Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany.
J Chem Phys. 2009 Oct 21;131(15):154702. doi: 10.1063/1.3246844.
Combining computer simulations and experiments we address the impact of charged surfaces on the solvation forces of a confined, charged colloidal suspension (slit-pore geometry). Investigations based on the colloidal-probe atomic-force-microscope technique indicate that an increase in surface charges markedly enhances the oscillations of the force in terms of their amplitude. To understand this effect on a theoretical level we perform grand-canonical Monte-Carlo simulations (GCMC) of a coarse-grained model system. It turns out that various established approaches of the interaction between a charged colloid and a charged wall, such as linearized Poisson-Boltzmann (PB) theory involving the bulk screening length, do not reproduce the experimental observations. We thus introduce a modified PB potential with a space-dependent screening parameter. The latter takes into account, in an approximate way, the fact that the charged walls release additional (wall) counterions which accumulate in a thin layer at the surface(s). The resulting, still purely repulsive fluid-wall potential displays a nonmonotonic behavior as function of the surface potential with respect to the strength and range of repulsion. GCMC simulations based on this potential reproduce the experimentally observed charge-induced enhancement in the force oscillations. We also show, both by experiment and by simulations, that the asymptotic wave- and decay length of the oscillating force do not change with the wall charge, in agreement with predictions from density functional theory.
我们结合计算机模拟和实验研究了带电表面对受限带电胶体悬浮液（狭缝孔几何形状）溶剂化力的影响。基于胶体探针原子力显微镜技术的研究表明，表面电荷的增加显著增强了力的振荡幅度。为了在理论层面上理解这种效应，我们对粗粒化模型系统进行了巨正则蒙特卡罗模拟（GCMC）。结果表明，各种已建立的带电胶体与带电壁之间相互作用的方法，例如涉及体相屏蔽长度的线性化泊松-玻尔兹曼（PB）理论，无法重现实验观测结果。因此，我们引入了一个具有空间相关屏蔽参数的修正 PB 势。该参数以近似的方式考虑了带电壁释放额外（壁）抗衡离子的事实，这些抗衡离子在表面附近的薄层中积聚。所得的、仍然纯粹排斥的流体-壁势表现出与表面电势有关的非单调行为，其强度和排斥范围。基于该势的 GCMC 模拟再现了实验观察到的电荷诱导的力振荡增强。我们还通过实验和模拟表明，振荡力的渐近波和衰减长度不会随壁电荷而变化，这与密度泛函理论的预测一致。
