Rah Kyunil, Freed Karl F, Dudowicz Jacek, Douglas Jack F
The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA.
J Chem Phys. 2006 Apr 14;124(14):144906. doi: 10.1063/1.2181138.
Dynamic clustering associated with self-assembly in many complex fluids can qualitatively alter the shape of phase boundaries and produce large changes in the scale of critical fluctuations that are difficult to comprehend within the existing framework of theories of critical phenomena for nonassociating fluids. In order to elucidate the scattering and critical properties of associating fluids, we consider several models of equilibrium polymerization that describe widely occurring types of associating fluids at equilibrium and that exhibit the well defined cluster geometry of linear polymer chains. Specifically, a Flory-Huggins-type lattice theory is used, in conjunction with the random phase approximation, to compute the correlation length amplitude xi(o) and the Ginzburg number Gi corresponding, respectively, to the scale of composition fluctuations and to a parameter characterizing the temperature range over which Ising critical behavior is exhibited. Our calculations indicate that upon increasing the interparticle association energy, the polymer chains become increasingly long in the vicinity of the critical point, leading naturally to a more asymmetric phase boundary. This increase in the average degree of polymerization implies, in turn, a larger xi(o) and a drastically reduced width of the critical region (as measured by Gi). We thus obtain insight into the common appearance of asymmetric phase boundaries in a wide range of "complex" fluids and into the observation of apparent mean field critical behavior even rather close to the critical point.
在许多复杂流体中，与自组装相关的动态聚集能够定性地改变相界的形状，并在临界涨落尺度上产生巨大变化，而这在非缔合流体临界现象的现有理论框架内是难以理解的。为了阐明缔合流体的散射和临界性质，我们考虑了几种平衡聚合模型，这些模型描述了平衡状态下广泛存在的缔合流体类型，并展现出线性聚合物链明确的聚集几何结构。具体而言，我们使用弗洛里 - 哈金斯型晶格理论，并结合随机相位近似，来计算分别对应于成分涨落尺度和表征呈现伊辛临界行为的温度范围的一个参数的关联长度振幅ξ(0)和金兹堡数Gi。我们的计算表明，随着粒子间缔合能的增加，聚合物链在临界点附近变得越来越长，自然导致相界更加不对称。平均聚合度的这种增加反过来意味着更大的ξ(0)和临界区域宽度的大幅减小（由Gi衡量）。因此，我们深入了解了在广泛的“复杂”流体中不对称相界的普遍出现，以及即使在相当接近临界点处也能观察到明显的平均场临界行为的现象。
