Department of Physical Chemistry, University of Chemistry and Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic.
J Phys Chem B. 2020 Mar 26;124(12):2495-2504. doi: 10.1021/acs.jpcb.0c00413. Epub 2020 Mar 12.
The behavior of thermoresponsive polymer poly(-isopropylacrylamide) (PNiPAM), an essential building block in the design of smart soft materials, in aqueous solutions has attracted much interest, which contrasts with our knowledge of -isopropylacrylamide (NiPAM) monomer. Strikingly, the physicochemical properties of aqueous NiPAM are similarly rich, and their understanding is far from being complete. This stems from the lack of accurate thermodynamic data and quantitative model for atomistic simulations. In this joint study, we have probed the thermodynamic behavior of aqueous NiPAM by experimental methods, molecular dynamics (MD) simulations, and Kirkwood-Buff (KB) analysis at ambient conditions. From the partial molar volumes and simultaneously correlated osmotic coefficients, with excess partial molar enthalpies of NiPAM in water, the concentration and temperature dependence of KB integrals was determined. For the purpose of this work, we have developed and employed a novel NiPAM force field, which not only reproduces KB integrals () and adequately captures macroscopic thermodynamic quantities but also provides more accurate structural insight than the original force fields. We revealed in the vicinity of NiPAM the competing effect of amide hydration with interaction between nonpolar regions. This microscopic picture is reflected in the experimentally observed NiPAM-NiPAM association, which is present from highly dilute conditions up to the solubility limit and is evidenced by . From intermediate concentrations, it is accompanied by the existence of apparent dense-water regions, as indicated by positive values. The here-employed KB-based framework provided a mutually consistent thermodynamic and microscopic insight into the NiPAM solution and may be further extended for ion-specific effects. Moreover, our findings contribute to the understanding of thermodynamic grounds behind PNiPAM collapse transition.
温敏聚合物聚（异丙基丙烯酰胺）（PNiPAM）是设计智能软材料的基本组成部分，其在水溶液中的行为引起了广泛关注，这与我们对异丙基丙烯酰胺（NiPAM）单体的认识形成了鲜明对比。引人注目的是，NiPAM 水溶液的物理化学性质同样丰富，但其理解还远未完成。这源于缺乏准确的热力学数据和原子模拟的定量模型。在这项联合研究中，我们通过实验方法、分子动力学（MD）模拟和环境条件下的 Kirkwood-Buff（KB）分析研究了 NiPAM 水溶液的热力学行为。从偏摩尔体积和同时相关的渗透压系数，以及 NiPAM 在水中的超额偏摩尔焓，确定了 KB 积分随浓度和温度的变化。为此，我们开发并采用了一种新的 NiPAM 力场，该力场不仅再现了 KB 积分（）并充分捕获了宏观热力学量，而且比原始力场提供了更准确的结构洞察力。我们在 NiPAM 附近揭示了酰胺水合与非极性区域之间相互作用的竞争效应。这种微观图景反映在实验观察到的 NiPAM-NiPAM 缔合中，缔合存在于高度稀释条件直至溶解度极限，并由 证实。从中等浓度开始，它伴随着明显的致密水区域的存在，这由正值 表明。这里采用的基于 KB 的框架为 NiPAM 溶液提供了相互一致的热力学和微观见解，并可进一步扩展到离子特异性效应。此外，我们的发现有助于理解 PNiPAM 塌陷转变背后的热力学基础。
