Molecular Dynamics Simulation Studies of Self-Assembly of Racemic (R)/(S)-2-Bromohexadecanoic Acid on a Graphite Surface: Enantio-pure or Enantio-mixed Domains?

The morphological enantio-identity of racemic ( R )/( S )-2-bromohexadecaonic acid self-assemblies on graphite is examined according to molecular dynamics simulations of the optimized potential for liquid simulations (OPLS) force field. Racemic ( R )/( S )-2-bromohexadecaonic acid segregates at the graphite−solvent interface into coexisting domains characterized by approximately 50° clockwise and 50° counter-clockwise lamellae-trough angles. The “upward” orientation (away from the graphite surface) of bromine atoms has been observed experimentally to be thermodynamically more stable under solvent for a wide range of bromo-substituted organic molecules, suggesting that the domains of racemic ( R )/( S )-2-bromohexadecaonic acid observed by scanning tunneling microscopy under phenyl octane solvent consist of pure enantiomers. Our calculations, performed under vacuum conditions, suggest a competition between the stability gained by mixing “upward” and “downward” pairs of ( R ) and ( S ) enantiomers and the symmetry breaking that favors the “upward” orientation of the bromine atoms at the graphite−solvent interface. The calculations indicate that racemic ( R )/( S )-2-bromohexadecaonic acid is likely to form enantio-mixed domains at the graphite−vacuum interface. The possibility that the domains formed by racemic ( R )/( S )-2-bromohexadecaonic acid at the graphite−solvent interface are not enantio-pure is discussed in this paper.