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

Thermodynamic analysis of adsorption and retention behaviors in normal-phase liquid chromatography

This study investigated the adsorption mechanisms in a normal-phase system using a cyano-based stationary phase as the sorbent. The minor disturbance method was used to measure the adsorption isotherms of acetone and alcohols with various structures. Excluding data in pure n-hexane revealed that the adsorption behaviors on cyano sites were well described by the Langmuir model. The adsorption equilibrium constants, ranging from 8.86 to 11.15 at 25 °C, showed no significant differences across alcohol structures and decreased with increasing temperature. The saturation adsorption concentration decreased with increasing alcohol molecule size, with branched-chain alcohols showing a lower saturation adsorption amount compared to straight-chain alcohols. The standard state adsorption enthalpies and entropies calculated from the equilibrium constants for various alcohols ranged from -29 to -22 kJ/mol and -78 to -55 J/K·mol, respectively, showing enthalpy-entropy compensation. A discrepancy was observed between these adsorption enthalpies and those obtained from the retention factors of alcohols using pure n-hexane as the mobile phase. This discrepancy may result from the affinity energy distribution of the adsorbent. In pure n-hexane, the adsorption behaviors of adsorbates were considerably affected by high-affinity sites. Moreover, acetone and these alcohol molecules were used as solvent modifiers to investigate the relationship between the retention factor, modifier concentration, and temperature for various solutes with distinct functional groups. The retention curves were converted to enthalpic curves using the van't Hoff equation. A theoretical model was proposed to describe the relationship between the van't Hoff enthalpy change and mobile phase composition. The proposed model effectively described the enthalpic curves, indicating that the enthalpy change follows a saturation curve with increasing modifier concentration. This trend is primarily due to competitive adsorption and complexation behaviors between the solute and modifier molecules.

Effect of mobile-phase modifiers on the enantioselective retention behavior of methyl mandelate with an amylose 3,5-dimethylphenylcarbamate chiral stationary phase under reversed-phase conditions

In this study, methanol, ethanol, n-propyl alcohol, isopropyl alcohol, acetone, and tert-butanol were used as organic modifiers in reversed-phase mode chiral liquid-chromatography to systematically investigate the effects of mobile phase components on the enantioselective retention behavior of methyl mandelate with immobilized amylose 3,5-dimethylphenylcarbamate-based sorbent called Chiralpak IA. A two-site enantioselective model was used to obtain information on the recognition mechanisms by observing the dependence of the enantioselectivity and retention factor difference on the modifier content. Similar enantioselective retention behaviors were observed for all modifiers, and characteristic modifier concentration points (P_L , P_M , and P_H ) were identified. At modifier concentrations up to P_M , the weakened hydrophobic environment resulted in polymer structural relaxation, which changed the recognition mechanisms. By contrast, at concentrations beyond P_H , considerably different enantioselectivity behaviors were observed, indicating that the existence of dipole-dipole interaction, which was stronger at higher modifier concentrations, contributed to the retention mechanisms. The concentrations at which these characteristic points occurred were dependent on the carbon number of the modifier molecule. Modifiers with more carbon numbers facilitated the transition in the enantioselective behaviors. These results demonstrated that the proposed method can provide a physically consistent quantitative description of enantioselective retention behavior in reversed-phase mode.

Retention modeling and adsorption mechanisms in reversed-phase liquid chromatography

To interpret the dependence of solute retention behavior on modifier content in reversed-phase liquid chromatography, a theoretical framework, based on the concentration dependence of solvophobic forces imposed on solutes and the competitive adsorptions of solutes and solvent modifiers, was proposed. The generality of the developed model was demonstrated by comparing the model with conventional retention models. The linear dependence of the Gibbs energy change of solute adsorption with respect to the modifier concentration was assumed, and the model was fitted to the experimental results, with good agreement demonstrated between the experimental data and the model. Retention behaviors were inferred to be determined by two key dimensionless groups that represented the reductions in the retention factors resulting from a weakened solvophobic interaction and modifier competitive adsorption. The retention behaviors were successfully deconvoluted for each contribution as a function of the modifier concentration by using the fitted parameters. The effects of both contributions on the retention behaviors were enhanced for the solutes with aromatic groups. The standard Gibbs energy change _SL^o of benzene adsorption was found to depend linearly on the number of modifier molecules present but independent of modifier identity. For the solutes associated with hydrogen-bonding groups, the degree of reduction in the solvophobic interactions was considerably reduced. Hence, the relative contributions of both mechanisms to solute retention depend greatly on the solute structure. Perturbation method was performed to investigate the modifier adsorption mechanisms. The results show that the standard Gibbs energy change _SL^o for the first-layer adsorption of modifiers changed linearly with the carbon number of modifier molecule. These results demonstrated that the proposed model can offer a physically consistent quantitative description of retention when solvent composition is varied.

Effect of 2-propanol content on solute retention mechanisms determined using amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase under normal- and reversed-phase conditions

The electrostatic interactions between chiral solutes and polysaccharide (PS)-based chiral selectors are the key to achieving chiral recognition; however, PS-based sorbents, derivatized of phenyl moieties, can exhibit considerably non-polar characteristics, and they are also useful for the separation of enantiomers in the reversed-phase mode. In this study, an immobilized amylose 3,5-dimethylphenylcarbamate-based sorbent was used to investigate the balance between electrostatic interactions and solvophobic interactions, with complementary effects on solute retention behavior when the isopropanol (IPA) concentration was altered. It was proposed that in both normal- and reversed-phase modes, information on the retention mechanisms could be obtained by observing the curvature of the logarithm of the retention factor versus the logarithm of the IPA concentration, and the slope values of the curves were related to the number of displaced IPA molecules upon solute adsorption. Using the proposed model and the two-site adsorption model, the retention behaviors of pantolactone (PL) enantiomers in both normal- and reversed-phase modes were investigated. The PL-sorbent interactions were classified into four types: electrostatic/enantioselective, electrostatic/nonselective, solvophobic/enantioselective, and solvophobic/nonselective. At IPA concentrations below 50 vol.% in n-hexane, the retention behaviors of PL were dominated by electrostatic/enantioselective sites, whereas at IPA concentrations beyond 50 vol.%, the solvophobic interactions of PL-sorbent were strengthened and mostly nonselective. By contrast, in the reversed-phase mode, a reverse in the enantiomeric elution order of PL was observed at 10 vol.% IPA, and considerably different enantioselectivity behaviors were found below and above 20 vol.%, indicating an abrupt change in the sorbent molecular environment. At IPA concentrations beyond 40 vol.%, the presence of PL-sorbent electrostatic interactions enhanced chiral recognition.

How mobile phase composition and column temperature affect enantiomer elution order of liquid crystals on amylose tris(3-chloro-5-methylphenylcarbamate) as chiral selector

A comprehensive study into the effects of mobile phase composition and column temperature on enantiomer elution order was conducted with a set of chiral rod-like liquid crystalline materials. The analytes were structurally similar and comprised variances such as length of terminal alkyl chain, presence of chlorine, number of phenyl rings, and type of chiral center. Experiments were carried out in polar organic and reversed-phase modes using amylose tris(3-chloro-5-methylphenylcarbamate) immobilized on silica gel as the chiral stationary phase. For all liquid crystals, reversal of elution order of enantiomers was observed based on type of used cosolvent and/or its content in the mobile phase; for some of the liquid crystals a temperature-induced reversal was also observed. Both linear and nonlinear dependencies of natural logarithm of enantioselectivity on temperature were found. Tested mobile phases comprised pure organic solvents and binary and tertiary mixtures of acetonitrile with organic solvents and/or water. Effect of acidic/basic mobile phase additives was also tested. Effect of structure of chiral selector is briefly discussed.

Effect of solvents on the chiral recognition mechanisms of immobilized cellulose-based chiral stationary phase

The effect of solvents on the enantioselectivities of four structurally similar chiral solutes with a cellulose derivative-based chiral stationary phase, Chiralpak IB, were studied using acetone (AC), 2-propanol (IPA), and tert-butanol (TBA) separately as polar modifiers. The enantioselectivities α of benzoin and methyl mandelate decrease with an increase in modifier concentration C_M, whereas the enantioselectivity of pantolactone increased with increasing AC concentration. These results were attributed to the heterogeneous adsorption mechanisms of enantiomers. To interpret the dependence of enantioselectivity on modifier content, an enantioselectivity model based on a two-site adsorption model was proposed. The dependence of α on C_M was inferred to be mainly due to the distinct modulating effects of modifier concentration on the two adsorption sites: the nonselective type-I site and enantioselective type-II site. The model fitted the benzoin data satisfactorily over a wide TBA concentration range. The retention factors as a function of TBA concentration were successfully deconvoluted for each site. With the use of the proposed model, it was inferred that the chiral recognitions of benzoin and methyl mandelate were mainly achieved by the presence of an aromatic group adjacent to the hydroxyl group. When using IPA and TBA separately as modifiers, the presence of an aromatic group adjacent to the ketone group mainly contributed to the nonselective π interactions and enantioselective steric interactions, respectively. These results, along with those of the modifier adsorption isotherms, determined using the perturbation method, as well as the retention behaviors of various achiral solutes, indicate that the molecular recognition mechanism of IB sorbent is highly sensitive to the adsorbate's molecular geometry. The molecular environment of the sorbent can be controlled using different modifiers, leading to distinct adsorption and retention mechanisms.