LAYERED ALKYLTRIMETHYLAMMONIUM CHROMATES - THERMAL AND STRUCTURAL INVESTIGATIONS AND CRYSTAL-STRUCTURE OF THE ANHYDROUS BISOCTYLTRIMETHYLAMMONIUM DICHROMATE

New mesostructured bisalkyltrimethylammonium dichromates of formula (C nH2n+1(CH3)(3)N)(2)Cr2O7.xH(2)O (n = 12, 14, 16, 18; 0 less than or eq ual to x less than or equal to 2) were prepared at 80 degrees C from a n aqueous solution of alkyltrimethylammonium salt and K2Cr2O7. The che mical composition and phase transitions have beers determined by therm ogravimetric analysis (TGA) coupled with differential scanning calorim etry (DSC) and mass spectrometry (MS). As suggested by X-ray diffracti on and transmission electron microscopy studies, the lamellar hydrated and anhydrous bisalkyltrimethylammonium dichromates crystallize in th e triclinic system, with space group P-1. The structure of the anhydro us bisoctyltrimethylammonium dichromate, (C18H37(CH3)(3)N)(2) Cr2O7, w as determined from single-crystal X-ray diffraction data. The compound crystallizes in the triclinic system, space group P-1, with 2 formula units in a cell a = 7.197(1)Angstrom, b = 8.816(2)Angstrom, c = 43.40 0(9)Angstrom, alpha = 93.43(3)degrees, beta = 90.00(3)degrees, gamma = 113,98(3)degrees. The structure consists of discrete dichromate anion s stacking up in a layer, separated by a double layer of octyltrimethy lammonium surfactant chains lying in parallel. The interlayer spacing of 43.4 Angstrom, smaller than the expected value far the fully extend ed molecular model, is achieved through a tilting of the surfactant ch ains of about 37.5 degrees from the normal to the (Cr2O7)(2-) plane. ( C) 1998 Academic Press.