【作者】 李丽妍；

【导师】 蔡钧；

【作者基本信息】 华东理工大学 ， 物理化学， 2014， 博士

【摘要】 本文通过巨正则系综下的Monte Carlo模拟研究了具有方阱势能的流体的相平衡性质和临界非对称性。通过完整标度下的Q参数方法结合Histogram reweighting技术以及有限尺寸标度分析预测了阱宽为=1.5,链长分别为2,4,8,16的方阱链的临界温度值和临界密度值,临界温度比不完整标度理论下得到的结果要低。并根据Flory-Huggins理论进行外推获得了无限长的方阱链的临界温度。采用完整标度下Q参数方法重新模拟了阱宽分别为λ=1.25,1.75,2.0的方阱单体的临界温度和临界密度,并采用近临界标度算法获得了在近临界区域的气液相平衡曲线,分析了不同阱宽方阱流体的临界非对称性质,结果表明,阱宽越小,系统的临界非对称性越强。通过对共存曲线直径拟合发现,阱宽越小,|t|2β项与|t|1-α项的系数比越大。说明相互作用范围越小,|t|2β项的贡献越重要,与已有分析一致。本文还采用该标度算法还预测了阱宽为=1.5的方阱二聚体和链长为4的方阱链的近临界相平衡性质以及临界非对称性。通过考察共存曲线直径的临界奇异性发现,方阱二聚体的共存曲线直径|t|2β项的作用比方阱单体更明显,说明分子体积越大,|t|2β项越占主导地位。但是链长为4的方阱链的共存曲线直径中|t|2β项的作用比方阱二聚体要弱,而是|t|1-α项占主导地位。可能的原因是链长为4的方阱链分子团在一起,缩小了分子体积,而不是以伸展的形式存在。因此,我们认为除了分子相互作用范围和分子体积之外,链状分子的分子构型也会对系统的临界非对称性产生影响。我们还利用Hyper parallel tempering MC模拟获得了方阱二聚体远离临界区域的相平衡曲线,其中共存密度由密度概率分布双峰等面积准则来确定。获得的共存曲线与已有模拟结果相吻合。最终给出了方阱二聚体完整的气液相平衡曲线。更多还原

【Abstract】 The vapor-liquid phase equilibrium properties and critical asymmetry of fluids with Square-Well potential were investigated by grand-canonical ensemble Monte Carlo simulation based on complete scaling theory. With the help of histogram reweighting technique and Finite-Size Scaling theory, we obtained the critical temperatures and critical densities of Square-Well chain molecules with λ=1.5and2,4,8,16segments. The values of critical temperatures are lower than that presented before based on incompelete scaling theory. And the critical temperature for an infinitely long Square-Well chain was estimated.The critical properties of Square-Well fluids with λ=1.25,1.75,2.0were reexamined using the Q-parameter method. The vapor-liquid coexistence curve in near-critical region for Square-Well monomers with λ=1.25,1.75,2.0were obtained by a scaling algorithms for precise simulation of fluid coexistence close to criticality. The results implied that the system with a smaller value of λ showed stronger critical asymmetry. Analyzing the coexistence diameter by fitting to a scaling relation made us concluded that the radio of coefficients in terms|t|2β and|t|1-α in Square-Well fluid with1=1.25was the largest, which means the smaller of inaction range, the more important in|t|2β term to coexistence diameter. This fact is in agreement with the conclusion in other papers.The vapor-liquid phase equilibrium properties and critical asymmetry of Square-Well dimer and tetramer with λ=1.5were studied by the same scaling algorithms. It showed that the term of|t|2β contributed more to coexistence diameter for Square-Well dimer than that for Square-Well monomer. That means fluids with a bigger molecule size had a much more dominating effect to the singular of coexistence diameter. While analyzing the results for Square-Well tetramer, we found that the domination of|t|2β term to the coexistence diameter was smaller for Square-Well tetramer fluid than that for Square-Well dimer fluid, and|t|1-α term dominate the singular behavior of coexistence diameter for Square-Well tetramer. The reason may be that the Square-Well tetramer molecule existed not in the form of a linear chain but in the form of a curly chain. Therefore, except the interacting range and molecule size, the molecule configuration of chain fluids was expected to impact the critical asymmetry. We also performed hyper parallel tempering Monte Carlo simulations for Square-Well dimer to predict coexistence properties far from critical point. The coexistence vapor and liquid densities were located by equal-area criterion for the double-peak of density probability distribution. The results are in good agreement with previous ones. Finally, the complete vapor-liquid coexistence curve of Square-Well dimer was showed.更多还原