Elucidation of adsorption mechanisms of solvent molecules with distinct functional groups on amylose tris(3,5-dimethylphenylcarbamate)-based sorbent

Abstract

Although polysaccharide derivative-based sorbents have been widely used for chiral separation, for a long time it remained unclear how these CSPs interact with the molecules associated with different functional groups. In this study, six molecules were chosen for retention behavior studies: acetone (AC), tetrahydrofuran (THF), methanol (MET), isopropanol (IPA), tert-butanol (TBA), and benzene (BZN). An immobilized amylose carbamate stationary phase, amylose tris(3,5-dimethylphenylcarbamate)-based sorbent, or Chiralpak IA, was used. Van’t Hoff plots of ln k versus 1/T showed that alcohol molecules may simultaneously form two H-bonds with the IA sorbent. The results of density functional theory simulations and IR spectra support this inference showing that alcohol may bind with amide groups in three possible configurations. Frontal tests of AC and IPA were performed to estimate adsorbed solute concentration. Langmuir isotherm for IPA adsorption and mass action model for IPA self-aggregation were used for analyzing the IPA frontal results. Average IPA aggregation numbers range from 1.4 to 2.3. More than fifty percent of IPA molecules were found to be in aggregate form. From the frontal test results, thermodynamic properties of the adsorptions were determined. Retention behaviors of the five solutes as a function of IPA concentration were investigated. The absolute values B of the slopes from plots of the logarithms of the retention factor versus the logarithms of the IPA concentration increase in the order THF<AC<TBA<MET. U-shaped retention curves were observed for BZN, indicating that π-interactions of BZN aromatic group with the IA sorbent may involve both electrostatic and hydrophobic interactions. For THF, the slightly concave upward retention curves also suggested potential hydrophobic interactions between THF and IA. A monovalent retention model was used to estimate equilibrium constants (KSL-A) of solute-IPA complexation. The retention factors of the solutes with larger KSL-A values increase more rapidly with a decrease in the IPA concentration. Hence, a combination of the thermodynamic analysis and retention model has been shown to be reliable approach for elucidating the adsorption mechanisms of monovalent molecule containing different functional groups.