Crystalline polysilicate magadiite with intercalated n-alkylmonoamine and some correlations involving thermochemical data

Synthesized hydrated lamellar acidic crystalline magadiite (H₂Si₁₄O₂₉·2H₂O) nanocompound was used as host for intercalation of polar n-alkylmonoamine molecules of the general formula H₃C(CH₂)_nNH₂ (n = 1–6) in aqueous solution. The original interlayer distance (d) of 1500 pm, determined by X-ray powder diffraction patterns, increases after intercalation. The values correlated with the number of aliphatic amine carbon (n_c) atoms: d = [(1312 ± 11) + (21 ± 2)]n_c. The amount of intercalated amines (N_s), decreased as n_c increased: N_s = [(5.82 ± 0.04) − (0.45 ± 0.01)]n_c. The acidic layered nanocompound was calorimetrically titrated with the amines and the thermodynamic data gave exothermic values for all guest molecules, as shown by the correlation: Δ_intH = −[(24.45 ± 0.49) − (1.91 ± 0.10)]n_c and d = [(1576 ± 16) − (10.8 ± 1.0)]Δ_intH. The negative values of the Gibbs energies and the positive entropies also presented the correlations: Δ_intG = −[(22.8 ± 0.2) − (0.2 ± 0.1)]n_c and Δ_intS = [(6 ± 1) + (5 ± 1)]n_c, respectively.     

Rearrangement of cationic surfactants in magadiite

Partly loaded magadiite samples have been prepared from a synthetic sodium magadiite, 0.9 Na₂O · 13.9 SiO₂ · 9.3 H₂O, by a two-step intercalation process using n-cetylpyridinium (CP) chloride as a model surfactant. Usually, partly loading with long-chain organic cations yields a non-uniform distribution of the surfactant molecules in the interlamellar space of the layered silicates. The resulting samples contain fully expanded crystals or zones within the crystals besides unreacted crystals or domains. After equilibration in water the partly loaded samples transform into products with a uniform expansion of all interlayer spaces due to rearrangement of the CP cations within and between the interlayer spaces. Received: 10 June 1998 Accepted in revised form: 5 November 1998     

Intercalative reaction of a cobalt(III) cage complex,Co(sep)3+, with magadiite,a layered sodium silicate

The cationic metal cage complex (1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]cicosane)cobalt(III), Co(sep)³⁺ has been investigated as a potential pillaring reagent for Na⁺-magadiite (Na_1.7Si₁₄O_27.9(OH)_1.9 · 7.6 H₂O) a synthetic layered sodium silicate. Reaction of Na⁺-magadiite with aqueous solutions of Co(sep)Cl₃ at 25°C resulted in the binding of Co(sep)³⁺ cations to the external crystalline surface of the layered silicate. In contrast, an intercalated product exhibiting a 17.6 Å basal spacing was generated by reaction at 100°C.²⁹Si MAS NMR and FT-IR spectroscopy indicate that Co(sep)³⁺ intercalated reaction products retain the magadiite layer structure. Moreover, scanning electron micrographs of the reaction products showed retention of the original particle morphology, suggesting a topotactic intercalation. However, during intercalation, some of the Co(sep)³⁺ was found to undergo an unusual demetalation reaction leaving a combination of Co(II) and Co(sep)³⁺ between the layers. Nitrogen surface area analysis showed that only a small amount of microporous surface existed in the Co(sep)³⁺ intercalated derivative, suggesting that most of the interlayer space is stuffed with cobalt species.     

Preparation of silylated magadiite thin-film-containing covalently attached pyrene chromophores

Thin-film of magadiite (Mag) silylated by [2-(perfluorohexyl)-ethyl]trichlorosilane was prepared by casting the chloroform/ethylacetate nanosheet solution of it. The addition of alkylamine and perfluoroalkyl groups to silylated magadiite greatly changed the surface morphology of the resulting sample and the transparent thin-film with relatively smooth surface was obtained. Amino groups were introduced into it by using 3-aminopropyltriethoxysilane after removal of alkylamine. The reaction of 1-pyrenebutanoic acid succimidyl ester with the thin-film of silylated magadiite-containing amino groups in N,N-dimethylforamide (DMF) resulted in the covalent attachment of pyrene chromophores to it.     

Effect of thermal dehydration and rehydration on Na-magadiite structure

The effect caused by dehydration and rehydration of the synthetic Na-magadiite was investigated by TGA, XRD, SEM, and ²⁹Si NMR. Thermal analysis of Na-magadiite presented two well-defined loss mass stages between 20 and 150 °C and another between 270 and 310 °C, both related to the removal of interlayer water. The swelling behavior of Na-magadiite was studied by thermal dehydration data obtained at 150 and 300 °C, and respective rehydration by water addition. X-ray patterns showed that the dehydration of Na-magadiite at 150 and 300 °C provoked the basal spacing decrease. The XRD also showed that only the material treated at 150 °C returned to the original structure with the rehydration. ²⁹Si NMR spectra showed that after rehydration, the Q₃/Q₄ relationship presented the same value for Na-magadiite treated at 150 °C. However, this Q₃/Q₄ value decreased when the treatment was done at 300 °C. Kinetic studies of thermal decomposition showed that the dehydration of magadiite is based on a phase boundary-controlled reaction, caused by contracting areas. The exfoliation of lamellas with thermal treatment can explain this behavior, as observed in SEM images.