Isotropic-polar phase transitions in an amphiphilic fluid: density functional theory versus computer simulations.
Isotropic-polar phase transitions in an amphiphilic fluid: density functional theory versus computer simulations.
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DOI：
10.1103/physreve.87.012313
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发表时间：
2013-01
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影响因子：
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通讯作者：
Stefano Giura;B. G. Márkus;S. Klapp;M. Schoen
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作者：
Stefano Giura;B. G. Márkus;S. Klapp;M. Schoen
We investigate the critical line separating isotropic from polar phases in an amphiphilic bulk fluid by means of density functional theory (DFT) and Monte Carlo (MC) simulations in the isothermal-isobaric ensemble.分子间相互作用由 Lennard-Jones 势来描述，其中吸引力的贡献由方向相关函数修改。后者由两项组成：第一项具有经典三维海森堡相互作用的方向依赖性，而第二项具有经典偶极-偶极相互作用的方向依赖性。然而，这两种贡献都是短期的。将 DFT 与方向相关对相关函数的修正平均场 (MMF) 近似结合使用，我们得出了分离各向同性与极性类液体相的临界线的解析表达式。在并行MC模拟中，我们通过分析极阶参数的Binder二阶累积量来定位临界点线。与 DFT 的比较表明，类偶极子的贡献与各向同性-极性相变无关。就海森堡贡献而言，MC 数据与 DFT 预测半定量一致，表明分子取向之间存在足够强的耦合。对于较弱的耦合，MMF 处理低估了临界密度与温度之比 ρ(c)/T(c) 随海森堡耦合常数 ε(H) 的变化。 MC 结果表明，这是因为 ρ(c) 随着 ε(H) 的减小而增大，使得 MMF 方法所依据的假设在弱耦合极限中变得不太适用。
We investigate the critical line separating isotropic from polar phases in an amphiphilic bulk fluid by means of density functional theory (DFT) and Monte Carlo (MC) simulations in the isothermal-isobaric ensemble. The intermolecular interactions are described by a Lennard-Jones potential in which the attractive contribution is modified by an orientation-dependent function. The latter consists of two terms: The first one has the orientation dependence of a classical three-dimensional Heisenberg interaction, whereas, the second one has the orientation dependence of a classical dipole-dipole interaction. However, both contributions are short range. Employing DFT together with a modified mean-field (MMF) approximation for the orientation-dependent pair correlation function, we derive an analytical expression for the critical line separating isotropic from polar liquidlike phases. In parallel MC simulations, we locate the line of critical points through an analysis of Binder's second-order cumulant of the polar-order parameter. Comparison with DFT shows that the dipolelike contribution is irrelevant for the isotropic-polar phase transition. As far as the Heisenberg contribution is concerned, the MC data are in semiquantitative agreement with the DFT predictions for sufficiently strong coupling between molecular orientations. For weaker coupling, the variation in the ratio of critical density and temperature ρ(c)/T(c) with the Heisenberg coupling constant ε(H) is underestimated by the MMF treatment. The MC results suggest that this is because ρ(c) increases with decreasing ε(H) such that the assumption on which the MMF approach rests becomes less applicable in the weaker-coupling limit.