Barnes Brian C, Siderius Daniel W, Gelb Lev D
Department of Chemistry and Center for Materials Innovation, Washington University in St. Louis, St. Louis, Missouri 63130, USA.
Langmuir. 2009 Jun 16;25(12):6702-16. doi: 10.1021/la900196b.
To better understand the self-assembly of small molecules and nanoparticles adsorbed at interfaces, we have performed extensive Monte Carlo simulations of a simple lattice model based on the seven hard "tetrominoes", connected shapes that occupy four lattice sites. The equations of state of the pure fluids and all of the binary mixtures are determined over a wide range of density, and a large selection of multicomponent mixtures are also studied at selected conditions. Calculations are performed in the grand canonical ensemble and are analogous to real systems in which molecules or nanoparticles reversibly adsorb to a surface or interface from a bulk reservoir. The model studied is athermal; objects in these simulations avoid overlap but otherwise do not interact. As a result, all of the behavior observed is entropically driven. The one-component fluids all exhibit marked self-ordering tendencies at higher densities, with quite complex structures formed in some cases. Significant clustering of objects with the same rotational state (orientation) is also observed in some of the pure fluids. In all of the binary mixtures, the two species are fully miscible at large scales, but exhibit strong species-specific clustering (segregation) at small scales. This behavior persists in multicomponent mixtures; even in seven-component mixtures of all the shapes there is significant association between objects of the same shape. To better understand these phenomena, we calculate the second virial coefficients of the tetrominoes and related quantities, extract thermodynamic volume of mixing data from the simulations of binary mixtures, and determine Henry's law solubilities for each shape in a variety of solvents. The overall picture obtained is one in which complementarity of both the shapes of individual objects and the characteristic structures of different fluids are important in determining the overall behavior of a fluid of a given composition, with sometimes counterintuitive results. Finally, we note that no sharp phase transitions are observed but that this appears to be due to the small size of the objects considered. It is likely that complex phase behavior may be found in systems of larger polyominoes.
为了更好地理解吸附在界面处的小分子和纳米颗粒的自组装过程，我们基于七种硬“四连块”（占据四个晶格位置的相连形状）对一个简单晶格模型进行了广泛的蒙特卡罗模拟。在很宽的密度范围内确定了纯流体以及所有二元混合物的状态方程，并且还在选定条件下研究了大量的多组分混合物。计算是在巨正则系综中进行的，类似于真实系统，其中分子或纳米颗粒从本体储库中可逆地吸附到表面或界面上。所研究的模型是无热的；这些模拟中的物体避免重叠，但除此之外不相互作用。因此，观察到的所有行为都是由熵驱动的。单组分流体在较高密度下都表现出明显的自排序趋势，在某些情况下会形成相当复杂的结构。在一些纯流体中还观察到具有相同旋转状态（取向）的物体有显著的聚集现象。在所有二元混合物中，两种组分在大尺度上完全互溶，但在小尺度上表现出强烈的物种特异性聚集（分离）。这种行为在多组分混合物中持续存在；即使在所有形状的七组分混合物中，相同形状的物体之间也存在显著的关联。为了更好地理解这些现象，我们计算了四连块的第二维里系数和相关量，从二元混合物的模拟中提取混合热力学体积数据，并确定每种形状在各种溶剂中的亨利定律溶解度。所得到的总体情况是，单个物体的形状以及不同流体的特征结构的互补性在决定给定组成流体的整体行为中都很重要，有时会产生违反直觉的结果。最后，我们注意到没有观察到尖锐的相变，但这似乎是由于所考虑物体的尺寸较小。在更大的多连块系统中可能会发现复杂的相行为。
