Micro-Raman observation on the H_2PO_4 ~- association structures in a supersaturated droplet of potassium dihydrogen phosphate (KH_2PO_4)

摘要

The efflorescence of an individual KH_2PO_4 droplet on Teflon substrate was investigated by micro-Raman spectroscopy. With the decrease of relative humidity (RH) from 98.0% to 73.0%, the KH_2PO_4 droplet lost water gradually and entered into supersaturated state, which was reflected by the area ratio between the water stretching band to the sum of ν_s-PO_2 and ν_s-P(OH)_2 bands of the H_2PO_4 ~- (AH_2O/(A(ν_s-PO _2+Aνs-P(OH)_2))). In 1.0 mol l~(-1) KH _2PO_4 solution, the ν_s-P(OH)_2 and ν_s-PO_2 bands appeared at 877 and 1077 cm~(-1). In the KH_2PO_4 droplet, the two bands shifted to 894 and 1039 cm~(-1) at 98.0% RH, to 899 and 1031 cm~(-1) at 89.6% RH, and then to 904 and 997 cm~(-1) at 73.0% RH. Moreover, the aggregation process between the H_2PO_4 ~- ions was observed from the spectral characteristic of the ν_s-P(OH)_2 band in the concentration process, including the transitions of the H _2PO_4 ~- ions from monomer in bulk solutions (0.5-1.0 mol l~(-1)) to possible dimers at 98.0% RH and then further to oligomers in the droplet with the RH decrease, which were indicated by the blueshift of the νs-P(OH)_2 band and its full width at half-height as a function of the RH. When the RH reached at 72.0%, the anhydrous crystal was obtained. A strong peak appeared at 928 cm~(-1), implying that the four oxygen atoms of the H_2PO_4 ~- were all hydrogen bonding through the bridge hydrogen atoms to get the extensive hydrogen-bonded network structure of the H_2PO_4 ~- association, leading to the symmetric increase of the H_2PO _4 ~- ion from C_(2v) in dilute solution to quasi-Td in the anhydrous crystal.