(525d) Phase Behavior of Binary Systems of Liquid Crystals with CO2

Rod-like liquid crystals (LCs) are compounds having a structured liquid phase, called either nematic (N) or smectic phase (Sm) depending on the degree of ordering. In the phase diagram, this structured liquid phase is located between the solid (S) and the usual isotropic liquid phase (I). The first order phase transition between the structured and isotropic liquid can be controlled by varying the temperature. CO₂ is less soluble in the structured liquid than in the isotropic liquid. Therefore, a liquid crystal may be used as a solubility switch: CO₂ can be dissolved in the isotropic liquid phase and by cooling the isotropic mixture a few degrees, the structured phase is formed and CO₂ is released again. The pressure is kept constant during this process. This CO₂ capture process has the potential to consume less energy than current processes. Nematic liquid crystals are most interesting for capturing CO₂ because the viscosity of nematic LCs is much lower than that of the smectic LCs.

Two parameters are of particular importance for the functioning of this solubility switch: the solubility of CO₂ in the isotropic liquid phase and the solubility difference between the structured- and isotropic liquid phase. The solubility of CO₂ in liquid crystals is determined by the chemical structure of the liquid crystal. Solubility predictions with the Predictive Soave-Redlich-Kwong (PSRK) equation of state indicate that the presence of ether or ester groups leads to a higher CO₂ solubility than for carboxylic acid and alcohol groups. The solubility difference between the structured and isotropic liquid phases is determined by the phase transition enthalpy. A larger transition enthalpy leads to a larger solubility difference. 

Phase diagrams (p,T) were measured for samples of pure liquid crystals and for mixtures of liquid crystals with up to 5% CO₂ in a closed environment using a Cailletet setup. The liquid crystals tested can be divided in three different classes: apolar liquid crystals, polar liquid crystals and weakly polar liquid crystals. Preliminary results confirm part of the predictions concerning the solubility in the isotropic liquid: the solubility of CO₂ is influenced by the polarity of the liquid crystal. The apolar liquid crystals have the lowest mass based solubility and weakly poloar molecules  the highest solubility. However, the presence of an ether group next to a benzene ring lead to a lower instead of higher solubility. A possible explanation for this behavior is a distorted quadrupolar moment of the benzene ring, leading to decreased affinity for CO₂.

The relative solubility difference depends on the enthalpy: the liquid crystal with the highest enthalpy shows a larger change in the pressure needed to keep the CO₂ dissolved in the two phases. Further research is needed to optimize these parameters.