Lal Moti, Plummer Martin, Smith William
Centre for Nanoscale Science, The Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, United Kingdom.
J Phys Chem B. 2006 Oct 26;110(42):20879-88. doi: 10.1021/jp0633650.
In exploring the effects of solvent density on the mode and the degree of solvation of the bare and passivated 38-atom gold particle in supercritical ethane, we have extended the molecular dynamics simulations of the system, reported previously,(34) to cover a range of isotherms in the T > T(c) regime, where T(c) is the critical temperature of the solvent. Consonant with our previous observations, the modes of solvation of the bare and the passivated particle, deduced from the radial distribution of the solvent about the metal core center of mass, are found to be vastly different from each other at all solvent densities: while the molecules solvating the bare particle form a well-defined, two-region layer around it, those solvating the passivated particle are loosely dispersed in the passivating layer. For the bare particle, the degree of solvation (vartheta) as a function of solvent density passes through a maximum occurring in the close vicinity of the critical point, consistent with our previous results and in agreement with Debenedetti's theoretical analysis,(22,23) which predicts a solvation enhancement effect in the critical region for systems where the unlike solvent/solute interaction is much stronger than the solvent/solvent interaction. Taking the degree of solvation (vartheta) as a measure of solvent quality, we have investigated how the solvent quality would vary along the solvent-density isotherms. In the solvent-density regime rho > rho(c), the solvent quality is found to be a decreasing function of the density as a result of progressive dominance of the excluded volume effect over the attractive particle/solvent interactions. The particle/solvent affinity is greatly reduced in the presence of the passivating layer, resulting in considerable shrinkage of the good-solvent-quality domain in the supercritical regime. The solvent environment and the presence of the passivating chains produce significant disorder in the equilibrium structure assumed by the nanoparticle core.
在探究溶剂密度对超临界乙烷中裸露的和钝化的38原子金颗粒的溶剂化模式及程度的影响时，我们扩展了先前报道的该系统的分子动力学模拟(34)，以涵盖T > T(c)区域内的一系列等温线，其中T(c)是溶剂的临界温度。与我们之前的观察结果一致，从溶剂围绕金属核心质心的径向分布推断出的裸露颗粒和钝化颗粒的溶剂化模式，在所有溶剂密度下都彼此大不相同：溶剂化裸露颗粒的分子在其周围形成一个明确的双区域层，而溶剂化钝化颗粒的分子则松散地分散在钝化层中。对于裸露颗粒，溶剂化程度(ϑ)作为溶剂密度的函数在临界点附近出现最大值，这与我们之前的结果一致，也与德贝内代蒂的理论分析(22,23)相符，该分析预测对于异种溶剂/溶质相互作用远强于溶剂/溶剂相互作用的系统，在临界区域会出现溶剂化增强效应。以溶剂化程度(ϑ)作为溶剂质量的度量，我们研究了溶剂质量如何沿溶剂密度等温线变化。在溶剂密度范围ρ > ρ(c)内，由于排除体积效应逐渐超过吸引性的颗粒/溶剂相互作用，溶剂质量是密度的递减函数。在存在钝化层的情况下，颗粒/溶剂亲和力大大降低，导致超临界区域中良溶剂质量区域显著缩小。溶剂环境和钝化链的存在使纳米颗粒核心所假定的平衡结构产生了显著的无序。
