，只有凝聚的 Ca(2+)离子在聚合物链内提供强桥接效应。我们观察到与离子凝聚相关的水分子的氧振动谱向更高频率的显著移动，而氢谱仅向更高频率稍有移动。
