Molecular association of heteronuclear vibrating square-well dumbbells in liquid-vapor phase equilibrium

摘要

Molecular aggregates are formed by heteronuclear vibrating square-well dumbbells. In a recent article [G. A. Chapela and J. Alejandre, J. Chem. Phys., 132(10), 104704 (2010)], it is shown that heteronuclear vibrating square-well dumbbells with a diameter ratio between particles of 1/2 and interacting potential ratio of 4 form micelles of different sizes and shapes which manifest themselves in both the liquid and vapor phases, up to and above the critical point. This means that micellization and phase separation are present simultaneously in this simple model. These systems present a maximum in the critical temperature when plotted against the potential well depth of the second particle ε2. In the same publication, it was speculated that the formation of micelles was responsible for the appearance of the maximum. A thorough study on this phenomena is presented here and it is found that there is a threshold on the size of the second particle and its corresponding depth of interaction potential, where the micelles are formed. If the diameter and well depth of the second particle are small enough for the first and deep enough for the second, micelles are formed. For σ2/σ1 between 0.25 and 0.65 and ε2/ε1 larger than 5.7, micelles are formed up to and above the critical temperature. Outside these ranges micelles appear only at temperatures lower than the critical point. There is a strong temperature dependence on the formation and persistence of the aggregates. For the deepest wells and large enough second particles, a gel interconnected aggregate is obtained. In this work, the micelles are formed at temperatures as low as the triple point and as high as the critical point and, in some cases, persist well above it. The presence of these maxima in critical temperatures Tc when plotted against ε2 as follows. At lower values of ε2, an increase of Tc is obtained as is expected by the increase of the attractive volume as indicated by the principle of corresponding states. As ε2 increases further, the formation of molecular aggregates produce a saturation effect of the deepening of the potential well by encapsulating the particles of the second kind inside the micelles, so the resulting Tc represents a new poly disperse system of molecular aggregates and not the original heteronuclear vibrating square-well dumbbells. The surface tension is also analyzed for these systems, and it is shown that decreases with increasing attraction due to the formation of molecular aggregates. © 2011 American Institute of Physics Article Outline INTRODUCTION TECHNICAL DETAILS Vibrating square-well dumbbells and interaction potential Discontinuous molecular dynamics CALCULATED PROPERTIES Surface tension and vapor pressure Density profile and orthobaric densities Critical temperature and density Radial distribution function, cluster size, and radius of gyration RESULTS Maxima in Tc* vs ε2* for σ2* constant Maxima in Tc* vs σ2* for ε2* = 1 Triple-point crystal and the existence of gel-like structures Finite-size effects CONCLUDING REMARKS