Micro-Raman observation on the HPO ₄ ^2- association structures in an individual dipotassium hydrogen phosphate (K ₂HPO ₄) droplet

A single K ₂HPO 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 ₂HPO 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 ₂HPO 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 ₂HPO ₄ solutions. After 25.1% RH, the solid was proved to be K ₂HPO ₄•3H ₂O according to the Raman spectral features. Furthermore, the O-H stretching envelope of a K ₂HPO 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.