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

Abstract

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-t...