Supramolecular chiral surface of nickel sulfate hexahydrate crystals and its ability to chirally recognize enantiomers by adsorption data†
The cause of chiral symmetry breaking, leading to the chirality of all living things, remains one of the mysteries of chemistry. One possible source of inducing chirality may be enantiomorphic crystals. The surface of such crystals does not have chirality centers; however, it has supramolecular chirality. In this work, we evaluated the ability of the surface of nickel sulfate hexahydrate crystals, obtained under Viedma ripening conditions, for chiral recognition. For this, enantiomer-adsorption on the surface of α-NiSO4·6H2O crystals was studied. To obtain adsorption isotherms, two approaches were used: adsorption from solutions and vapor adsorption under inverse gas chromatography conditions. It was found that the initial regions of the adsorption isotherms of the enantiomers coincide. At the same time, at high adsorbate concentrations, a significant difference in adsorption isotherms was observed. In the case of menthol adsorption from solutions in n-heptane, it was found that the difference in adsorption isotherms was observed in the range of 0.75 < θ < 1.75. The ability of the studied surface to cause chiral separation under the conditions of both gas and liquid chromatography was found. It was concluded that in the case of the α-NiSO4·6H2O supramolecular chiral surface, the recognition of enantiomers is possible when an enantiomer layer is formed on it, i.e., the chirality of the supramolecular level turned out to be capable of recognizing the chirality of only the same level. The occurrence of similar processes of enantiomer adsorption on enantiomorphic crystals under Archean Earth conditions could be the cause of global chiral symmetry breaking.