Simulation Study of the Link between Molecular Association and Reentrant Miscibility for a Mixture of Molecules with Directional Interactions

Abstract

The reentrant liquid-liquid miscibility of a symmetrical mixture with highly directional bonding interactions is studied by Gibbs ensemble Monte Carlo simulation. The resulting closed loop of immiscibility and the corresponding lower critical solution temperature are shown to be a direct consequence of the dramatic increase in association between unlike components as the temperature is lowered. Our exact calculations for an off-lattice system with a well-defined anisotropic potential confirm the findings of previous theoretical studies. [S0031-9007(99) PACS numbers: 61.20.Qg, 05.20.Jj Reentrant phase behavior is one of the most characteristic and beguiling signatures of hydrogen bonding in simple and complex fluids alike. The so-called closedloop liquid-liquid immiscibility found in a wide variety of fluid mixtures is a common example of the relationship between hydrogen-bonding association and reentrant behavior: here the region of liquid-liquid immiscibility is bounded above and below by upper and lower critical solution temperatures (UCST and LCST) (see Refs. [1,2] for general reviews). Closed-loop immiscibility was first reported for the nicotine-water mixture at the turn of the 20th Century [3]. Other well-known examples of this type of reentrant behavior include aqueous solutions of butan-2-ol [4], alkylpyridines [5], and alkylpolyoxyethylene nonionic surfactants [6], as well as a number of polymer solutions [7,8]. The common feature of these systems is the possibility of hydrogen bonding between the two components of the mixture. An increase in temperature overcomes the unfavorable enthalpic contribution (incompatibility of the two components) leading to miscibility above the UCST. The low-temperature miscibility below the LCST can be explained in terms of hydrogen bonding between unlike components which becomes the dominant feature of the free energy of the system, and results in a favorable heat of solution. One should note that in the case of polymer solutions the principal feature responsible for the LCST is the large difference in size between the polymer molecule and the solvent.Hirshfelder et al. [9] were the first to suggest directional attractive forces as a direct cause of the LCST. This view is supported by the theoretical predictions of Barker and Fock [10], who analyzed a simple lattice model with interactions that depend on the relative orientations of the molecules using the Bethe-Guggenheim quasichemical approximation; an appropriate choice of parameters for the unlike directional interactions gives rise to closedloop behavior. A more rigorous treatment of similar lattice models, using appropriate decoration transformations to the equivalent Ising models of ferromagnets [11] or alternatively using the Migdal-Kadanoff approximations of renormalization group (RG) theory [12], has also been applied to study closed-loop immiscibility in systems with highly asymmetrical interactions (e.g., see Refs. [13][14][15][16][17][18]). The results of the more recent calculations are in reasonable q...

Phase behavior of a binary fluid mixture of quadrupolar molecules

“…Phase diagram exponents β ≃ 1/3 for the three-dimensional Ising universality class [44] are observed for both critical points (see Appendix C for details on the critical temperatures and exponents). Similar result has been reported in a previous work [12]. In our simulations, the magnitude of the quadrupole moment |Q| is an important parameter which controls the phase behavior.…”

“…Hence, understanding molecular details of the behavior of liquids with the anisotropic interactions is one of the central issues of materials science. Based on the above idea, previous studies have adopted a shortranged anisotropic interaction between different species into their models [10][11][12], with which they succeeded in reproducing the closed-loop phase diagram. As these existing studies have been carried out with on-lattice-based structures and/or spatially discretized anisotropic interactions, however, the shape and the statistical properties of the molecular complexes at temperatures below the LCST can be artifacts of the discretizations assumed in their models.…”

Re-entrant phase behavior for systems with competition between phase separation and self-assembly

“…Liquid-liquid immiscibility leading to such phase diagrams is often attributed to hydrogen bonding. 43,44 Complete mixing will occur if the temperature is sufficiently high so as to overcome the enthalpic incompatibility of mixing. However, at sufficiently low temperatures, miscibility results from hydrogen bonding between different components in the system.…”

Liquid–liquid phase separation of N-isopropylpropionamide aqueous solutions above the lower critical solution temperature

“…Computer simulations can provide molecular-level insight into phase transition behaviors, as demonstrated in the cases of water and aqueous solutions 29 30 31 32 ; however, to date there are few simulation studies that demonstrate heat-induced spontaneous phase separation of aqueous amphiphilic solutions and try to answer the question as to which factors, hydrogen bonds between unlike molecules or the hydrophobic effect, play a critical role in demixing upon heating 33 34 35 36 .…”