A laser diffraction and nuclear magnetic resonance investigation of the cholesteric potassium N-dodecanoylalaninate mesophase system

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Abstract
Nematic and twisted nematic mesophases prepared from various mixtures of potassium N-dodecanoyl-l-alaninate and its racemate. The induced pitch was determined as a function of optical purity as were NMR spectra from a variety of additives including several inorganic salts and d,l-alanine. Dipolar couplings from the CHCH3 moiety of the alaninate headgroup were studied, as were quadrupole splittings from the N-D of the amido group and the D2O of the water. It was proposed that with development of cholesteric behavior on proceeding in a stepwise fashion from the d,l to l amphiphile, the individual disk-like micelles twisted into a chiral shape, then aggregated to form domain-like structures. The major axis of these aggregates is the axis along which the helix propagates and this local axis serves as the director axis for the aggregates. The individual aggregates reorient as a unit when placed in the magnetic field of an NMR spectrometer. As a consequence of this, the diamagnetic anisotropy of the mesophase prepared from the l amphiphile is positive in sign while that of the d,l analog is negative. Despite this observed change in diamagnetic anisotropy, the alignment of the individual micelles is not different for the chiral and racemic materials.

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