Retention models and interaction mechanisms of benzene and other aromatic molecules with an amylose-based sorbent.

Stoichiometric displacement models have been widely used for understanding the adsorption mechanisms of solutes in chromatography systems. Such models are used for interpreting plots of solute retention factor versus concentrations of polar modifier in an inert solvent. However, these models often assume that dispersion forces are negligible and they are unable to account for solutes with significant aromatic interactions. In this study, a systematic investigation of the relationship between retention behavior and aromatic groups was performed using five simple aromatic molecules-benzene, naphthalene, mesitylene, durene, and toluene-with a commercially available amylose tris(3,5-dimethylphenylcarbamate)-based sorbent. The enthalpy changes of adsorption, determined from van't Hoff plots, were obtained separately in pure n-hexane and in pure isopropanol (IPA). In pure n-hexane, the solute adsorptions were driven by electrostatic interactions, favoring a T-shaped binding configuration (edge-to-face π-π interaction). The order of enthalpy change indicated the amount of effective T-shaped π-interactions. In pure IPA, solute adsorption was dominated by dispersion forces, favoring a sandwich binding configuration (face-to-face π-π interaction). The adsorption isotherms of toluene revealed that in pure IPA and in pure n-hexane, the isotherms were linear. The results suggested that the high solvent strength of IPA weakened the interactions between aromatic molecules. The retention behavior of the benzene, naphthalene, mesitylene, and durene as a function of IPA concentration was investigated. U-shaped retention curves were found for all aromatic solutes. A new retention model for monovalent aromatic solutes was developed for describing the U-shaped curves. Three key dimensionless groups were revealed to control the retention behavior. The models suggested that solvophobic interactions should be accounted for in the retention models used to investigate the retention behaviors of solutes associated with aromatic groups.

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

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