Department of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.
J Colloid Interface Sci. 2012 Jul 1;377(1):94-104. doi: 10.1016/j.jcis.2012.03.046. Epub 2012 Mar 28.
The detailed study of the isotropic-nematic phase transition in a system of discotic particles of aspect ratios L/D≤0.1 presented here is relevant to a broad range of colloidal suspensions of chemically modified clay particles. Using Monte Carlo simulation techniques the equation of state, radial distribution functions, structure factors and normalized scattering intensities are calculated for each phase. The results are interpreted and related to previously reported free energy calculations [Fartaria and Sweatman, Chem. Phys. Lett. 478 (2009) 150], suggesting a nearly continuous isotropic-nematic transition for lower aspect ratios. Given this behavior we examined the structural information for each phase to determine how experimental scattering data might be used to distinguish the two phases. The radial distribution functions in each phase depend strongly on aspect ratio, and for larger aspect ratios a dramatic increase in the local ordering of discotic particles (represented here as cut-spheres) is observed just before the phase transition. However, this nearest-neighbor ordering seen in g(r) around r/D=0.1 would hardly be discernible in experimental scattering data subject to usual statistical errors. The structure factors and scattering intensities were calculated for L/D=0.1, 0.04 and 0.01 for the isotropic and nematic phases at and away from the isotropic-nematic transition. While the isotropic-nematic phase transition can be detected from the height and shape of the first scattering peak around 7QD for larger aspect ratios, this feature becomes much less discriminatory with decreasing aspect ratio. Instead, scattering intensities at low scattering vector amplitudes (Q→0) can be used for detection of the phase transition at low aspect ratios. These results provide useful insight to guide interpretation of X-ray and light scattering measurements for colloidal dispersions of thin platelets undergoing isotropic-nematic transitions.
本文详细研究了长宽比 L/D≤0.1 的盘状颗粒系统中的各向同性-向列相转变，这与经过化学修饰的粘土颗粒的广泛胶体悬浮液有关。使用蒙特卡罗模拟技术，计算了各相的状态方程、径向分布函数、结构因子和归一化散射强度。结果进行了解释，并与之前报道的自由能计算结果[Fartaria 和 Sweatman，Chem. Phys. Lett. 478（2009）150]相关联，表明对于较低的长宽比，几乎存在连续的各向同性-向列相转变。鉴于这种行为，我们检查了各相的结构信息，以确定实验散射数据如何用于区分这两个相。各相中的径向分布函数强烈依赖于长宽比，对于较大的长宽比，在相变之前观察到盘状颗粒（此处表示为截球体）的局部有序性急剧增加。然而，在实验散射数据中，围绕 r/D=0.1 的 g(r) 中看到的这种最近邻有序性几乎无法通过通常的统计误差来识别。计算了各向同性和向列相在 L/D=0.1、0.04 和 0.01 时的结构因子和散射强度，在各向同性-向列相变处和远离各向同性-向列相变处。虽然对于较大的长宽比，可以从大约 7QD 的第一散射峰的高度和形状检测到各向同性-向列相变，但随着长宽比的减小，该特征变得不那么具有区分性。相反，在低散射矢量幅度（Q→0）处的散射强度可用于检测低长宽比下的相变。这些结果为指导薄片状胶体分散体经历各向同性-向列转变的 X 射线和光散射测量的解释提供了有用的见解。
Zotero 插件