Thermal and Structural Investigations of the Bis-Dihexadecyldimethylammonium Dichromate

✍ Scribed by Nathalie Fosse; Luc Brohan

• Publisher: Elsevier Science
• Year: 1999
• Tongue: English
• Weight: 787 KB
• Volume: 145
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1999.8277

✦ Synopsis

New mesostructured bis-dialkyldimethylammonium-dichromate of formula [(C 16 H 33 ) 2 (CH 3 ) 2 N] 2 Cr 2 O 7 was prepared at 803C from an aqueous solution of hexadecyldimethylammonium salt and potassium dichromate. Phase transitions have been evidenced by means of thermogravimetric analysis coupled with di4erential scanning calorimetry and by optical microscopy. As suggested by a preliminary powder X-ray di4raction study, the compound exhibits a lamellar structure. The detailed conformation and molecular packing of the compound were determined from single crystal X-ray di4raction data analysis. [(C 16 H 33 ) 2 (CH 3 ) 2 N] 2 Cr 2 O 7 crystallizes in the triclinic system (P1 space group, Z ‫؍‬ 1), with cell parameters a ‫؍‬ 34.330(7) A s , b ‫؍‬ 7.800(2) A s , c ‫؍‬ 7.250(1) A s , ‫؍‬ 103.69(3)3, ‫؍‬ 89.86(3)3, ‫؍‬ 94.12(3). The structure consists of discrete dichromate anions, statistically distributed in a layer, separated by a doublelayer 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 A s , b s ‫؍‬ 7.80 A s , c s ‫؍‬ 2.57 A s , s ‫؍‬ 90.93, s ‫؍‬ 98.93, s ‫؍‬ 104.03. The packing cross-section of the hydrophilic part is 54.9 A s 2 per molecule of surfactant in the layer plane. Therefore, the chains axis forms a tilt angle of 463 (cos ‫؍‬ (a s ؋ b s ؋ sin s )/a ؋ b ؋ sin ) relative to the normal of the polar plane.