Evidence of liquid crystal phase transitions in mesostructured alkyltrimethylammonium chromates

Using amphiphilic molecules as templating director agents, we have recently synthesized, under soft chemistry conditions, new mesostructured bis-alkyl trimethylammonium dichromates of formula (CnH2n+1(CH3)(3)N)(2)Cr2O7. x H2O where n = 12, 14, 16, 18, and 0 less than or equal to x less than or equal to 2. These layered compounds crystallise in the triclinic system, space gr oup P-1. The structure consists of discrete dichromate anions stacking up i n a layer, separated by a double layer of alkyltrimethylammonium cationic m olecules lying in parallel. The d-spacing of these compounds linearly incre ase with the number of carbons in the alkyl chain, and depend on the number of water molecules. Phase transitions were investigated by mean of thermog ravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, optical microscopy and X-ray powder diffraction experiments. The hydrated bis-alkyltrimethylammonium dichromates exhibits a mesophasic behavior and the temperature domain in which the smectic A mesophase exists decreases when the length of the alkyl chains increases.