Thermal stability of mesoscopic compounds of cetyltrimethylammonium and Keggin metatungstates

Abstract

A hybrid surfactant/polyoxometalate compound was synthesized by combining isopolytungstate anions with the cationic surfactant cetyltrimethylammonium bromide (CTA-Br) to produce a hierarchical compound that we identify as (CTA)[HWO]Cl·2HO. At room temperature the compound consisted of hexagonally ordered sheets of Keggin ions, with an intervening gallery containing alkyl-chains of the organic cations. The synthesis was highly dependent on solution pH, reaction time and the order in which the reactants were added. We examined the effect of temperature on the stability of (CTA)[HWO]Cl·2HO using thermal gravimetric analysis, differential scanning calorimetry, FT-IR spectroscopy and in situ synchrotron X-ray diffraction, and found a step-wise conversion to monoclinic WOvia a series of intermediates. Heating under nitrogen atmospheres accelerated transition events by ∼100 °C when compared to heating in air. During heating, the interplanar gallery at first expanded in a series of steps starting at 90 °C as the CTA amphiphiles changed orientation, before collapsing rapidly at 240 °C, a temperature coinciding with the removal of about 40% of the organic material. Between 240 and 320 °C, the material consisted of fragments of the Keggin ion cores, arranged in 2D hexagonally-packed sheets. At ∼330 °C, the Keggin ions were completely destroyed and replaced by bulk WO which, upon further heating, transformed to bulk WO or WO depending on the environment.

Photomechanical photochromism in a cetyltrimethylammonium isopolytungstate

“…The peak at d ¼ 9.80Å is assigned to diffraction between Keggin ions within each plane. 21 Previous work has shown that two Keggin ions in a unit cell with a ¼ 12.40 AE 0.10Å, b ¼ 16.45 AE 0.10Å, c ¼ 24.90 AE 0.20Å, a ¼ 90 , b ¼ 101.5 AE 0.5 and g ¼ 90 will produce a diffraction pattern similar to that observed. 21 The peaks at 24.5Å and 9.80 A correspond to (001) and (110) respectively if indexed on the above unit cell.…”

“…An interplanar distance of 24.5Å was also measured using low-angle powder XRD. 21 The length of one CTA + surfactant ion is reported to be 24Å, 41 and the metatungstate anion has a diameter of 10Å, 42 indicating that a signicant degree of interdigitation of the CTA + tailgroups occurs within the structure. In a related POM structure, 38 (CTA) 5 (CH 3 CN) 4 [H x SiMo 12 O 40 ], the CTA + tail groups were interdigitated by an average of 13.3 carbons per chain with some CTA + cations completely interdigitated (16 carbons) and others only by 9 carbons.…”

“…For convenience, we have designated this compound as 'CTA-W 12 '. 21 Compounds of this type may be produced by an efficient and simple synthetic method: [22][23][24] they self-assemble in aqueous solution via electrostatic and hydrophobic-hydrophilic interactions between the organic cationic surfactants and the inorganic anionic POMs. The outcome is a highly predictable hierarchical nanostructure.…”

“…schematic depiction of the structure of CTA-W 12 showing planes of hexagonally-packed Keggin ions with intercalated and interdigitated cetyltrimethylammonium chains (see Colusso et al21 for further discussion. ).…”

X‐ray‐induced reduction of a surfactant/polyoxotungstate hybrid compound

“…The synthesis of (C 19 H 42 N) 7 Cl[H 2 W 12 O 42 ] 2H 2 O (designated in this work as CTA‐W 12 ) has been described elsewhere . X‐ray photoelectron spectroscopy measurements were conducted using a Thermo Scientific ESCALAB250Xi, utilising monochromated Al K‐α (1486.68 keV) radiation at a power of 120 W (13.8 kV × 8.7 mA).…”