UC Irvine

The fundamental properties of 1-n-alkyl-3-methylimidazolium ([CnMIM+], n = number of carbons on the linear alkyl chain)-based ionic liquids (ILs) are not fully characterized, including quantification of the amount of water or carbon dioxide (CO2) absorbed from the atmosphere and the nature of the IL‒water or IL‒CO2 interactions. For example, interactions of bulk ILs and water in previous studies were limited by slow equilibration times of the ILs when exposed to water vapor. This is the first study of the interaction of water or CO2 with aerosolized IL nanoparticles. The particles’ small size ensures IL‒water vapor equilibrium is achieved quickly. Additionally, ILs may have the ability to absorb large amounts of CO2 and can be used for CO2 sequestration in the environment.

The combined experimental and theoretical techniques of tandem nano-differential mobility analysis (TDMA), attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and molecular dynamics (MD) simulations were used to provide a comprehensive picture of the behavior of absorbed water or CO2 in IL nanoparticles and thin films. [CnMIM]+ (n = 2, 4, or 6) paired with Cl‒ or BF4‒ were studied to examine the effect of alkyl chain length on IL hygroscopicity and CO2 uptake. Particles' growth from water or CO2 uptake was quantified using TDMA. For the hygroscopicity studies, growth curves show steady uptake of water vapor with increasing relative humidity. Water molar fractions (χw) agreed well with those estimated from ATR-FTIR data. Additionally, the performance of several liquid activity coefficient models were evaluated by fits to the experimental data. MD simulations, used to model IL−water interactions, revealed considerably stronger interactions between Cl− and water compared to BF4− and water. The increase in growth factor after CO2 exposure showed little change for the IL studied. These combined experimental, theoretical, and modeling techniques help provide a more complete picture for these two families of ILs in the presence of water or CO2. The techniques also provide a quantitative parameterization of the amount of water vapor that ILs with different alkyl chain lengths can absorb under equilibrium conditions.

The use of ILs as suitable seed particles for secondary organic formation and growth from α-pinene ozonolysis in an environmental smog chamber was also explored. Additionally, we investigated the sensitivity of the nanoparticles’ chemical composition to varying operation conditions in electrospray aerosol generators.