Thermal and structural investigations of the bis-dihexadecyldimethylammonium dichromate

New mesostructured bis-dialkyldimethylammonium-dichromate of formula [(C16H33)(2)(CH3)(2)N](2)Cr2O7 was prepared at 80 degrees C from an aqueous solution of hexadecyldimethylammonium salt and potassium dichromate. Phase transitions have been evidenced by means of thermogravimetric analysis coupled with differential scanning calorimetry and by optical microscopy. As suggested by a preliminary powder X-ray diffraction study, the compound exhibits a lamellar structure. The detailed conformation and molecular packing of the compound were determined from single crystal X-ray diffraction data analysis. [(C16H33)(2) (CH3)(2)N](2) Cr2O7 crystallizes in the triclinic system (P1 space group, Z = 1), with cell parameters a 34.330(7) Angstrom, b = 7.800(2) Angstrom, c = 7.250(1) Angstrom, alpha = 103.69(3)degrees, beta = 89.86(3)degrees, gamma = 94.12(3). The structure consists of discrete dichromate anions, statistically distributed in a layer, separated by a double-layer of dihexadecyldimethylammonium cationic molecules in which the alkyl chains are lying in parallel. The hydrocarbon chains packing can be described using a triclinic subcell with dimensions a(s) 5.01 Angstrom, b(s) = 7.80 Angstrom, c(s) = 2.57 Angstrom, alpha(s) = 90.9 degrees, beta(s) = 98.9 degrees, gamma(s) = 104.0 degrees. The packing cross-section of the hydrophilic part is 54.9 Angstrom(2) per molecule of surfactant in the layer plane. Therefore, the chains axis forms a tilt angle of 46 degrees (cos phi = (a(s) x b(s) x sin gamma(s))/a x b x sin gamma) relative to the normal of the polar plane. (C) 1999 Academic Press.