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SILICIC ACID MAGADIITE AS A HOST FOR N-ALKYLDIAMINE GUEST MOLECULES AND FEATURES RELATED TO THE THERMODYNAMICS OF INTERCALATION

Instituto de Química, Universidade Estadual de Campinas, Caixa Postal 6154, 13084-971 Campinas, São Paulo, Brazil

^* E-mail address of corresponding author: airoldi{at}iqm.unicamp.br

Some n-alkyldiamines with the general formulae H₂N(CH₂)_nNH₂ (n = 2–5) were intercalated into the layered silicic acid magadiite, from aqueous solution, causing an increase in the original interlayer distance of 1172 pm. The synthetic magadiite and all intercalated compounds were characterized by elemental analysis, infrared vibrational spectroscopy, X-ray diffractometry, ²⁹Si nuclear magnetic resonance in the solid state, thermogravimetry, scanning electron microscopy, surface area and porosity. The intercalation was followed through a batch-wise method at 298±1 K and gave the maximum amounts 3.70, 2.80, 1.75 and 1.18 mmol g^–1, for n varying from 2 to 5, respectively. The well characterized magadiite was calorimetrically titrated in a heterogeneous medium, to obtain the thermodynamic data of intercalation at the solid/liquid interface. Linear correlations were obtained for the number of moles intercalated (N_f), the interlamellar distance (d) and the specific enthalpy (_inth) values of the interactive process as a function of the number of C atoms of the aliphatic organic chains (n_C) for n-alkyldiamine: N_f = (5.36±0.25) – (0.86±0.07)n_C, d = (1406.6±1.9) + (20.9±0.5)n_C and _inth = (5.96±0.25) + (0.06±0.01)n_C. The basic N guest atom/silanol acidic center interactions inside the host nanospace gallery gave exothermic enthalpies, positive entropies and negative Gibbs free energy values. This set of data suggests the spontaneity of these intercalation reactions.

Key Words: Calorimetry • Intercalation • Magadiite • n-alkyldiamines • Silicic Acid