Synthesis, characterization and thermodynamics of the reaction of calcium methylphosphonate with n-alkylmonoamines

Abstract

Lamellar crystalline calcium methylphosphonate, Ca[(HO)O₂PCH₃]₂.H₂O reacted with a series of n-monoalkylamines to yield the compounds Ca[(HO)O₂PCH₃]₂·xH₂N(CH₂)_nCH₃·(1−x)H₂O (n=0–4). Intense bands of the phosphonate group in the 1095–995 cm⁻¹ region in the infrared spectrum were detected. X-ray diffraction patterns showed a sharp and intense peak with an interlamellar distance of 907 pm. ³¹P NMR spectrum gave a peak at 27.38 ppm for phosphonate groups. On heating methylphosphonate water molecules, organic moiety and P₂O₅ were released to yield a residue of pyrophosphate. The aminated compound presented the same sequence of mass loss, with amine being lost in the first stage. ³C NMR spectrum presented peaks at 15 and 17 ppm for methylphosphonate groups. The original crystallinity of the compound is disturbed by the reaction with the n-alkylmonoamine, giving constant interlamellar distance. The energetic effect caused by amine was determined through reaction-solution calorimetry in the solid–liquid interface from aqueous solution. The thermodynamic data showed that the system is favored by enthalpy, Gibbs free energy, and entropy values.