Amine-Enhanced Methanesulfonic Acid-Driven Nucleation: Predictive Model and Cluster Formation Mechanism

Atmospheric amines are considered to be an effective enhancer for methanesulfonic acid (MSA)-driven nucleation. However, out of the 195 detected atmospheric amines, the enhancing potential (EP) has so far only been studied for five amines. This severely hinders the understanding of the contribution of amines to MSA-driven nucleation. Herein, a two-step procedure was employed to probe the EP of various amines on MSA-driven nucleation. Initially, the formation free energies (ΔG) of 50 MSA-amine dimer clusters were calculated. Based on the calculated ΔG values, a robust quantitative structure-activity relationship (QSAR) model was built and utilized to predict the ΔG values of the remaining 145 amines. The QSAR model identified two guanidino-containing compounds as the potentially strongest enhancer for MSA-driven nucleation. Second, the EP of guanidino-containing compounds was studied by employing larger clusters and selecting guanidine (Gud) as a representative. The results indicate that Gud indeed has the strongest EP. The Gud-MSA system presents a unique clustering mechanism, proceeding via the initial formation of the (Gud)₁(MSA)₁ cluster, and subsequently by cluster collisions with either a (Gud)₁(MSA)₁ or (Gud)₂(MSA)₂ cluster. The developed QSAR model and the identification of amines with the strongest EP provide a foundation for comprehensively evaluating the contribution of atmospheric amines to MSA-driven nucleation.