Calorimetric study of intercalation of n-alkyldiamines into α-titanium hydrogenphosphate

Abstract

A series of n-alkyldiamines of general formula H₂N(CH₂)_nNH₂(n= 2-9) has been intercalated into the crystalline lamellar compound α-Ti(HPO₄)₂· H₂O (TiP) from aqueous solution. The amount intercalated was followed batchwise at 298 ± 1 K and the variation of the original interlayer distance for TiP (756 pm) was followed by X-ray powder diffraction. Linear correlations with good fits were obtained for the interlamellar distance (d) or for the number of moles intercalated (n_int) as a function of the number of carbon atoms in the aliphatic chain (n_c): d=(883.14 ± 12.76)+(108.51 ± 2.20)n_c and n_int=(5.79 ± 0.12)–(0.28 ± 0.02)n_c. The exothermic enthalpies for the intercalation are related to the monomolecular layer arrangement with a longitudinal axis inclined by 58° to the inorganic sheets. The enthalpies for the overall reaction 2O₃P–OH(c)+ H₂N(CH₂)_nNH₂(c,l)= O₃P–O^–+H₃N(CH₂)_nNH₃^+–O–PO₃(c); Δ_intH, determined by reaction-solution calorimetry at 298.15 ± 0.02 K are correlated with the number of carbons in the aliphatic chain or the interlamellar distance, by the equations ΔH=–(56.16 ± 0.67)–(2.06 ± 0.12)n_c and Δ_intH=–(39.41 ± 1.41)–(1.80 × 10^–2± 0.10 × 10^–2)d. The enthalpic value for n_c= 0 gave – 56.17 ± 0.67 kJ mol^–1 which corresponds to the intercalation of two moles of ammonium cation.