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-alkyltrimethylammonium dichromates of formula (C n H 2n+1 (CH 3 ) 3 N) 2 Cr 2 O 7 · × H 2 O where n = 12, 14, 16, 18, and 0 ≤ × ≤ 2. These layered compounds crystallise in the triclinic system, space group P-1. The structure consists of discrete dichromate anions stacking up in a layer, separated by a double layer of alkyltrimethylammonium cationic molecules lying in parallel. The d-spacing of these compounds linearly increase with the number of carbons in the alkyl chain, and depend on the number of water molecules. Phase transitions were investigated by mean of thermogravimetric 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.