Eduard-Zintl-Institut für Anorganische und Physikalische Chemie , Technische Universität Darmstadt , D-64287 Darmstadt , Germany.
J Phys Chem B. 2019 May 2;123(17):3875-3883. doi: 10.1021/acs.jpcb.9b01644. Epub 2019 Apr 24.
We test the OPLS/AA force field for a single PNIPAM 40-mer in aqueous solution using replica exchange molecular dynamics simulations and find that the force field fails to reproduce the experimental temperature behavior. To resolve this issue, we apply a modification on the partial charges previously suggested to reproduce the liquid-liquid phase separation of NIPAM aqueous solutions. The modified force field features stronger amide-water electrostatic interactions than the original OPLS model, predicts a weaker water-mediated monomer-monomer attraction, and reproduces the experimental coil-globule collapse enthalpy of PNIPAM in water. We revisit the cononsolvency problem of PNIPAM in methanol/water mixtures with the modified model and show that the dependence of the coil-globule collapse enthalpy on methanol concentration follows the experimental trend of the lower critical solution temperature. The calculations with the modified force field confirm that polymer dehydration is the determining factor for chain collapse in the cononsolvency regime.
我们使用复制交换分子动力学模拟测试了水溶液中的单个 PNIPAM 40 -mer 的 OPLS/AA 力场，并发现该力场无法重现实验的温度行为。为了解决这个问题，我们对以前建议的部分电荷进行了修改，以重现 NIPAM 水溶液的液-液相分离。修改后的力场具有比原始 OPLS 模型更强的酰胺-水静电相互作用，预测出较弱的水介导单体-单体吸引力，并重现了 PNIPAM 在水中的实验卷曲-球-collapse 焓。我们使用修改后的模型重新研究了 PNIPAM 在甲醇/水混合物中的共溶剂问题，并表明卷曲-球-collapse 焓对甲醇浓度的依赖性遵循实验上的下临界溶液温度趋势。修改后的力场的计算结果证实，聚合物脱水是共溶剂区链塌陷的决定因素。
