The occurrence of azeotropes poses great challenges for separation processes based on vapor-liquid equilibria (VLE), such as distillation, where the liquid and vapor phases have identical compositions. Despite the abundance of experimental data of many azeotropic systems, understanding into the molecular mechanism is lacking which is impeding the design of more efficient separation processes. In this study, we apply the united atom transferable potentials for phase equilibria (TraPPE-UA) force-field to model the polar-nonpolar and polar-polar positive azeotropic mixtures. Results of our Gibbs ensemble Monte Carlo simulation well reproduce the experimental vapor-liquid equilibrium diagrams. The molecular origin for azeotrope formation is investigated through the cohesive energies between molecules. A comprehensive framework is proposed based on the different molecular arrangement patterns between the two components at different concentration regimes. It is shown that changing molecular arrangement is responsible for the drastic change in the relative volatilities of the components and thus the azeotrope. The framework is then tested with a detailed liquid structure analysis.
