Micro-Raman study on the conformation behavior of succinate in supersaturated sodium succinate aerosols

Micro-Raman spectra of supersaturated aerosols of sodium succinate were obtained. The conformation behavior of the succinate dianion as a function of relative humidity (RH) was investigated by combining micro-Raman spectroscopy with theoretical calculations. A shoulder at 968 cm^-1 of the v(C-CO₂^-) band on the rise in more concentrated droplets was believed indicative of conformation transformations. The intensity ratio (I₉₆₃/I₉₉₇) of the v(C-CO₂^-) band at 963 cm^-1 to the v(C-C) band at 997 cm^-1, versus the molar water-to-solute ratio (WSR), was used to fathom the equilibrium between gauche and trans conformations. Before saturation (WSR = 25.8) for the droplets, the ratio of I₉₆₃/I₉₉₇ retains a value of ∼2.6 independent of WSR, indicating that the equilibrium was not disturbed in the dilute droplets. In supersaturated droplets (WSR < 25.8), however, the ratio sharply decreases from ∼2.6 to ∼1.1 at WSR = 9.6, which was attributed to the formation of contact ion pairs (CIPs).