(147ak) Understanding Chromatographic Retention and Fluid Phase/Sorption Equilibria By Molecular Simulations and Thermodynamic Modeling

My research focuses on understanding less energy-intensive separation processes, such as chromatography and adsorption, by molecular simulations to provide molecular-level understanding. The phase behavior of associating fluids and ionic liquids is also part of my interests since constructing more accurate phase diagrams is instrumental for the design of efficient separation processes.

Retention mechanism in chromatography

Supercritical fluid chromatography (SFC) is a resurgent mode of chromatography because of, compared to other modes of liquid chromatography, its applicability to a wider range of solute polarities, facile solvent recovery, and faster analysis speed due to lower viscosity that combine to make SFC more sustainable. Gibbs ensemble Monte Carlo simulations with advanced sampling techniques, such as configurational bias Monte Carlo and aggregation-volume-bias Monte Carlo, have been carried out to tackle the retention thermodynamics of linear analytes and aromatic compounds. The simulations capture the temperature, pressure, and mobile-phase-composition effects on retention. By decoupling the retention into stationary phase and mobile phase contributions, we can distinguish which phase is more relevant to the retention behavior.

Gas adsorption for pore characterization

The characterization of stationary phase materials (SPM) relies on adsorption isotherms because for the estimation of accessible pore volume, surface area, and surface chemistry. Several SPMs ranging from a very hydrophobic C18 phase to a very hydrophilic amorphous silica phase have been examined by gas adsorption with argon, nitrogen, n-butane, CO_2, and water as the adsorbates. The simulated isotherms can be further utilized as the input for the non-local density functional theory (NLDFT) to obtain the pore size distributions of the model SPMs and to provide guidance for further development of NLDFT.

Refinement of the COSMO-SAC model

The COSMO-SAC model determines the activity coefficients in mixtures based on electronic structure calculations. Many efforts have been made to describe the hydrogen bonding (HB) interactions more accurately. The directionality of HB interactions for proton acceptors have been considered based on the Molecular Electrostatic Potential (MESP) map. The results have shown improvement in vapor–liquid, liquid–liquid equilibria, infinite dilution activity coefficient, and octanol–water partition coefficient.

Activity coefficient modelling of ionic liquids

To model charged species such as ionic liquids, the Pitzer–Debye–Hückel (PDH) model is commonly used to describe the long-range interactions. However, the original version of PDH excludes the ionic species from the solvent. This may not be appropriate at high concentration of ionic species. Consequently, an extension of the PDH model has been proposed to consider the dielectric response of ionic liquids at high concentrations.