Kinetic features of the adsorption of menthol enantiomers on o-toluylic acid and CsCuCl3 crystals with supramolecular chirality
Authors: Belonogov Eduard V. , Muslimov Idris N. , Zinoviev Ilya M. , Guskov Vladimir Yu. 
Published: July 2024

Chirality plays a key role in modern science because it is the distinguishing feature of molecules and crystals. The spontaneous emergence of chirality in the absence of detectable chiral physical and chemical sources has recently advanced significantly due to the deracemization of conglomerates through Viedma ripening. As a result, systems based on supramolecular chirality are obtained. Of particular importance to this type of chirality is the fact that supramolecular chirality underlies the formation of life on Earth. One manifestation of supramolecular chirality is enantiomorphic crystals.
Previously, we studied the mechanism of supramolecular chiral recognition for enantiomorphic crystals in the case of adsorption of optically active substances on them. However, for a more detailed study of the mechanism, it is required to study a large number of chiral crystals, which differ in their physicochemical properties. In this work, we studied the adsorption kinetics of menthol enantiomers on the surface of enantiomorphic crystals of o-toluic acid and CsCuCl3 with supramolecular chirality.
The Viedma ripening method was used to obtain homochiral crystals. The crystals obtained in this way were deposited on the surface of the ASKG silica gel. The kinetics of adsorption has been studied by describing chemical models of Lagergren's pseudo-first order, Ho and McKay's pseudo-second order, and the Elovich model, simplified by Chen and Clayton.
From an analysis of the adsorption curves of menthol enantiomers, it is noticeable that the curves differ on the crystals under consideration. The enantioselectivity coefficient on crystals of o-toluic acid and CsCuCl3 α, calculated as the ratio of higher adsorption to lower one, is 1.04-1.07 and 1.34-1.36 respectively. Using the t-test, the adsorption values of menthol enantiomers on enantiomorphic crystals of o-toluic acid and CsCuCl3 were processed until equilibrium was reached. The variances of all experimental data were checked for homogeneity using the F-test. It follows from the obtained data that the difference in adsorption values is statistically significant for the sample modified with both o-toluic acid and CsCuCl3.
Based on the results obtained in the process of modeling the adsorption kinetics of menthol enantiomers by chemical models, the adsorption rate constants were calculated. It was found that in all methods the rate constants of D-menthol and L-menthol are different, and D-menthol is adsorbed faster than L-. From the data obtained, it is noticeable that the adsorption equilibrium in the case of both samples is shifted to the right.
Thus, the difference in adsorption rate constants, as well as significant differences in the adsorption values of enantiomers in the region until adsorption–desorption equilibrium is reached, indicates that the adsorption rate of menthol enantiomers on crystals of o-toluic acid and CsCuCl3 obtained under Viedma ripening conditions is different.