Resolution of Binary Enantiomeric Mixtures in Two Dimensions

The chirality of a molecule can play an important role during surface self-assembly. For example, adsorption on an achiral surface can lead to chiral resolution of enantiomeric mixtures. The resulting structures are often extended homochiral domains or two-dimensional chiral aggregates. Such processes hold promise not only as a means of directly achieving enantiomeric resolution but also for use in asymmetric catalysis and for building chiral surfaces for further chiral adsorption. We examined the mixing phase behavior of two model chiral molecules, in intermediate-density monolayers, restricted to assemble in two dimensions. In bulk solutions, the separation of enantiomers may occur at crystallization. On surfaces, the relevant phase change is a two-dimensional condensation. We found that this dimensional restriction led to higher-entropy phase separation regimes and resulted in a rich phase behavior for these systems. One chiral model is a two-dimensional chiral conglomerate which behaves similarly to, but with more nuanced transitions than, respective bulk analogues. The second model exhibits a more complex structural evolution with enantiomeric excess and a hybrid “racemic compound”/pseudoracemic mixing behavior.