Micro-Raman observation on the H₂PO₄^- association structures in a supersaturated droplet of potassium dihydrogen phosphate (KH₂PO₄)

The efflorescence of an individual KH₂PO₄ droplet on Teflon substrate was investigated by micro-Raman spectroscopy. With the decrease of relative humidity (RH) from 98.0% to 73.0%, the KH₂PO₄ 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-PO₂ and ν_s-P(OH)₂ bands of the H₂PO₄^- (AH₂O/(A(νs-PO ₂+Aνs-P(OH)₂))). In 1.0 mol l^-1 KH ₂PO₄ solution, the ν_s-P(OH)₂ and ν_s-PO₂ bands appeared at 877 and 1077 cm^-1. In the KH₂PO₄ 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 H₂PO₄^- 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 ₂PO₄^- 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 ν_s-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 H₂PO₄^- were all hydrogen bonding through the bridge hydrogen atoms to get the extensive hydrogen-bonded network structure of the H₂PO₄^- association, leading to the symmetric increase of the H₂PO ₄^- ion from C_2v in dilute solution to quasi-T_d in the anhydrous crystal.