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Abstract
The liquidcrystalline thermotropic behaviour of a series of long alkyl chain dimethylammonioalkoxydicyanoethenolates of the type CH3-(CH2)n-1-N+(CH3)2-(CH2)p-O-CO-C-(CN)2 (n=10-18, even values only, with p=2, 3; p=4 for n=16) was analysed through differential scanning calorimetry, X-ray diffraction and polarizing optical microscopy. Smectic A, columnar and cubic mesophases were observed in addition to a crystalline phase. The crystalline structure corresponds to a single layered lamellar arrangement of molecules characterized by a tilt angle of the paraffin chains of about 50 degrees and 56 degrees for p=2 and 3, respectively. Above the melting point (70-100 degrees C), all amphiphiles n-p, except 10-2 and 12-3 which became immediately isotropic, display one or more mesophases: smectic A for 14-2, 16-2, 16-3, 18-2 and 18-3 (zwitterionic bilayers with a thickness of about 12A), a hexagonal columnar phase for 12-2, 14-3 and 16-4 (monotropic for 12-2, zwitterionic cylinders of radius of about 12A), and an inverted bicontinuous cubic phase (Ia3dspace group) for 14-2, 16-3 and 18-3 (rod-like elements of radius of about 10A). For 14-2 and 18-3, the cubic mesophase tends to form slowly from the smectic A phases both on heating and cooling. The amphiphiles displaying the columnar mesophase possess the lowest and an identical ratio of alkyl tail length to zwitterion length. The temperature range of mesophase stability is a strongly increasing function of n and a decreasing function of p. For the longest alkyl tail, mesophases extend over more than 130 degrees C up to the degradation temperatures (T 210 degrees C). Strong dipolar interactions between the zwitterionic groups in their stretched conformation (mu>25 D) within the liquid apolar and incompatible paraffin matrix are the driving force for the thermotropic behaviour of these new amphiphiles.
