NMR and FTIR studies of clustering of water molecules: From low-temperature matrices to nano-structured materials used in innovative medicine

Abstract

H-bond clustering of water molecules confined in an argon matrix and in nano-structured calcium hydroxyapatites (Ca10(PO4)6(OH)2, CaHA) was studied by means of FTIR and 1H MAS NMR spectroscopy. FTIR spectra of water in matrix were measured from T=9K to 40K stepping by 2K with sequent 2D correlation analysis (2DCOR) carried out in the range of O–H stretching vibrations 3000–3800cm−1. The peaks of monomer, dimer and the H-bond clusters from trimer to hexamer as well as the surface (dangling) modes were resolved. Some peaks in 2DCOR spectra drop out from the water clustering scheme. They were related to the enhancement of vibration–rotation motion due to increasing temperature and volume of the cavities in argon matrix. Water clusters were revealed in nano-CaHAs in FTIR spectra for the first time. Beside the sharp peak at 3570cm−1 of the stretching vibration of O–H− ion in the bulk, the bands of water dimer, tetramer, hexamer and several surface O–H and P–O–H modes were resolved at 2900–3900cm−1. They were assigned comparing with the spectra of water in argon matrix and in hydrophobic solvent (CCl4). The cluster structure and the size depends on the hydration level. Hexamer- or even higher structures were observed only in the samples containing significant amount of adsorbed water. The amount of adsorbed water in the studied nano-CaHAs was probed using the intensity of the broad 1H signal at ca 5.1ppm normalized respect to OH– peak at 0ppm. Two narrow signals observed in 1H MAS NMR spectra at 0.8 and 1.3ppm are originated from the surface structured H2O molecules. The NMR data perfectly correlate with those obtained by FTIR spectroscopy.