Schoen Martin, Giura Stefano, Klapp Sabine H L
Stranski-Laboratorium für Physikalische und Theoretische Chemie, Fakultät für Mathematik und Naturwissenschaften, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany and Department of Chemical and Biomolecular Engineering, Engineering Building I, Box 7905, North Carolina State University, 911 Partners Way, Raleigh, North Carolina 27695, USA.
Stranski-Laboratorium für Physikalische und Theoretische Chemie, Fakultät für Mathematik und Naturwissenschaften, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany.
Phys Rev E Stat Nonlin Soft Matter Phys. 2014 Jan;89(1):012310. doi: 10.1103/PhysRevE.89.012310. Epub 2014 Jan 31.
We invoke mean-field density functional theory (DFT) to investigate the phase behavior of an amphiphilic fluid composed of a hard-sphere core plus a superimposed anisometric Lennard-Jones perturbation. The orientation dependence of the interactions consists of a contribution analogous to the interaction potential between a pair of "spins" in the classical, three-dimensional Heisenberg fluid and another one reminiscent of the interaction between (electric or magnetic) point dipoles. At fixed orientation both contributions are short-range in nature decaying as r-6 (r being the separation between the centers of mass of a pair of amphiphiles). Based upon two mean-field-like approximations for the pair correlation function that differ in the degree of sophistication we derive expressions for the phase boundaries between various isotropic and polar phases that we solve numerically by the Newton-Raphson method. For sufficiently strong coupling between the Heisenberg "spins" both mean-field approximations generate three topologically different and generic types of phase diagrams that are observed in agreement with earlier work [see, for example, Tavares et al., Phys. Rev. E 52, 1915 (1995)]. Whereas the dipolar contribution alone is incapable of stabilizing polar phases on account of its short-range nature it is nevertheless important for details of the phase diagram such as location of the gas-isotropic liquid critical point, triple, and tricritical points. By tuning the dipolar coupling constant suitably one may, in fact, switch between topologically different phase diagrams. Employing also Monte Carlo simulations in the isothermal-isobaric ensemble the general topology of the DFT phase diagrams is confirmed.
我们运用平均场密度泛函理论（DFT）来研究一种两亲流体的相行为，该流体由硬球核心加上叠加的各向异性 Lennard-Jones 微扰组成。相互作用的取向依赖性包括类似于经典三维海森堡流体中一对“自旋”之间相互作用势的贡献，以及另一个让人联想到（电或磁）点偶极子之间相互作用的贡献。在固定取向下，这两种贡献本质上都是短程的，随 r⁻⁶ 衰减（r 是一对两亲分子质心之间的间距）。基于对成对关联函数的两种不同精细程度的类似平均场近似，我们推导了各种各向同性相和极性相之间相边界的表达式，并通过牛顿 - 拉夫逊方法进行数值求解。对于海森堡“自旋”之间足够强的耦合，两种平均场近似都产生了三种拓扑不同且通用类型的相图，这与早期工作一致[例如，见 Tavares 等人，《物理评论 E》52，1915（1995）]。虽然仅偶极贡献因其短程性质无法稳定极性相，但它对于相图的细节仍然很重要，例如气 - 各向同性液体临界点、三相点和三临界点的位置。通过适当地调整偶极耦合常数，实际上可以在拓扑不同的相图之间切换。在等温 - 等压系综中还进行了蒙特卡罗模拟，证实了 DFT 相图的一般拓扑结构。
