Structural heterogeneity of alkylammonium-exchanged, synthetic fluorotetrasilicic mica
A series of organo-silicates was obtained from synthetic Na-fluorotetrasilicic mica (Na-TSM) by ion exchange with dodecyl-,tetradecyl-, hexadecyl- and octadecyltrimethylammonium (C12, C14, C16 and C18) bromides. W hen the concentration of the alkylammonium ions has varied from 0.43 to 3.4 mmol/g of the silicate, the maximum amount adsorbed on the mica was 0.92 mmol/g. As shown by 23Na MAS NMR, ~0.30 mmol/g of Na+ ions were non-exchangeable, even if a large excess of the organic ions was used. The XRD patterns of all the samples obtained show that alkylammonium ions are distributed inhomogeneously within the mica interlayers. As a consequence, the organo-micas contain several phases consisting of randomly and regularly interstratified layers differing in terms of the amount of alkylammonium ions adsorbed. These different layers swell in ethylene glycol in the same manner and form new interlayers with identical spacings. The d 001 values of individual organo-TSMs after glycolation suggest that the alkylammonium ions are oriented perpendicular or almost perpendicular to the mica layers. The Na-TSM layers were not detected in organo-TSMs by XRD. The presence of organic and inorganic (Na) mixed-ion interlayers with spacings controlled by the arrangements of the organic ions is consistent with the results obtained. These data also suggest that the alkylammonium/Na+ ion ratios vary in alternate interlayers.
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Document Type: Research Article
Publication date: 2003-06-01