Supramolecular Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan.
J Chem Phys. 2010 Apr 14;132(14):144503. doi: 10.1063/1.3373404.
We have systematically investigated the diffusion behavior of silica nanoparticles within supercritical ethanol, in terms of solvent properties by varying temperature (T) and pressure (P), to elucidate how the inhomogeneous solvent structures and density fluctuations in the solvent affect the diffusion behavior of solute particles. Results show that at a constant pressure, the diffusion coefficient (D) of the particles increases with increasing temperature, reaches the maximum (D(max)) within the gaslike supercritical fluid (slightly below the ridge), and finally decreases abruptly at very low fluid density when temperature is increased further. Results reveal that D is appreciably larger than the theoretical prediction (Einstein-Stokes relationship) in the vicinity of the critical density (rho(c)) of the solvent. We interestingly observed that D becomes maximum (D(max)) at a particular thermodynamic condition (T(i),P(i)), which is expressed by the empirical formula T(ri)=P(ri) (0.16) (for T(ri)>1, P(ri)>1). Here, T(ri)=T(i)/T(c) and P(ri)=P(i)/P(c); T(c) and P(c) are the temperature and the pressure at critical point, respectively. Results further reveal that D(max) increases significantly with decreasing solvent density within the gaslike supercritical fluid where the changes in viscosities are negligible. These findings are unique, novel, and intriguing. We suggest that the enhancement of the diffusion coefficient in the vicinity of the critical density and the abrupt decrease in the diffusion coefficient in very low density gaslike fluid are associated with the change in the solvent-solvent and solute-solvent direct correlation function (related to the effective interaction potential) upon density change when the fluid crosses the ridge of density fluctuations and within the gaslike fluid.
我们系统地研究了二氧化硅纳米粒子在超临界乙醇中的扩散行为，通过改变温度（T）和压力（P）来研究溶剂性质，以阐明溶剂不均匀结构和密度涨落如何影响溶质粒子的扩散行为。结果表明，在恒定压力下，粒子的扩散系数（D）随温度升高而增大，在类气态超临界流体（略低于脊线）中达到最大值（D(max)），当温度进一步升高时，在非常低的流体密度下急剧下降。结果表明，在溶剂的临界密度（rho(c)）附近，D 明显大于理论预测（爱因斯坦-斯托克斯关系）。我们有趣地观察到，D 在特定的热力学条件（T(i),P(i)）下达到最大值（D(max)），这可以用经验公式 T(ri)=P(ri) (0.16)（对于 T(ri)>1, P(ri)>1）来表示。这里，T(ri)=T(i)/T(c)，P(ri)=P(i)/P(c)；T(c)和 P(c)分别是临界点的温度和压力。结果进一步表明，在类气态超临界流体中，当粘度变化可以忽略时，随着溶剂密度的降低，D(max)显著增加。这些发现是独特的、新颖的和有趣的。我们认为，在临界密度附近扩散系数的增强以及在非常低密度的类气态流体中扩散系数的急剧下降，与流体穿过密度涨落的脊线以及在类气态流体中时密度变化引起的溶剂-溶剂和溶质-溶剂直接相关函数（与有效相互作用势能有关）的变化有关。
