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.