(694c) Retention Models and Interaction Mechanisms of Acetone and Related Molecules with Amylose Tris[(S)-α-Methylbenzylcarbamate] Sorbent

This polymeric sorbent is capable of separating chiral enantiomer solutes, such as benzoin.* HPLC results show a substantial effect of the solvent or mobile phase composition on the retention factors k_R and k_S and the enantioselectivity S= k_R/ k_S. A typical solvent is n-hexane containing isopropanol (IPA) with IPA volume fractions ϕ=0.01 to ϕ=0.1. The retention factor dependence on ϕ follows the empirical relationship ln (k) =A-B ln ϕ (1), where the intercept A and the slope B are dimensionless constants. Values of B range from 0.4 to 0.8 for benzoin and related chiral solutes. These slopes are related to solute-sorbent, IPA-sorbent, and IPA-solute interactions. Because such interactions are quite complex for chiral solutes, several simple non-chiral solutes, such as acetone and phenylacetaldehyde, each containing one C=O functional group which can bind with the NH group of the AS sorbent, were examined. The retention factors for these solutes also follow Eq. (1), with slopes B ranging from 0.25 to about 0.4. Thermodynamic models reveal that the above three types of interactions alone cannot lead to slopes B below 1.0. Chromatography simulations, which take into account competitive adsorption of solute and IPA and IPA-solute binding, support this conclusion. Our hypothesis for explaining values of the slopes B below 1 involves accounting also for substantial IPA aggregation in solution. This phenomenon leads to a reduction in the IPA thermodynamic activity, which affects IPA adsorption and IPA-solute interactions. Chromatography simulations with added IPA aggregation support this hypothesis, which is confirmed further by Infrared Spectroscopy (IR) data and Density Functional Theory (DFT) simulations of the IPA-IPA and IPA-solute interactions. The results apply also to the data of the chiral solutes.

 * H.-W. Tsui, J.N. Willing, R.B. Kasat, N.-H.L. Wang, E.I. Franses, J. Phys. Chem. B, 2011, 115, 12785-12800.