(100a) Effects of Alcohol Modifier Aggregation and Complexation On the Retention Factors of Chiral Solutes With An Amylose-Based Sorbent

Various stoichiometric displacement models in the literature have been widely used for understanding the competitive adsorption mechanisms of solutes and the polar modifiers of the mobile phase. The models were used to explain the often-observed linear log-log plots of the solute retention factor k versus the concentration C_I⁰   of the modifier. The slopes of the plots were sometimes inferred to be equal to the number of the displaced modifier molecules upon adsorption of one solute molecule, or upon adsorption of the solute-modifier complex, and were generally found to be greater than 1. In this study, the retention factors and enantioselectivities of ethyl lactate (EL) and pantolactone (PL) enantiomer pairs were measured for the amylose tris[(S)-α-methylbenzylcarbamate] sorbent, or AS, with isopropanol (IPA) in n-hexane at 25 ˚C. The slopes range from less than 1, from 0.43 to 0.76,  at C_I⁰ = 0.13 to 1.3 M, to slightly more than 1 (from 1.0 to 1.25) at the higher concentrations used (6 to 7 M). The literature models cannot account for such slopes. To understand such slopes, five achiral monovalent solutes were chosen and studied in detail; see Tsui, Franses, and Wang, J. Chromatography A, 1279, 36-48 (2013). Infrared Spectroscopy (IR) and Density Functional Theory (DFT) simulations provided indications that there is significant IPA aggregation with an average aggregation number of n=3. A new retention model for the monovalent solutes has been developed, to take into account the IPA aggregation and the solute-IPA complexation. The small slopes can only be explained if the alcohol forms aggregates in solution. The model and the HPLC data for acetone show that the limiting slopes at high IPA concentrations approach the value of =0.67. The data for acetone were fitted to the model to determine the IPA aggregation number and equilibrium constant. More complex multivalent models have been developed for chiral molecules, accounting for multivalent solute adsorption, with an average  number of binding sites x,  multivalent solute-alcohol complexation with number of complexation sites y, solute intra hydrogen-bonding, monovalent alcohol adsorption, and, most importantly, alcohol aggregation in the mobile phase. The chromatographic data of EL and PL enantiomers were fitted to the model, to determine the retention factors at zero IPA concentration (for PL only, because for EL k was measured directly), the equilibrium constants of complexation and intra hydrogen bond formation K_intra, and the values of x and y. The equilibrium constant K_intra was higher for EL than for PL, as indicated by IR spectra, apparently  because the EL molecule is more flexible. The values of y were the same for each pair of EL enantiomers, y=2.9, and for each pair of PL enantiomers, y=2.7. Hence there are about three binding sites, on average, for solute/IPA complexation. The value of x was higher for PL, 1.8, than for EL, 1.3. The limiting slopes, at the highest IPA concentrations used approach the theoretically expected values of (x+y)/n=1.4 for EL and 1.5 for PL The equilibrium constant for PL adsorption was also higher than that of EL. These results can account for the much higher retention factors measured for PL than for EL, and help explain the solute-IPA and the solute-sorbent interaction mechanisms. The method used here has the potential for estimating the average number of binding sites for any solute in a mobile phase containing a polar modifier.