Orientational Order in the Lyomesophases of the Disodiumchromoglycate-Water Systme by 2D, 17O and 23Na NMR

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Abstract
Deuterium, oxygen-17 and sodium-23 NMR measurements in the mesophase region of the disodiumchromoglycate (DSCG)-Water system are reported. The results indicate that, depending on the temperature and concentration of the DSCG, at least three different mesophases can exist in the system: At temperature below -4°C a relatively highly ordered smectic-like phase exist (phase III) which was not reported previously. Above - 4°C two phases, labeled N and M appear. The dividing line between these two phases is at approximately 18 wt.% DSCG and depends slightly on temperature. Phase N which prevails on the low concentration side of the dividing line, appears to be nematic-like as manifested by the fact that it reorients in a magnetic field, while phase M which appears on the high concentration side of the dividing line seems to be smectic-like. There is no discontinuity in the ordering characteristics of the water solvent in the N-M transition region, suggesting that these two phases have similar ordered structures. These are believed to be columns of stacked highly hydrated DSCG molecules. In the M-phase these columns are arranged in ordered arrays, whereas in the N-phase they are relatively free to move within the bulk water while retaining their orientational order. The phase transition from the N, M phases to phase III is accompanied by a marked change in the water ordering indicating that the smectic structures in the latter phase are quite distinct from those in the N, M phases. Sample rotation experiments in a magnetic field show that all three phases are uniaxial and of type II (i.e. Δx < 0) indicating that in all the lyotropic structures the DSCG molecules prefer an orientation in which the aromatic planes lie perpendicular to the director.