The efflorescence of an individual KH(2)PO(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(2)PO(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(2)PO(4)(-) (A(H(2)O)/(A((ν(s)-PO(2))+A(ν(s)-P(OH)(2))))). In 1.0 mol l(-1) KH(2)PO(4) solution, the ν(s)-P(OH)(2) and ν(s)-PO(2) bands appeared at 877 and 1077 cm(-1). In the KH(2)PO(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(2)PO(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(2)PO(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 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 H(2)PO(4 (-) were all hydrogen bonding through the bridge hydrogen atoms to get the extensive hydrogen-bonded network structure of the H(2)PO(4)(-) association, leading to the symmetric increase of the H(2)PO(4)(-) ion from C(2v) in dilute solution to quasi-T(d) in the anhydrous crystal.

A single K(2)HPO(4) droplet with size of ∼50 μm on a Teflon substrate was forced to enter into the supersaturated state by decreasing the relative humidity (RH), allowing accurate control over the concentration of the solute within a droplet of a nanogram. The K(2)HPO(4) solutions from dilute (0.1-1.0 mol·L(-1) bulk) to concentrated state (a droplet from RH 98.2% to 25.1%) were studied through micro-Raman spectroscopy in the spectral region of about 200-4000 cm(-1). The area ratio between the water stretching band to the sum of the ν(1)-PO(3), ν(2)-POH, and ν(4)-PO(3) bands of the HPO(4)(2-) at various RHs was used to describe the dehydration behavior of a microsized single K(2)HPO(4) droplet in dehumidifying process. The peak position of the v(1)-PO(3) band for the 1 mol·L(-1) bulk solution appeared at 991 cm(-1) and moved to 986 cm(-1) at 98.2% RH, to 978 cm(-1) at 70.2% RH, and then to 964 cm(-1) at 30.0% RH for a droplet, accompanying an increase of the full width at half-height (fwhh) of this peak from 16.3 to 17.2, 22.2, and then to 24.2 cm(-1), indicating transition of the HPO(4)(2-) anions from monomers to dimers/trimers/oligomers and then to polyanions with chain structures in the K(2)HPO(4) solutions. After 25.1% RH, the solid was proved to be K(2)HPO(4)·3H(2)O according to the Raman spectral features. Furthermore, the O-H stretching envelope of a K(2)HPO(4) droplet showed that the intensity ratios of the strong hydrogen bonding component (3255 cm(-1)) to the weak one (3417 cm(-1)) and the cage-like water (2925 cm(-1)) to the weak one (3417 cm(-1)) were sensitive to the HPO(4)(2-) association structures, which can be used to understand the effects of dimers/trimers/oligomers and chain structures of the HPO(4)(2-) associations on the hydrogen bonding of water molecules.