Liquid-crystalline and thermochromic behaviour of 4-substituted 1-methylpyridinium iodide surfactants

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Abstract
The mesogenic behaviour of a series of thirty-one 1-alkyl-4-(or 2-)alkylpyridinium salts and of a homologous series of four 1-methyl-4-n-alkoxycarbonylpyridinium iodides is described. The occurrence and stability range of the thermotropic phases depend dramatically on the structure of the surfactants. Mesophases are only observed if the 1-alkyl group is methyl. Furthermore, an unusually strong dependence of the mesogenic behaviour on the number of carbon atoms in an unbranched alkyl chain is found. Alkyl chain branching, especially close to the headgroup, in 1-methy1-4-(C12-alky1)Pyridinium iodides lowers the clearing point and, to a lesser extent, the melting point. Apparently, the mesophase is destabilized due to the lack of space to accommodate a protruding side chain. The mesophases of the unbranched as well as the branched compounds are identical to the smectic A phase found for carbohydrate mesogens with one alkyl chain (smectic Ad). The mesophase of the peg-shaped compound 1-methyl-4-(17-tritriacontyl)-pyridinium iodide could not be assigned with certainty. Based on molecular shape, textural characteristics and the observed miscibility with the columnar hexagonal phase of a non-ionic carbohydrate amphiphile, the compound most likely exhibits a Dhd phase. However, the single reflection, corresponding to a d-spacing of just over one molecular length, suggests a smectic phase. It is assumed that all compounds described belong to the general class of amphiphilic mesogens. 1-Methyl-4-n-alkoxycarbonylpyridinium iodides are visibly thermochromic, whereas the corresponding 4-alkyl derivatives are not. This probably stems from a shift of the chargetransfer absorption band to higher wavelengths, induced by the presence of the ester group in conjugation with the pyridinium ring. Lyotropic liquid-crystalline phases of some surfactants were also examined. There is no apparent correlation between the occurrence of thermotropic and lyotropic phases in individual compounds. The types of lyotropic phases observed are dependent on the overall shape of the anhydrous molecule, but, surprisingly, it was not always possible to correlate the mesophase behaviour at maximum hydration with the type of aggregate formed in dilute solution.