Department of Chemistry, The University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, USA.
J Chem Phys. 2010 Feb 7;132(5):054706. doi: 10.1063/1.3308625.
The physical properties of a liquid in contact with a solid are largely determined by the solid-liquid surface tension. This is especially true for nanoscale systems with high surface area to volume ratios. While experimental techniques can only measure surface tension indirectly for nanoscale systems, computer simulations offer the possibility of a direct evaluation of solid-liquid surface tension although reliable methods are still under development. Here we show that using a mean field approach yields great physical insight into the calculation of surface tension and into the precise relationship between surface tension and excess solvation free energy per unit surface area for nanoscale interfaces. Previous simulation studies of nanoscale interfaces measure either excess solvation free energy or surface tension, but these two quantities are only equal for macroscopic interfaces. We model the solid as a continuum of uniform density in analogy to Hamaker's treatment of colloidal particles. As a result, the Hamiltonian of the system is imbued with parametric dependence on the size of the solid object through the integration limits for the solid-liquid interaction energy. Since the solid-liquid surface area is a function of the size of the solid, and the surface tension is the derivative of the system free energy with respect to this surface area, we obtain a simple expression for the surface tension of an interface of arbitrary shape. We illustrate our method by modeling a thin nanoribbon and a solid spherical nanoparticle. Although the calculation of solid-liquid surface tension is a demanding task, the method presented herein offers new insight into the problem, and may prove useful in opening new avenues of investigation.
与固体接触的液体的物理性质在很大程度上取决于固液表面张力。对于具有高表面积与体积比的纳米尺度系统来说尤其如此。虽然实验技术只能间接地测量纳米尺度系统的表面张力，但计算机模拟提供了直接评估固液表面张力的可能性，尽管可靠的方法仍在开发中。在这里，我们表明，使用平均场方法可以深入了解表面张力的计算，以及表面张力与单位表面积过剩溶剂化自由能之间的精确关系，对于纳米尺度界面来说。纳米尺度界面的先前模拟研究要么测量过剩溶剂化自由能，要么测量表面张力，但这两个量仅在宏观界面上相等。我们将固体模拟为均匀密度的连续体，类似于哈马克（Hamaker）对胶体颗粒的处理。结果，通过固体-液体相互作用能的积分极限，系统的哈密顿量具有对固体物体大小的参数依赖性。由于固液界面面积是固体大小的函数，并且表面张力是系统自由能相对于该界面面积的导数，因此我们获得了任意形状界面的表面张力的简单表达式。我们通过模拟一个薄纳米带和一个固体球形纳米颗粒来演示我们的方法。虽然计算固液表面张力是一项艰巨的任务，但本文提出的方法为该问题提供了新的见解，并可能有助于开辟新的研究途径。
