Solid state nuclear magnetic resonance studies of synthetic mineral surfaces

Solid state 1H, 31P and 27Al Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR) was used in studies of surface species formed on a synthetic fluorapatite in aqueous suspensions at different pH with or without ions or ion complexes of iron or aluminium in the suspension. Different sites related to the PO43- ions at the mineral surface were suggested, POx, POxH and POxH2. Also, three sites formed of Ca2+, H+ and OH- ions at the fluorapatite surface CaOH2+, CaOH and Ca(OH) 2- were suggested. Both single-pulse and cross polarization MAS NMR experiments were used to measure the 1H, 31P and 27Al isotropic chemical shifts. In the 1H to 31P cross polarization experiments the contact time was varied in order to differentiate the resonance liners corresponding to different surface sites from the resonance lines assigned to the PO43- ions in the crystal structure of fluorapatite. In the solid state dipole-dipole recoupling MAS NMR experiments the sequence XY8-DRAMA was used in studies of distances between O,O'- dialkyldithiophosphates adsorbed on synthetic galena (PbS). The sequence was tested experimentally and gave highest double-quantum excitation efficiency of the tested dipole-dipole recoupling sequences tested at the spinning frequency 4.2 kHz. A new sequence IRS-DRAMA was derived analytically and showed high double quantum excitation efficiency in a broad interval of the 31P resonance frequencies at the spinning frequency 2.1 kHz.