Crystalline-hydrated barium phenylphosphonate as a host for n-alkyldiamine intercalation
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series of n-alkyldiamines with the general formula H2N(CH2)nNH2 (n ⳱ 2–6) has been intercalated into crystalline lamellar-hydrated barium phenylphosphonate from ethanolic solutions. The amount intercalated was followed batchwise and the variation in interlayer distance (d) for the original host and the respective intercalated compounds was followed by x-ray diffraction patterns. Linear correlation with good fits were obtained for (d) or for the number of moles intercalated (nf) as a function of the number of carbon atoms in the aliphatic chains (nc): d ⳱ (1436 ± 65) + (108 ± 14)nc and nf ⳱ (2.24 ± 0.15) – (0.31 ± 0.03)nc. The energetic effect caused by amine intercalation was determined through reaction-solution calorimetry at the solid-liquid interface from ethanolic solutions. The resulting exothermic enthalpies for intercalation give rise to a monomolecular layer guest molecule arrangement with a longitudinal axis inclined by 58° to the inorganic sheets. The thermodynamic data showed that the system is favored by exothermic enthalpy, negative Gibbs free energy, and positive entropic values.
I. INTRODUCTION
Crystalline-layered metal phosphonates comprise a class of compounds consisting of alternating inorganic and organic layers, and they have received increasing interest over the past few years. Early investigations were focused on the tetravalent metals,1,2 but, progressively, a variety of divalent and trivalent metals were explored to establish a relevant field of research, mainly from considerable interest in the intercalation behavior of these layered phosphonate compounds.3,5 Great advances in the intercalation field were obtained from observing whether any property of the lamellar compounds changed, such as those related to the host, as well as to the inserted guest molecules, at the end of the process.6,7 On the other hand, intercalation behavior depends on several features,7,8 among them the size and the pKa values of the guest molecules. However, other aspects, such as an increase in temperature or the use of a convenient solvent, can accelerate the intercalation kinetics several times. In other cases, the success in intercalating weak bases and/or even large molecules is strongly dependent on a previous expansion of the interlayer
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0293 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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distance through a convenient preceding molecule to make the final insertion easier.8 Some investigations concerning the intercalation process are connected to insertion of a variety of amines such as secondary, tertiary, cyclic, or arylic into phosphonate compounds.9–11 The intercalation of these species is of interest to further clarify the intercalation chemistry of layered metal phosphonates and also because the process leads to the preparation and characterization of new intercalated phases. These new phases are suitable for studying the interlayer organization of the in
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