Specific Heat Capacity of Non-Functional and Functional Ionic Liquids during the Absorption of SO₂

Ionic liquids (ILs), a new type of absorbents, are widely used to absorb sulfur dioxide (SO₂) in flue gas. However, the lack of data on the physical properties of ILs has severely hampered their industrial applications. Unlike previous reports on the specific heat capacity of pure ILs, this work not only focuses on measuring the specific heat capacity of pure ILs but also provides the specific heat capacity of IL and SO₂ mixtures during the absorption process. First, we measured the specific heat capacities of six ILs including functional ILs, 1,1,3,3-tetramethylguanidinium lactate ([TMG]­[Lac]), [TMG]­[suberate], [TMG]­[dodecanedioate], and 1-butyl-3-methylimidazolium acetate ([Bmim]­[Ac]), and non-functional ILs, [TMG]­[tetrafluoroborate] and [Bmim]­[tetrafluoroborate], at 303.15–333.15 K. We found that the specific heat capacities of the ILs with or without absorbed SO₂ increase with temperature. The specific heat capacity of ILs increases with the increase of the amount of absorbed SO₂. Then, we investigated the effect of the length of anionic alkyl side chains on the specific heat capacity of SO₂ absorption and found that the specific heat capacity of ILs with shorter alkyl side chains was smaller than that of ILs with longer alkyl side chains. We simulated the absorption process using Gaussian software and found that the specific heat capacities of ILs and SO₂ systems were mainly caused by the vibration of molecules.