The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkyls
ulphate series were studied by X-ray diffraction and dilatometry. All of th
ese compounds exhibit single smectic A phases whose layer spacings are betw
een 0.64 and 0.8 times the molecular length. Polarizing optical microscopy
observation shows that these peculiar layer spacings are not due to the til
ting of the molecules; a new packing model is proposed to explain the parti
cular structure of these thermotropic ionic liquid crystal phases.