Krishnan S H, Ayappa K G
Department of Chemical Engineering, Indian Institute of Science, Bangalore-India 560012.
J Phys Chem B. 2005 Dec 15;109(49):23237-49. doi: 10.1021/jp054402a.
The density of states for bulk and confined fluids have been modeled using a recently proposed gamma distribution (Krishnan, S. H.; Ayappa, K. G. J. Chem. Phys. 2004, 121, 3197). The gamma distribution results in a closed form analytical expression for the velocity autocorrelation function and the relaxation time of the fluid. The two parameters of the gamma distribution are related analytically to the second and fourth frequency moments of the fluid using short time expansions. The predictions by the proposed gamma model are compared with the velocity autocorrelation functions obtained using the theory of instantaneous normal modes (INMs) and from molecular dynamics simulations. The model is applied to a bulk soft sphere liquid and fluids confined in a spherical cavity and slit-shaped pores. The gamma model is able to capture the resulting changes in relaxation time due to changes in density and temperature extremely well for both the bulk liquid and confined inhomogeneous fluid situations. In all cases, the predictions by the gamma model are superior to those obtained from the INM theory. In the case of the fluid confined in a slit pore, the loadings were obtained from a grand canonical Monte Carlo simulation where the pore is equilibrated with a bulk fluid. This is similar to a confinement situation in a surface force apparatus. The predicted relaxation times vs pore widths from the gamma model are seen to accurately capture the oscillations due to formation and disruption of layers within the slit pore.
已使用最近提出的伽马分布（Krishnan, S. H.; Ayappa, K. G. J. Chem. Phys. 2004, 121, 3197）对体相流体和受限流体的态密度进行了建模。伽马分布给出了速度自相关函数和流体弛豫时间的封闭形式解析表达式。利用短时展开，伽马分布的两个参数与流体的二阶和四阶频率矩解析相关。将所提出的伽马模型的预测结果与使用瞬时正则模（INMs）理论以及分子动力学模拟得到的速度自相关函数进行了比较。该模型应用于体相软球液体以及受限在球形腔和狭缝形孔中的流体。对于体相液体和受限非均匀流体情况，伽马模型都能够非常好地捕捉由于密度和温度变化导致的弛豫时间变化。在所有情况下，伽马模型的预测都优于从INM理论获得的结果。在受限在狭缝孔中的流体的情况下，负载是通过大正则蒙特卡罗模拟获得的，其中孔与体相流体达到平衡。这类似于表面力装置中的受限情况。从伽马模型预测的弛豫时间与孔宽度的关系可以准确地捕捉到由于狭缝孔内层的形成和破坏而产生的振荡。
