The presence of chirality at an interface enables a surface to distinguish between enantiomers. The mechanism by which this selectivity occurs is complicated by the surface morphology, the possible involvement of the solvent, and the characteristics of the chiral molecules at the surface. In this article, atomic force microscopy (AFM), chemical force titrations, density functional theory, and molecular dynamics simulations are employed to examine surfaces terminated by (S)-and (R)-N-(1-phenylethyl)-N′-[3-(triethoxysilyl)propylurea (PEPU). A "brush-type" chiral interface is formed by attaching the PEPU molecules to either an oxidized poly(dimethylsiloxane) or oxidized Si(111) substrate. Using AFM, the morphologies of the resulting chiral surfaces are obtained. Chemical force measurements of the chiral selectivity in the presence of water, methanol, hexane, and CS 2 have been performed. Ab initio studies provide descriptions of the hydrogen bonding between PEPU molecules at the interface. Molecular dynamics simulations are employed to examine the distribution of water, methanol, and CS 2 near PEPU interfaces.

A theoretical study of the solvation of ( R)- N-(3,5-dinitrobenzoyl)phenylglycine- and ( R)- N-(3,5-dinitrobenzoyl)leucine-derived chiral stationary phases (CSPs) is presented. Semiflexible models of the chiral selectors are prepared from B3LYP/6-311G** calculations, and these are used in the molecular dynamics simulations of the corresponding interface. The chiral interface is examined for four solvents: 100% hexane, 90:10 hexane:2-propanol, 80:20 hexane:2-propanol, and 100% 2-propanol. Despite the similarities between phenylglycine and leucine, the interfaces are distinct both in terms of the selector orientations at the surface and in the number of hydrogen bonds formed with 2-propanol. We also find that an increase in alcohol concentration alters the preferred orientations of the selectors.

A theoretical and experimental study of alcohol/water and alcohol/alcohol solvent mixtures near a surface of N-(1-phenylethyl)-N'-[3-(triethoxysilyl)propyl]-urea (PEPU), a Pirkle-type chiral stationary phase, is presented. Molecular dynamics simulations are performed at room temperature for water/methanol, water/1-propanol, water/2-propanol, and methanol/1-propanol solvent mixtures confined between two PEPU surfaces. The interface was also prepared experimentally by attaching the PEPU molecules to atomic force microscopy tips and oxidized Si(111) substrates. Chemical force spectrometric measurements between such PEPU-terminated tips and samples were taken in the solvent mixtures, and the results are compared to the molecular dynamics study. We find that the extent of hydrogen bonding at the surface is the dominant contributor to the measured forces.

A study of chiral self-discrimination in two model chiral stationary phases, N- (3,5-dinitrobenzoyl-phenylglycine and N-(3,5-dinitrobenzoyl)leucine, is described. "Brush-type" chiral interfaces are obtained by attaching these compounds onto oxidized Si(111) samples and atomic force microscopy tips by using two sample preparation procedures: a direct and a two-step surface deposition of the chiral stationary phase onto the substrate. By using AFM, the morphologies of the resulting chiral interfaces are obtained. XPS and FTIR provide information regarding the relative distribution of the compounds on the surface. We analyze the N 1s peaks using XPS to ensure that all the NH 2 groups on the surface are capped. Chemical force spectrometric measurements of the chiral self-selectivity in 2-propanol demonstrate that chiral discrimination is present in both systems, but larger forces are observed for N-(3,5-dinitrobenzoyl)phenylglycine, consistent with a previous molecular dynamics study that demonstrated much weaker solvent interactions with this species.

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