The structural evolution of magnesium acetate complex in aerosols by FTIR–ATR spectra

The structural evolution of magnesium acetate complex in aerosols with the relative humidity (RH) has been studied by ATR–FTIR technique. When the RH is higher than 66%, the ν4 band lies at 929 cm−1 meaning the free CH3COO− ions in Mg(CH3COO)2 droplets. At the 66% RH, ν4 band positioned at 939 cm−1, accompanying the ν8 band shift to 1554 cm−1, which indicats that the free CH3COO− ions are bounded to Mg2+ ions to form [Mg(H2O)5(CH3COO)]+ species. At the 57.7% RH, the ν8-COO band shifts to 1556 cm−1 accompanying the ν3 band at 1421 cm−1 and the appearance of shoulder at 1452 cm−1, which suggests the formation of chain-structure connected by the bridging bidentate of Mg2(CH3COO)4(H2O)2. In the region of 57.7–18.7% RH, the shoulder at 1452 cm−1 increases with the decrease in RH, showing the increase of Mg2(CH3COO)4(H2O)2. From the water-content, the water-transfer from and to the surface of the aerosols became limited, showing the aerosols enter the gel state. Below 18.7%RH, water-loss becomes rapid and the ν8 band performs blue-shift. At 3.8%RH, the ν8 band positioned at 1581 cm−1, showing the anhydrous Mg(CH3COO)2 solid, which can be reflected by the ν4 band at 947 cm−1. During the humidification process, the reverse structural evolution can be found.