Prediction of the handedness of the chiral domains of amphiphilic monolayers: monolayers of amino acid amphiphiles

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The effect of chiral interaction on the morphology of the condensed phase domains of amino acid amphiphiles has been investigated theoretically in the present paper. The amphiphiles considered are N-palmitoyl aspartic acid, N-stearoyl serine methyl ester, N-palmitoyl-allo-threonine methyl ester and N-stearoyl-allo-threonine methyl ester. The domains of these amphiphiles show pronounced curvatures in the experiment, which are known to be driven by the chirality of the molecules concerned. We use the previously derived relations to calculate the intermolecular potential of a pair of chiral molecules of general shape, which is dependent on the intermolecular separation and the mutual orientation of the molecules. It is observed that the chiral interaction favors a mutual azimuthal orientation between the molecules in the unit cell for all amino acid amphiphiles considered. However, the azimuthal projections of the molecules within the domain are parallel as observed in the experiment. The energy to obtain parallel arrangement between the molecules is of the same order of the hydrogen bonding energy in the amino acid residues. Thus, it is suggested that the hydrogen bond cycles present among the molecules within the domain prevent the tendency of intermolecular twist due to chiral interaction. However, at the interface between the condensed phase and the fluid phase, the hydrogen bonding energy is not as strong as that within the domain as well as less direction specific. It is expected that the chiral interaction is dominating at the interface resulting the curved shape of the domain. The mutual orientation driven by chirality is cooperative in nature i.e. the mutual orientations at short and long axis of the unit cell occur in the same direction and does not oppose each other. The directions of the orientations of the molecules with respect to a reference molecule have the same senses (handedness) as those of the aggregates. The theoretically predicted senses of all four amino acid amphiphiles considered in the present study agree fairly well with the experimentally observed handedness using Brewster angle microscopy.