氯化胆碱-尿素（reline）深共晶体系非反常水致结构转变,Physical Chemistry Chemical Physics

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氯化胆碱-尿素（reline）深共晶体系非反常水致结构转变
Physical Chemistry Chemical Physics ( IF 2.9 ) Pub Date : 2022-11-22 , DOI: 10.1039/d2cp04139a
Hugo Monteiro ₁ , Alexandre Paiva ₁ , Ana Rita C Duarte ₁ , Nuno Galamba ₂

Affiliation

摩尔比 (1:2) 的氯化胆碱-尿素二元混合物，通常称为 reline，是低共熔溶剂 (DES) 中的原型溶剂。中子衍射 (ND) 和经验势结构细化 (EPSR) 结果提供的证据表明，再衬里在加水后表现出一种特殊的结构转变，表现为胆碱阳离子在约 51 wt% 的水下突然去湿，因此，非单调胆碱-水和胆碱-胆碱配位的变化。在这里，我们通过分子动力学 (MD) 研究了水对氯化胆碱结构的影响：尿素：水 DES (1 : 2 : ζ ; ζ= 0 到 40)，以进一步了解这种特殊结构转变的分子来源。研究了五个不同的力场。我们的结果表明，胆碱-胆碱配位的 ND/EPSR 非单调行为由那些更准确地描述 DES 动力学（即扩散和粘度系数）的力场定性再现。然而，在ζ处胆碱-胆碱配位明显增加> 10 (41 wt%) 与特定第一配位球的消耗有关，而不是与胆碱阳离子的去润湿有关，如 ND/EPSR 所预测的那样。因此，发现了胆碱-水配位的单调增加，以及水的四面体氢键网络的出现，这与 ND/EPSR 相反。尿素在胆碱周围的初始水合消耗率最高（~0.9 个尿素分子/ ζ），而胆碱则描述了最高的水合率（~2.5 个水分子/ ζ）。这些比率随ζ降低并且观察到稳定在 41 wt% 以上的结构转变，这与 ND/EPSR 结果一致。尽管力场存在局限性，但我们认为实验和模拟相反的图片可能与低估 EPSR 方法描绘的水的氢键网络有关。总体而言，与 ND/EPSR 相比，MD 描绘了从 DES 到 DES 组分水溶液的更平滑过渡。

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On the not so anomalous water-induced structural transformations of choline chloride–urea (reline) deep eutectic system

The choline chloride–urea binary mixture in the molar ratio (1 : 2), commonly known as reline, is an archetypal solvent among deep eutectic solvents (DES). Neutron diffraction (ND) and empirical potential structure refinement (EPSR) results provided evidence that reline exhibits a peculiar structural transformation upon water addition that manifests in a sudden dewetting of the choline cations at ∼51 wt% water and, therefore, a non-monotonic variation of the choline–water and choline–choline coordination. Here, we study, through molecular dynamics (MD), the influence of water on the structure of a choline chloride : urea : water DES (1 : 2 : ζ; ζ = 0 to 40), to gain additional insight into the molecular source of this peculiar structural transformation. Five different force fields were investigated. Our results show that the ND/EPSR non-monotonic behavior of the choline–choline coordination is qualitatively reproduced by those force fields that describe more accurately the dynamics of the DES, namely, the diffusion and viscosity coefficients. However, the apparent increase of the choline–choline coordination at ζ > 10 (41 wt%) is associated with the drain of a particular first coordination sphere, rather than with a dewetting of the choline cations, as predicted by ND/EPSR. Thus, a monotonic increase of the choline–water coordination is found instead, as well as the emergence of a tetrahedral hydrogen bond network of water, opposite to ND/EPSR. The highest rate of depletion upon initial hydration is found for urea around choline (∼0.9 urea molecules/ζ), whereas choline depicts the highest rate of hydration (∼2.5 water molecules/ζ). These rates decrease with ζ and a structural transition that stabilizes above 41 wt% is observed, consistent with ND/EPSR results. Despite limitations in the force fields, we argue that the experimental and simulation opposite pictures could be related to an underestimation of water's hydrogen bond network portrayed by the EPSR method. Overall, a smoother transition from a DES to an aqueous solution of the DES components is portrayed by MD, compared with ND/EPSR.

更新日期：2022-11-22

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