Yadav Hari O S, Shrivastav Gourav, Agarwal Manish, Chakravarty Charusita
Department of Chemistry, Indian Institute of Technology-Delhi, New Delhi 110016, India.
J Chem Phys. 2016 Jun 28;144(24):244901. doi: 10.1063/1.4954325.
The extent to which solvent-mediated effective interactions between nanoparticles can be predicted based on structure and associated thermodynamic estimators for bulk solvents and for solvation of single and pairs of nanoparticles is studied here. As a test of the approach, we analyse the strategy for creating temperature-independent solvent environments using a series of homologous chain fluids as solvents, as suggested by an experimental paper [M. I. Bodnarchuk et al., J. Am. Chem. Soc. 132, 11967 (2010)]. Our conclusions are based on molecular dynamics simulations of Au140(SC10H21)62 nanoparticles in n-alkane solvents, specifically hexane, octane, decane and dodecane, using the TraPPE-UA potential to model the alkanes and alkylthiols. The 140-atom gold core of the nanocrystal is held rigid in a truncated octahedral geometry and the gold-thiolate interaction is modeled using a Morse potential. The experimental observation was that the structural and rheological properties of n-alkane solvents are constant over a temperature range determined by equivalent solvent vapour pressures. We show that this is a consequence of the fact that long chain alkane liquids behave to a good approximation as simple liquids formed by packing of monomeric methyl/methylene units. Over the corresponding temperature range (233-361 K), the solvation environment is approximately constant at the single and pair nanoparticle levels under good solvent conditions. However, quantitative variations of the order of 10%-20% do exist in various quantities, such as molar volume of solute at infinite dilution, entropy of solvation, and onset distance for soft repulsions. In the opposite limit of a poor solvent, represented by vacuum in this study, the effective interactions between nanoparticles are no longer temperature-independent with attractive interactions increasing by up to 50% on decreasing the temperature from 361 K to 290 K, accompanied by an increase in emergent anisotropy due to correlation of mass dipoles on the two nanoparticles. One expects therefore that during self-assembly using solvent evaporation, temperature can be used as a structure-directing factor as long as good solvent conditions are maintained. It also suggests that disordered configurations may emerge as solvent quality decreases due to increasing role of short-range attractions and ligand fluctuation-driven anisotropy. The possibilities of using structural estimators of various thermodynamic quantities to analyse the interplay of ligand fluctuations and solvent quality in self-assembly as well as to design solvation environments are discussed.
本文研究了基于纳米颗粒的结构以及针对本体溶剂和单个及成对纳米颗粒溶剂化的相关热力学估算器，能够在多大程度上预测纳米颗粒之间溶剂介导的有效相互作用。作为该方法的一个测试，我们按照一篇实验论文[M. I. 博德纳丘克等人，《美国化学会志》132, 11967 (2010)]所建议的，分析了使用一系列同系链状流体作为溶剂来创建与温度无关的溶剂环境的策略。我们的结论基于对Au140(SC10H21)62纳米颗粒在正构烷烃溶剂（具体为己烷、辛烷、癸烷和十二烷）中的分子动力学模拟，使用TraPPE-UA势来模拟烷烃和烷基硫醇。纳米晶体的140原子金核保持在截角八面体几何形状中刚性不变，金 - 硫醇盐相互作用使用莫尔斯势进行模拟。实验观察到，在由等效溶剂蒸气压确定的温度范围内，正构烷烃溶剂的结构和流变性质是恒定的。我们表明，这是由于长链烷烃液体在很好的近似程度上表现为由单体甲基/亚甲基单元堆积形成的简单液体这一事实导致的。在相应的温度范围（233 - 361 K）内，在良溶剂条件下，单个和成对纳米颗粒水平的溶剂化环境近似恒定。然而，诸如无限稀释时溶质的摩尔体积、溶剂化熵以及软排斥的起始距离等各种量确实存在10% - 20%量级的定量变化。在本研究中以真空为代表的不良溶剂的相反极限情况下，纳米颗粒之间的有效相互作用不再与温度无关，随着温度从361 K降至290 K，吸引相互作用增加高达50%，同时由于两个纳米颗粒上质量偶极的相关性，出现的各向异性增加。因此可以预期，在使用溶剂蒸发进行自组装过程中，只要保持良溶剂条件，温度就可以用作结构导向因素。这也表明，由于短程吸引力和配体波动驱动的各向异性作用增强，随着溶剂质量下降可能会出现无序构型。讨论了使用各种热力学量的结构估算器来分析自组装中配体波动与溶剂质量的相互作用以及设计溶剂化环境的可能性。
