Center for Computational and Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Chem Phys. 2012 Jul 28;137(4):044505. doi: 10.1063/1.4737432.
The static microstructures and thermodynamics of a colloidal dispersion of dipolar Janus (DJ) particles-that is, dipolar spheres in which each hemisphere is specified by a different charge interaction-have been investigated through simulation. DJ particles are modeled at a high level of detail with pairwise potentials represented as a sum of a spherically symmetric soft repulsion and an orientation-dependent electrostatic component using continuous potentials. The latter is important because it allows for the use of conventional molecular dynamics simulations, and is in contrast to the patch model and dipolar hard sphere model, which are discontinuous and therefore do not. The electrostatics are represented through a rigorous pointwise (PW) covering of two different hemispheres filled by points of corresponding charge. An isotropic coarse-graining (CG) of the PW models serves as a limit of the structure wherein the orientations of the DJ particles can be pairwise averaged. Over the range of volume fractions and DJ charge densities studied-consistent with reversible structures absent of long-range correlations-the CG model agrees well with the PW model with respect to equilibrium structure (isotropic pair correlation) and ensemble free energy. Time-dependent relaxation simulations of the PW model suggest that chain structures are not expected in liquid phases in contrast to that which has been observed for point dipole models of simple polar fluids.
通过模拟研究了双极各向异性（DJ）粒子胶体分散体的静态微观结构和热力学，即每个半球由不同电荷相互作用指定的偶极球体。DJ 粒子采用高细节水平建模，使用连续势表示的对势能由球形对称软排斥和取向相关的静电分量之和组成。后者很重要，因为它允许使用常规的分子动力学模拟，与补丁模型和偶极硬球模型形成对比，后两者是不连续的，因此不能使用。通过用相应电荷的点填充两个不同半球的严格逐点 (PW) 覆盖来表示静电。PW 模型的各向同性粗粒化 (CG) 作为结构的极限，其中 DJ 粒子的取向可以进行两两平均。在所研究的体积分数和 DJ 电荷密度范围内——与不存在长程相关的可逆结构一致——CG 模型在平衡结构（各向同性对相关）和总体自由能方面与 PW 模型非常吻合。PW 模型的时变弛豫模拟表明，与简单极性流体的点偶极模型观察到的情况相反，在液相中预计不会出现链状结构。
