Department of Chemical Engineering, Yale University, New Haven, Connecticut 06520-8286, USA.
J Phys Chem B. 2009 Oct 22;113(42):13860-5. doi: 10.1021/jp902864t.
We employ the grand canonical Monte Carlo simulation technique to investigate the influence of charged nanoparticles (macro-ions) on the force between colloidal objects. Specifically, the structure and osmotic pressure of a system of screened Coulomb (Yukawa) particles confined between charged planar walls are simulated. We observe osmotic pressure to oscillate with wall separation and these oscillations to correspond to changes in the number of nanoparticle layers present in the slit pore. Using the Derjaguin approximation, we estimate the overall force between a colloidal sphere and a flat surface and compare our predictions to recent atomic force microscopy (AFM) results (Tulpar, A.; Van Tassel, P. R.; Walz, J. Y. Langmuir 2006, 22, 2876-2883). In excellent agreement with experiment, we find the wavelength of the force versus distance oscillations to scale as c(nu), with c being the bulk nanoparticle concentration and nu = -0.31 +/- 0.01; that is, slightly lower in magnitude from the expected value -1/3 based on average molecule spacing. By considering an order parameter measuring the extent to which neighboring particles form hexagonal symmetry, we show structural order within confined nanoparticle systems to be significantly enhanced as compared to that of bulk systems, despite being quite insensitive to wall separation. Wavelength scaling and order parameter analysis together suggest the confined macro-ion systems to be somewhat glasslike.
我们采用巨正则蒙特卡罗模拟技术来研究带电纳米粒子（宏观离子）对胶体物体间相互作用力的影响。具体而言，我们模拟了受限于带电平面壁之间的屏蔽库仑（Yukawa）粒子体系的结构和渗透压。我们观察到渗透压随壁分离而振荡，这些振荡与狭缝孔中存在的纳米粒子层数量的变化相对应。我们使用德加古林近似法（Derjaguin approximation）来估算胶体球和平面之间的总相互作用力，并将我们的预测与最近的原子力显微镜（AFM）实验结果（Tulpar, A.; Van Tassel, P. R.; Walz, J. Y. Langmuir 2006, 22, 2876-2883）进行比较。与实验结果非常吻合，我们发现力与距离振荡的波长与 c(nu) 成正比，其中 c 是体相纳米粒子浓度，nu = -0.31 +/- 0.01；也就是说，与基于平均分子间距的预期值 -1/3 相比，其幅度略低。通过考虑一个测量相邻粒子形成六方对称性程度的序参量，我们表明受限纳米粒子体系的结构有序性与体相体系相比有显著提高，尽管对壁分离不太敏感。波长缩放和序参量分析表明，受限的宏观离子体系具有一定的玻璃化倾向。
