Hydration of the Sulfuric Acid–Methylamine Complex and Implications for Aerosol Formation

The binary H₂SO₄–H₂O nucleation is one of the most important pathways by which aerosols form in the atmosphere, and the presence of ternary species like amines increases aerosol formation rates. In this study, we focus on the hydration of a ternary system of sulfuric acid (H₂SO₄), methylamine (NH₂CH₃), and up to six waters to evaluate its implications for aerosol formation. By combining molecular dynamics (MD) sampling with high-level ab initio calculations, we determine the thermodynamics of forming H₂SO₄(NH₂CH₃)­(H₂O)_<i>n</i>, where <i>n</i> = 0–6. Because it is a strong acid–base system, H₂SO₄–NH₂CH₃ quickly forms a tightly bound HSO₄^––NH₃CH₃⁺ complex that condenses water more readily than H₂SO₄ alone. The electronic binding energy of H₂SO₄–NH₂CH₃ is −21.8 kcal mol^–1 compared with −16.8 kcal mol^–1 for H₂SO₄–NH₃ and −12.8 kcal mol^–1 for H₂SO₄–H₂O. Adding one to two water molecules to the H₂SO₄–NH₂CH₃ complex is more favorable than adding to H₂SO₄ alone, yet there is no systematic difference for <i>n</i> ≥ 3. However, the average number of water molecules around H₂SO₄–NH₂CH₃ is consistently higher than that of H₂SO₄, and it is fairly independent of temperature and relative humidity