Overduin S D, Perera Aurélien, Patey G N
Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada.
Laboratoire de Physique Théorique de la Matière Condensée (UMR CNRS 7600), Université Pierre et Marie Curie, 4 Place Jussieu, F75252 Paris Cedex 05, France.
J Chem Phys. 2019 May 14;150(18):184504. doi: 10.1063/1.5097011.
Large-scale molecular dynamics simulations are reported for aqueous t-butanol (TBA) solutions. The CHARMM generalized force field (CGenFF) for TBA is combined with the TIP4P/2005 model for water. Unlike many other common TBA models, the CGenFF model is miscible with water in all proportions at 300 K. The main purpose of this work is to investigate the existence and nature of a microheterogeneous structure in aqueous TBA solutions. Our simulations of large systems (128 000 and 256 000 particles) at TBA mole fractions of 0.06 and 0.1 clearly reveal the existence of long-range correlations (>10 nm) that show significant variations on long time scales (∼50 ns). We associate these long-range slowly varying correlations with the existence of supramolecular domainlike structures that consist of TBA-rich and water-rich regions. This structure is always present but continually changing in time, giving rise to long-range slowly varying pair correlation functions. We find that this behavior appears to have little influence on the single particle dynamics; the diffusion coefficients of both TBA and water molecules lie in the usual liquid state regime, and mean square displacements provide no indication of anomalous diffusion. Using our large system simulations, we are able to reliably calculate small angle x-ray scattering and small angle neutron scattering spectra, except at a very low wave vector, and the results agree well with recent experiments. However, this paper shows that simulation of the relatively simple TBA/water system remains challenging. This is particularly true if one wishes to obtain properties such as Kirkwood-Buff factors, or scattering functions at a low wave vector, which strongly depend on the long-range behavior of the pair correlations.
本文报道了叔丁醇（TBA）水溶液的大规模分子动力学模拟。TBA的CHARMM广义力场（CGenFF）与水的TIP4P/2005模型相结合。与许多其他常见的TBA模型不同，CGenFF模型在300 K下能与水以任意比例互溶。这项工作的主要目的是研究TBA水溶液中微观非均相结构的存在及其性质。我们对TBA摩尔分数为0.06和0.1的大型系统（128000和256000个粒子）进行的模拟清楚地揭示了长程相关性（>10 nm）的存在，这种相关性在长时间尺度（约50 ns）上表现出显著变化。我们将这些长程缓慢变化的相关性与由富TBA区域和富水区域组成的超分子域状结构的存在联系起来。这种结构始终存在，但随时间不断变化，从而产生长程缓慢变化的对关联函数。我们发现这种行为似乎对单粒子动力学影响很小；TBA和水分子的扩散系数都处于通常的液态范围，均方位移没有显示出反常扩散的迹象。利用我们的大型系统模拟，我们能够可靠地计算小角X射线散射和小角中子散射光谱，除了在非常低的波矢下，结果与最近的实验吻合得很好。然而，本文表明，相对简单的TBA/水系统的模拟仍然具有挑战性。如果希望获得诸如柯克伍德-布夫因子或低波矢下的散射函数等性质，情况尤其如此，因为这些性质强烈依赖于对关联的长程行为。
