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Chiral Separation: Mechanism Modeling in Two-Dimensional Systems
journal contribution
posted on 2007-03-28, 00:00 authored by Irina Paci, Igal Szleifer, Mark A. RatnerFluid phase separations of racemates are difficult because the subtle, short-ranged differences
in intermolecular interactions of like and unlike pairs of chiral molecules are typically smaller than the thermal
energy. A surface restricts the configurational space available to the pair of interacting molecules, thus
changing the effective interactions between them. Because of this restriction, a surface can promote chiral
separation of mixtures that are racemic in bulk. In this paper, we investigate chiral symmetry breaking
induced by an achiral surface in a racemate. A parallel tempering Monte Carlo algorithm with tempering
over the temperature domain is used to examine the interplay between molecular geometry and energetics
in promoting chiral separations. The system is restricted to evolve in two dimensions. By controlling the
balance between electrostatic and steric interactions, one can direct the surface assembly of the chiral
molecules toward formation of small clusters of identical molecules. When molecular shape asymmetry is
complemented by dipolar alignment, chiral micellar clusters of like molecules are assembled on the surface.
We examine the case of small model molecules for which the two-dimensional restriction of the pair potential
is sufficient to induce chiral segregation. An increase in molecular complexity can change the balance of
intermolecular interactions to the point that heterochiral pairs are energetically more favored. In this case,
we find conditions in which formation of homochiral micelles is still achieved, due to a combination of
multibody and entropic effects. In such systems, an examination of the pair potential alone is insufficient
to predict whether the multimolecular racemate will or will not segregate.