title: Micro-Raman observation on the HPO4(2-) association structures in an individual dipotassium hydrogen phosphate (K2HPO4) droplet.
abstract: 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.
authors.affiliation: Key Laboratory of Cluster Science, School of Science, Beijing Institute of Technology, Beijing 100081, China.
authors.forename: Kamran Ajmal
authors.initials: KA
authors.lastname: Syed
authors.forename: Shu-Feng
authors.initials: SF
authors.lastname: Pang
authors.forename: Yun
authors.initials: Y
authors.lastname: Zhang
authors.forename: Guang
authors.initials: G
authors.lastname: Zeng
authors.forename: Yun-Hong
authors.initials: YH
authors.lastname: Zhang