Pellicane G, Saija F, Caccamo C, Giaquinta P V
Dipartimento di Fisica, Università degli Studi di Messina, Contrada Papardo, 98166 Messina, Italy.
J Phys Chem B. 2006 Mar 9;110(9):4359-64. doi: 10.1021/jp056039d.
We studied the thermodynamic stability of fluid-fluid phase separation in binary nonadditive mixtures of hard-spheres for moderate size ratios. We are interested in elucidating the role played by small amounts of nonadditivity in determining the stability of fluid-fluid phase separation with respect to the fluid-solid phase transition. The demixing curves are built in the framework of the modified-hypernetted chain and of the Rogers-Young integral equation theories through the calculation of the Gibbs free energy. We also evaluated fluid-fluid phase equilibria within a first-order thermodynamic perturbation theory applied to an effective one-component potential obtained by integrating out the degrees of freedom of the small spheres. A qualitative agreement emerges between the two different approaches. We also addressed the determination of the freezing line by applying the first-order thermodynamic perturbation theory to the effective interaction between large spheres. Our results suggest that for intermediate size ratios a modest amount of nonadditivity, smaller than earlier thought, can be sufficient to drive the fluid-fluid critical point into the thermodinamically stable region of the phase diagram. These findings could be significant for rare-gas mixtures in extreme pressure and temperature conditions, where nonadditivity is expected to be rather small.
我们研究了中等尺寸比的硬球二元非加和混合物中流体-流体相分离的热力学稳定性。我们感兴趣的是阐明少量非加和性在决定流体-流体相分离相对于流体-固体相变的稳定性中所起的作用。通过计算吉布斯自由能，在修正超网链和罗杰斯-杨积分方程理论的框架内构建了分层曲线。我们还在一阶热力学微扰理论框架内评估了流体-流体相平衡，该理论应用于通过对小球自由度积分得到的有效单组分势。两种不同方法之间出现了定性的一致性。我们还通过将一阶热力学微扰理论应用于大球之间的有效相互作用来确定凝固线。我们的结果表明，对于中等尺寸比，比先前认为的更小的适量非加和性就足以将流体-流体临界点驱动到相图的热力学稳定区域。这些发现对于处于极端压力和温度条件下的稀有气体混合物可能具有重要意义，在这些条件下非加和性预计相当小。
