title: Micro-Raman observation on the H2PO4(-) association structures in a supersaturated droplet of potassium dihydrogen phosphate (KH2PO4).
abstract: 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.
authors: The Institute of Chemical Physics, Environmental Molecular Science Laboratory (EMSL), School of Chemistry, Beijing Institute of Technology, Beijing 100081, People's Republic of China. kamranajmalcn@gmail.com
authors: Kamran Ajmal
authors: KA
authors: Syed
authors: Shu-Feng
authors: SF
authors: Pang
authors: Yun
authors: Y
authors: Zhang
authors: Yun-Hong
authors: YH
authors: Zhang
mesh_terms: D000838:Anions / Q000737:chemistry*; D004867:Equipment Design; D006813:Humidity; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010710:Phosphates / Q000737:chemistry*; D011138:Polytetrafluoroethylene / Q000737:chemistry; D017680:Potassium Compounds / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013059:Spectrum Analysis, Raman / Q000295:instrumentation* / Q000379:methods; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D010710:Phosphates; D017680:Potassium Compounds; D012996:Solutions; D014867:Water; D006859:Hydrogen; D011138:Polytetrafluoroethylene; C013216:potassium phosphate