Layered Alkyltrimethylammonium Chromates: Thermal and Structural Investigations and Crystal Structure of the Anhydrous Bisoctyltrimethylammonium Dichromate

✍ Scribed by N. Fosse; M. Caldes; O. Joubert; M. Ganne; L. Brohan

• Publisher: Elsevier Science
• Year: 1998
• Tongue: English
• Weight: 608 KB
• Volume: 139
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1998.7847

✦ Synopsis

New mesostructured bisalkyltrimethylammonium dichromates of formula (C n H 2n؉1 (CH 3 ) 3 N) 2 Cr 2 O 7 • xH 2 O (n ‫؍‬ 12, 14, 16, 18; 04x42) were prepared at 80°C from an aqueous solution of alkyltrimethylammonium salt and K 2 Cr 2 O 7 . The chemical composition and phase transitions have been determined by thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) and mass spectrometry (MS). As suggested by X-ray diffraction and transmission electron microscopy studies, the lamellar hydrated and anhydrous bisalkyltrimethylammonium dichromates crystallize in the triclinic system, with space group P-1. The structure of the anhydrous bisoctyltrimethylammonium dichromate, (C 18 H 37 (CH 3 ) 3 N) 2 Cr 2 O 7 , was 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) A s , b ‫؍‬ 8.816(2) A s , c ‫؍‬ 43.400(9) A s , ‫؍‬ 93.43(3)°, ‫؍‬ 90.00(3)°, ‫؍‬ 113.98(3)°. The structure consists of discrete dichromate anions stacking up in a layer, separated by a double layer of octyltrimethylammonium surfactant chains lying in parallel. The interlayer spacing of 43.4 A s , smaller than the expected value for the fully extended molecular model, is achieved through a tilting of the surfactant chains of about 37.5°from the normal to the (Cr 2 O 7 ) 2؊ plane.