(231j) Molecular Fundamentals of the Solubility of Gases in Liquid Crystals

Liquid crystals are elongated and partially-rigid molecules which are able to form condensed phases with a certain degree of structural order. It has been observed that solubility of gases in liquid crystals decreases significantly when the fluid changes from the isotropic to a liquid crystal phase [1]. Furthermore, the transition between the isotropic and the nematic phase is associated with a very low enthalpy of transition (~1-10 kJ/mol) at temperatures close to ambient temperature [2]. A large solubility difference and a low enthalpy of transition make of liquid crystals an attractive candidate for solvents in gas separation applications. In this work, we disclose some of the molecular basis behind the solubility of gases in liquid crystals using molecular simulations techniques. Liquid crystal phases are identified by their degree of structural order, i.e. molecular orientation and position. In the isotropic or liquid phase, molecules are oriented and positioned randomly in space. In the nematic phase, molecules follow a preferred axis of orientation, however they are positioned randomly in space. In the smectic phase, molecules are distributed following both a preferred orientation and position, forming molecular layers in which molecules have a defined orientation. The isotropic and nematic phase are of special interest for the industrial application of liquid crystals. These phases remain largely fluid compared to the more viscous smectic phase. Therefore, we focus the study of solubility of gases in liquid crystals at the isotropic-nematic phase transition. Liquid crystal molecules have a typical structure formed by a rigid core with flexible terminal substituents at its ends. We model liquid crystal molecules as chain molecules formed by a rigid and a fully-flexible part, made of either hard-sphere or Lennard- Jones beads. Gas molecules are modeled as a single bead molecule. Hard-sphere molecules are used to understand the influence of free-volume in solubility while Lennard-Jones molecules are used to understand the role of intermolecular interactions. Monte Carlo simulations are performed in the NPT and in an Expanded-Gibbs ensemble to identify the phase behavior and phase equilibria of pure liquid crystals and mixtures of them. Solubility is expressed in terms of Henry's Law and is determined using the Widom test-particle insertion method. In hard systems solubility is determined solely by free-volume effects. We show that the solubility difference at the isotropic-nematic phase transition for hard systems is determined only by the packing fraction difference at the phase transition [3]. Moreover, for linear hard-sphere chains, a maximum in the free-volume difference at the isotropic-nematic phase transition is observed as a function of molecular length, indicating that an optimal chain length exists that maximizes the solubility difference between the isotropic and nematic phase. The effect of molecular flexibility is studied for chain molecules with constant total number of segments but with an increasing number of segments in the fully-flexible part. The effect of flexibility is reflected in a lower solubility difference, between the isotropic and the nematic phase. Molecular flexibility also destabilizes the nematic phase favoring a direct transition from the isotropic to the smectic phase. The behavior of mixtures and the relative importance of long-range interactions compared to free-volume effects is being studied at the moment. Results will be presented in the main part of this contribution.

References

[1]  M. de Groen, T.J.H. Vlugt, T.W. de Loos, J. Phys. Chem. B, 2012, 116, 9101-9106.

[2]  W.E. Acree, J.S. Chickos, J. Phys. Chem. Ref. Data, 2006, 35, 1051-1330.

[3]  B. Oyarzún, T. van Westen, T.J.H. Vlugt, J. Chem. Phys., 2013, 138, 204905.